Android Fundamentals:

Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apksuffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.

Once installed on a device, each Android application lives in its own security sandbox:

The Android operating system is a multi-user Linux system in which each application is a different user.
By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.
Each process has its own virtual machine (VM), so an application's code runs in isolation from other applications.
By default, every application runs in its own Linux process. Android starts the process when any of the application's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications.

In this way, the Android system implements the principle of least privilege. That is, each application, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an application cannot access parts of the system for which it is not given permission.

However, there are ways for an application to share data with other applications and for an application to access system services:

It's possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other's files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).
An application can request permission to access device data such as the user's contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.

That covers the basics regarding how an Android application exists within the system. The rest of this document introduces you to:

The core framework components that define your application.
The manifest file in which you declare components and required device features for your application.

Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations.

Application components are the essential building blocks of an Android application. Each component is a different point through which the system can enter your application. Not all components are actual entry points for the user and some depend on each other, but each one exists as its own entity and plays a specific role—each one is a unique building block that helps define your application's overall behavior.

There are four different types of application components. Each type serves a distinct purpose and has a distinct lifecycle that defines how the component is created and destroyed.

Here are the four types of application components:

Activities: An activity represents a single screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. Although the activities work together to form a cohesive user experience in the email application, each one is independent of the others. As such, a different application can start any one of these activities (if the email application allows it). For example, a camera application can start the activity in the email application that composes new mail, in order for the user to share a picture.
An activity is implemented as a subclass of Activity and you can learn more about it in the Activitiesdeveloper guide.
Services: A service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking user interaction with an activity. Another component, such as an activity, can start the service and let it run or bind to it in order to interact with it.
A service is implemented as a subclass of Service and you can learn more about it in the Servicesdeveloper guide.
Content providers: A content provider manages a shared set of application data. You can store the data in the file system, an SQLite database, on the web, or any other persistent storage location your application can access. Through the content provider, other applications can query or even modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the user's contact information. As such, any application with the proper permissions can query part of the content provider (such as ContactsContract.Data) to read and write information about a particular person.
Content providers are also useful for reading and writing data that is private to your application and not shared. For example, the Note Pad sample application uses a content provider to save notes.

A content provider is implemented as a subclass of ContentProvider and must implement a standard set of APIs that enable other applications to perform transactions. For more information, see the Content Providers developer guide.
Broadcast receivers: A broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the system—for example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. Applications can also initiate broadcasts—for example, to let other applications know that some data has been downloaded to the device and is available for them to use. Although broadcast receivers don't display a user interface, they may create a status bar notificationto alert the user when a broadcast event occurs. More commonly, though, a broadcast receiver is just a "gateway" to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event.
A broadcast receiver is implemented as a subclass of BroadcastReceiver and each broadcast is delivered as an Intent object. For more information, see the BroadcastReceiver class.

A unique aspect of the Android system design is that any application can start another application’s component. For example, if you want the user to capture a photo with the device camera, there's probably another application that does that and your application can use it, instead of developing an activity to capture a photo yourself. You don't need to incorporate or even link to the code from the camera application. Instead, you can simply start the activity in the camera application that captures a photo. When complete, the photo is even returned to your application so you can use it. To the user, it seems as if the camera is actually a part of your application.

When the system starts a component, it starts the process for that application (if it's not already running) and instantiates the classes needed for the component. For example, if your application starts the activity in the camera application that captures a photo, that activity runs in the process that belongs to the camera application, not in your application's process. Therefore, unlike applications on most other systems, Android applications don't have a single entry point (there's no main() function, for example).

Because the system runs each application in a separate process with file permissions that restrict access to other applications, your application cannot directly activate a component from another application. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies your intent to start a particular component. The system then activates the component for you.

Activating Components

Three of the four component types—activities, services, and broadcast receivers—are activated by an asynchronous message called an intent. Intents bind individual components to each other at runtime (you can think of them as the messengers that request an action from other components), whether the component belongs to your application or another.

An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of component—an intent can be either explicit or implicit, respectively.

For activities and services, an intent defines the action to perform (for example, to "view" or "send" something) and may specify the URI of the data to act on (among other things that the component being started might need to know). For example, an intent might convey a request for an activity to show an image or to open a web page. In some cases, you can start an activity to receive a result, in which case, the activity also returns the result in anIntent (for example, you can issue an intent to let the user pick a personal contact and have it returned to you—the return intent includes a URI pointing to the chosen contact).

For broadcast receivers, the intent simply defines the announcement being broadcast (for example, a broadcast to indicate the device battery is low includes only a known action string that indicates "battery is low").

The other component type, content provider, is not activated by intents. Rather, it is activated when targeted by a request from a ContentResolver. The content resolver handles all direct transactions with the content provider so that the component that's performing transactions with the provider doesn't need to and instead calls methods on the ContentResolver object. This leaves a layer of abstraction between the content provider and the component requesting information (for security).

There are separate methods for activating each type of component:

You can start an activity (or give it something new to do) by passing an Intent to startActivity() orstartActivityForResult() (when you want the activity to return a result).
You can start a service (or give new instructions to an ongoing service) by passing an Intent tostartService(). Or you can bind to the service by passing an Intent to bindService().
You can initiate a broadcast by passing an Intent to methods like sendBroadcast(),sendOrderedBroadcast(), or sendStickyBroadcast().
You can perform a query to a content provider by calling query() on a ContentResolver.

For more information about using intents, see the Intents and Intent Filters document. More information about activating specific components is also provided in the following documents: Activities, Services,BroadcastReceiver and Content Providers.

The Manifest File

Before the Android system can start an application component, the system must know that the component exists by reading the application's AndroidManifest.xml file (the "manifest" file). Your application must declare all its components in this file, which must be at the root of the application project directory.

The manifest does a number of things in addition to declaring the application's components, such as:

Identify any user permissions the application requires, such as Internet access or read-access to the user's contacts.
Declare the minimum API Level required by the application, based on which APIs the application uses.
Declare hardware and software features used or required by the application, such as a camera, bluetooth services, or a multitouch screen.
API libraries the application needs to be linked against (other than the Android framework APIs), such as theGoogle Maps library.

Declaring components

The primary task of the manifest is to inform the system about the application's components. For example, a manifest file can declare an activity as follows:

<?xml version="1.0" encoding="utf-8"?>
<manifest ... >
    <application android:icon="@drawable/app_icon.png" ... >
        <activity android:name="com.example.project.ExampleActivity"
                  android:label="@string/example_label" ... >
        </activity>
        ...
    </application>
</manifest>

In the <application> element, the android:icon attribute points to resources for an icon that identifies the application.

In the <activity> element, the android:name attribute specifies the fully qualified class name of theActivity subclass and the android:label attributes specifies a string to use as the user-visible label for the activity.

You must declare all application components this way:

<activity> elements for activities
<service> elements for services
<receiver> elements for broadcast receivers
<provider> elements for content providers

Activities, services, and content providers that you include in your source but do not declare in the manifest are not visible to the system and, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiver objects) and registered with the system by calling registerReceiver().

For more about how to structure the manifest file for your application, see The AndroidManifest.xml Filedocumentation.

Declaring component capabilities

As discussed above, in Activating Components, you can use an Intent to start activities, services, and broadcast receivers. You can do so by explicitly naming the target component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perform (and optionally, the data upon which you’d like to perform the action) and allow the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the user selects which one to use.

The way the system identifies the components that can respond to an intent is by comparing the intent received to the intent filters provided in the manifest file of other applications on the device.

When you declare a component in your application's manifest, you can optionally include intent filters that declare the capabilities of the component so it can respond to intents from other applications. You can declare an intent filter for your component by adding an <intent-filter> element as a child of the component's declaration element.

For example, an email application with an activity for composing a new email might declare an intent filter in its manifest entry to respond to "send" intents (in order to send email). An activity in your application can then create an intent with the “send” action (ACTION_SEND), which the system matches to the email application’s “send” activity and launches it when you invoke the intent with startActivity().

For more about creating intent filters, see the Intents and Intent Filters document.

Declaring application requirements

There are a variety of devices powered by Android and not all of them provide the same features and capabilities. In order to prevent your application from being installed on devices that lack features needed by your application, it's important that you clearly define a profile for the types of devices your application supports by declaring device and software requirements in your manifest file. Most of these declarations are informational only and the system does not read them, but external services such as Google Play do read them in order to provide filtering for users when they search for applications from their device.

For example, if your application requires a camera and uses APIs introduced in Android 2.1 (API Level 7), you should declare these as requirements in your manifest file. That way, devices that do not have a camera and have an Android version lower than 2.1 cannot install your application from Google Play.

However, you can also declare that your application uses the camera, but does not require it. In that case, your application must perform a check at runtime to determine if the device has a camera and disable any features that use the camera if one is not available.

Here are some of the important device characteristics that you should consider as you design and develop your application:

Screen size and density: In order to categorize devices by their screen type, Android defines two characteristics for each device: screen size (the physical dimensions of the screen) and screen density (the physical density of the pixels on the screen, or dpi—dots per inch). To simplify all the different types of screen configurations, the Android system generalizes them into select groups that make them easier to target.
The screen sizes are: small, normal, large, and extra large.
The screen densities are: low density, medium density, high density, and extra high density.

By default, your application is compatible with all screen sizes and densities, because the Android system makes the appropriate adjustments to your UI layout and image resources. However, you should create specialized layouts for certain screen sizes and provide specialized images for certain densities, using alternative layout resources, and by declaring in your manifest exactly which screen sizes your application supports with the <supports-screens> element.

For more information, see the Supporting Multiple Screens document.
Input configurations: Many devices provide a different type of user input mechanism, such as a hardware keyboard, a trackball, or a five-way navigation pad. If your application requires a particular kind of input hardware, then you should declare it in your manifest with the <uses-configuration> element. However, it is rare that an application should require a certain input configuration.
Device features: There are many hardware and software features that may or may not exist on a given Android-powered device, such as a camera, a light sensor, bluetooth, a certain version of OpenGL, or the fidelity of the touchscreen. You should never assume that a certain feature is available on all Android-powered devices (other than the availability of the standard Android library), so you should declare any features used by your application with the <uses-feature> element.
Platform Version: Different Android-powered devices often run different versions of the Android platform, such as Android 1.6 or Android 2.3. Each successive version often includes additional APIs not available in the previous version. In order to indicate which set of APIs are available, each platform version specifies an API Level(for example, Android 1.0 is API Level 1 and Android 2.3 is API Level 9). If you use any APIs that were added to the platform after version 1.0, you should declare the minimum API Level in which those APIs were introduced using the <uses-sdk> element.

It's important that you declare all such requirements for your application, because, when you distribute your application on Google Play, the store uses these declarations to filter which applications are available on each device. As such, your application should be available only to devices that meet all your application requirements.

For more information about how Google Play filters applications based on these (and other) requirements, see the Filters on Google Play document.

Application Resources

An Android application is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the application. For example, you should define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using application resources makes it easy to update various characteristics of your application without modifying code and—by providing sets of alternative resources—enables you to optimize your application for a variety of device configurations (such as different languages and screen sizes).

For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your application code or from other resources defined in XML. For example, if your application contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource ID named R.drawable.logo, which you can use to reference the image and insert it in your user interface.

One of the most important aspects of providing resources separate from your source code is the ability for you to provide alternative resources for different device configurations. For example, by defining UI strings in XML, you can translate the strings into other languages and save those strings in separate files. Then, based on a language qualifier that you append to the resource directory's name (such as res/values-fr/ for French string values) and the user's language setting, the Android system applies the appropriate language strings to your UI.

Android supports many different qualifiers for your alternative resources. The qualifier is a short string that you include in the name of your resource directories in order to define the device configuration for which those resources should be used. As another example, you should often create different layouts for your activities, depending on the device's screen orientation and size. For example, when the device screen is in portrait orientation (tall), you might want a layout with buttons to be vertical, but when the screen is in landscape orientation (wide), the buttons should be aligned horizontally. To change the layout depending on the orientation, you can define two different layouts and apply the appropriate qualifier to each layout's directory name. Then, the system automatically applies the appropriate layout depending on the current device orientation.

For more about the different kinds of resources you can include in your application and how to create alternative resources for various device configurations, see the Application Resources developer guide.

Activities

An Activity is an application component that provides a screen with which users can interact in order to do something, such as dial the phone, take a photo, send an email, or view a map. Each activity is given a window in which to draw its user interface. The window typically fills the screen, but may be smaller than the screen and float on top of other windows.

An application usually consists of multiple activities that are loosely bound to each other. Typically, one activity in an application is specified as the "main" activity, which is presented to the user when launching the application for the first time. Each activity can then start another activity in order to perform different actions. Each time a new activity starts, the previous activity is stopped, but the system preserves the activity in a stack (the "back stack"). When a new activity starts, it is pushed onto the back stack and takes user focus. The back stack abides to the basic "last in, first out" stack mechanism, so, when the user is done with the current activity and presses theBack button, it is popped from the stack (and destroyed) and the previous activity resumes. (The back stack is discussed more in theTasks and Back Stack document.)

When an activity is stopped because a new activity starts, it is notified of this change in state through the activity's lifecycle callback methods. There are several callback methods that an activity might receive, due to a change in its state—whether the system is creating it, stopping it, resuming it, or destroying it—and each callback provides you the opportunity to perform specific work that's appropriate to that state change. For instance, when stopped, your activity should release any large objects, such as network or database connections. When the activity resumes, you can reacquire the necessary resources and resume actions that were interrupted. These state transitions are all part of the activity lifecycle.

The rest of this document discusses the basics of how to build and use an activity, including a complete discussion of how the activity lifecycle works, so you can properly manage the transition between various activity states.

Creating an Activity

To create an activity, you must create a subclass of Activity (or an existing subclass of it). In your subclass, you need to implement callback methods that the system calls when the activity transitions between various states of its lifecycle, such as when the activity is being created, stopped, resumed, or destroyed. The two most important callback methods are:

onCreate(): You must implement this method. The system calls this when creating your activity. Within your implementation, you should initialize the essential components of your activity. Most importantly, this is where you must call setContentView() to define the layout for the activity's user interface.
onPause(): The system calls this method as the first indication that the user is leaving your activity (though it does not always mean the activity is being destroyed). This is usually where you should commit any changes that should be persisted beyond the current user session (because the user might not come back).

There are several other lifecycle callback methods that you should use in order to provide a fluid user experience between activities and handle unexpected interuptions that cause your activity to be stopped and even destroyed. All of the lifecycle callback methods are discussed later, in the section about Managing the Activity Lifecycle.

Implementing a user interface

The user interface for an activity is provided by a hierarchy of views—objects derived from the View class. Each view controls a particular rectangular space within the activity's window and can respond to user interaction. For example, a view might be a button that initiates an action when the user touches it.

Android provides a number of ready-made views that you can use to design and organize your layout. "Widgets" are views that provide a visual (and interactive) elements for the screen, such as a button, text field, checkbox, or just an image. "Layouts" are views derived from ViewGroup that provide a unique layout model for its child views, such as a linear layout, a grid layout, or relative layout. You can also subclass the View and ViewGroupclasses (or existing subclasses) to create your own widgets and layouts and apply them to your activity layout.

The most common way to define a layout using views is with an XML layout file saved in your application resources. This way, you can maintain the design of your user interface separately from the source code that defines the activity's behavior. You can set the layout as the UI for your activity with setContentView(), passing the resource ID for the layout. However, you can also create new Views in your activity code and build a view hierarchy by inserting new Views into a ViewGroup, then use that layout by passing the root ViewGroup tosetContentView().

For information about creating a user interface, see the User Interface documentation.

Declaring the activity in the manifest

You must declare your activity in the manifest file in order for it to be accessible to the system. To declare your activity, open your manifest file and add an <activity> element as a child of the <application> element. For example:

<manifest ... >
  <application ... >
      <activity android:name=".ExampleActivity" />
      ...
  </application ... >
  ...</manifest >

There are several other attributes that you can include in this element, to define properties such as the label for the activity, an icon for the activity, or a theme to style the activity's UI. The android:name attribute is the only required attribute—it specifies the class name of the activity. Once you publish your application, you should not change this name, because if you do, you might break some functionality, such as application shortcuts (read the blog post, Things That Cannot Change).

See the <activity> element reference for more information about declaring your activity in the manifest.

Using intent filters

An <activity> element can also specify various intent filters—using the <intent-filter> element—in order to declare how other application components may activate it.

When you create a new application using the Android SDK tools, the stub activity that's created for you automatically includes an intent filter that declares the activity responds to the "main" action and should be placed in the "launcher" category. The intent filter looks like this:

<activity android:name=".ExampleActivity" android:icon="@drawable/app_icon">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

The <action> element specifies that this is the "main" entry point to the application. The <category> element specifies that this activity should be listed in the system's application launcher (to allow users to launch this activity).

If you intend for your application to be self-contained and not allow other applications to activate its activities, then you don't need any other intent filters. Only one activity should have the "main" action and "launcher" category, as in the previous example. Activities that you don't want to make available to other applications should have no intent filters and you can start them yourself using explicit intents (as discussed in the following section).

However, if you want your activity to respond to implicit intents that are delivered from other applications (and your own), then you must define additional intent filters for your activity. For each type of intent to which you want to respond, you must include an <intent-filter> that includes an <action> element and, optionally, a<category> element and/or a <data> element. These elements specify the type of intent to which your activity can respond.

For more information about how your activities can respond to intents, see the Intents and Intent Filtersdocument.

Starting an Activity

You can start another activity by calling startActivity(), passing it an Intent that describes the activity you want to start. The intent specifies either the exact activity you want to start or describes the type of action you want to perform (and the system selects the appropriate activity for you, which can even be from a different application). An intent can also carry small amounts of data to be used by the activity that is started.

When working within your own application, you'll often need to simply launch a known activity. You can do so by creating an intent that explicitly defines the activity you want to start, using the class name. For example, here's how one activity starts another activity named SignInActivity:

Intent intent = new Intent(this, SignInActivity.class);
startActivity(intent);

However, your application might also want to perform some action, such as send an email, text message, or status update, using data from your activity. In this case, your application might not have its own activities to perform such actions, so you can instead leverage the activities provided by other applications on the device, which can perform the actions for you. This is where intents are really valuable—you can create an intent that describes an action you want to perform and the system launches the appropriate activity from another application. If there are multiple activities that can handle the intent, then the user can select which one to use. For example, if you want to allow the user to send an email message, you can create the following intent:

Intent intent = new Intent(Intent.ACTION_SEND);
intent.putExtra(Intent.EXTRA_EMAIL, recipientArray);
startActivity(intent);

The EXTRA_EMAIL extra added to the intent is a string array of email addresses to which the email should be sent. When an email application responds to this intent, it reads the string array provided in the extra and places them in the "to" field of the email composition form. In this situation, the email application's activity starts and when the user is done, your activity resumes.

Starting an activity for a result

Sometimes, you might want to receive a result from the activity that you start. In that case, start the activity by calling startActivityForResult() (instead of startActivity()). To then receive the result from the subsequent activity, implement the onActivityResult() callback method. When the subsequent activity is done, it returns a result in an Intent to your onActivityResult() method.

For example, perhaps you want the user to pick one of their contacts, so your activity can do something with the information in that contact. Here's how you can create such an intent and handle the result:

private void pickContact() {
    // Create an intent to "pick" a contact, as defined by the content provider URI
    Intent intent = new Intent(Intent.ACTION_PICK, Contacts.CONTENT_URI);
    startActivityForResult(intent, PICK_CONTACT_REQUEST);
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    // If the request went well (OK) and the request was PICK_CONTACT_REQUEST
    if (resultCode == Activity.RESULT_OK && requestCode == PICK_CONTACT_REQUEST) {
        // Perform a query to the contact's content provider for the contact's name
        Cursor cursor = getContentResolver().query(data.getData(),
        new String[] {Contacts.DISPLAY_NAME}, null, null, null);
        if (cursor.moveToFirst()) { // True if the cursor is not empty
            int columnIndex = cursor.getColumnIndex(Contacts.DISPLAY_NAME);
            String name = cursor.getString(columnIndex);
            // Do something with the selected contact's name...
        }
    }
}

This example shows the basic logic you should use in your onActivityResult() method in order to handle an activity result. The first condition checks whether the request was successful—if it was, then the resultCodewill be RESULT_OK—and whether the request to which this result is responding is known—in this case, therequestCode matches the second parameter sent with startActivityForResult(). From there, the code handles the activity result by querying the data returned in an Intent (the data parameter).

What happens is, a ContentResolver performs a query against a content provider, which returns a Cursorthat allows the queried data to be read. For more information, see the Content Providers document.

For more information about using intents, see the Intents and Intent Filters document.

Shutting Down an Activity

You can shut down an activity by calling its finish() method. You can also shut down a separate activity that you previously started by calling finishActivity().

Note: In most cases, you should not explicitly finish an activity using these methods. As discussed in the following section about the activity lifecycle, the Android system manages the life of an activity for you, so you do not need to finish your own activities. Calling these methods could adversely affect the expected user experience and should only be used when you absolutely do not want the user to return to this instance of the activity.

Managing the Activity Lifecycle

Managing the lifecycle of your activities by implementing callback methods is crucial to developing a strong and flexible application. The lifecycle of an activity is directly affected by its association with other activities, its task and back stack.

An activity can exist in essentially three states:

Resumed: The activity is in the foreground of the screen and has user focus. (This state is also sometimes referred to as "running".)
Paused: Another activity is in the foreground and has focus, but this one is still visible. That is, another activity is visible on top of this one and that activity is partially transparent or doesn't cover the entire screen. A paused activity is completely alive (the Activity object is retained in memory, it maintains all state and member information, and remains attached to the window manager), but can be killed by the system in extremely low memory situations.
Stopped: The activity is completely obscured by another activity (the activity is now in the "background"). A stopped activity is also still alive (the Activity object is retained in memory, it maintains all state and member information, but is not attached to the window manager). However, it is no longer visible to the user and it can be killed by the system when memory is needed elsewhere.

If an activity is paused or stopped, the system can drop it from memory either by asking it to finish (calling itsfinish() method), or simply killing its process. When the activity is opened again (after being finished or killed), it must be created all over.

Implementing the lifecycle callbacks

When an activity transitions into and out of the different states described above, it is notified through various callback methods. All of the callback methods are hooks that you can override to do appropriate work when the state of your activity changes. The following skeleton activity includes each of the fundamental lifecycle methods:

public class ExampleActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // The activity is being created.
    }
    @Override
    protected void onStart() {
        super.onStart();
        // The activity is about to become visible.
    }
    @Override
    protected void onResume() {
        super.onResume();
        // The activity has become visible (it is now "resumed").
    }
    @Override
    protected void onPause() {
        super.onPause();
        // Another activity is taking focus (this activity is about to be "paused").
    }
    @Override
    protected void onStop() {
        super.onStop();
        // The activity is no longer visible (it is now "stopped")
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        // The activity is about to be destroyed.
    }
}

Note: Your implementation of these lifecycle methods must always call the superclass implementation before doing any work, as shown in the examples above.

Taken together, these methods define the entire lifecycle of an activity. By implementing these methods, you can monitor three nested loops in the activity lifecycle:

The entire lifetime of an activity happens between the call to onCreate() and the call to onDestroy(). Your activity should perform setup of "global" state (such as defining layout) in onCreate(), and release all remaining resources in onDestroy(). For example, if your activity has a thread running in the background to download data from the network, it might create that thread in onCreate() and then stop the thread inonDestroy().
The visible lifetime of an activity happens between the call to onStart() and the call to onStop(). During this time, the user can see the activity on-screen and interact with it. For example, onStop() is called when a new activity starts and this one is no longer visible. Between these two methods, you can maintain resources that are needed to show the activity to the user. For example, you can register a BroadcastReceiver inonStart() to monitor changes that impact your UI, and unregister it in onStop() when the user can no longer see what you are displaying. The system might call onStart() and onStop() multiple times during the entire lifetime of the activity, as the activity alternates between being visible and hidden to the user.
The foreground lifetime of an activity happens between the call to onResume() and the call to onPause(). During this time, the activity is in front of all other activities on screen and has user input focus. An activity can frequently transition in and out of the foreground—for example, onPause() is called when the device goes to sleep or when a dialog appears. Because this state can transition often, the code in these two methods should be fairly lightweight in order to avoid slow transitions that make the user wait.

Figure 1 illustrates these loops and the paths an activity might take between states. The rectangles represent the callback methods you can implement to perform operations when the activity transitions between states.

Figure 1. The activity lifecycle.

The same lifecycle callback methods are listed in table 1, which describes each of the callback methods in more detail and locates each one within the activity's overall lifecycle, including whether the system can kill the activity after the callback method completes.

Table 1. A summary of the activity lifecycle's callback methods.

Method			Description	Killable after?	Next
`onCreate()`			Called when the activity is first created. This is where you should do all of your normal static set up — create views, bind data to lists, and so on. This method is passed a Bundle object containing the activity's previous state, if that state was captured (see Saving Activity State, later).Always followed by `onStart()`.	No	`onStart()`
	`onRestart()`		Called after the activity has been stopped, just prior to it being started again.Always followed by `onStart()`	No	`onStart()`
	`onStart()`		Called just before the activity becomes visible to the user.Followed by `onResume()` if the activity comes to the foreground, or `onStop()` if it becomes hidden.	No	`onResume()` or `onStop()`
		`onResume()`	Called just before the activity starts interacting with the user. At this point the activity is at the top of the activity stack, with user input going to it.Always followed by `onPause()`.	No	`onPause()`
		`onPause()`	Called when the system is about to start resuming another activity. This method is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, and so on. It should do whatever it does very quickly, because the next activity will not be resumed until it returns.Followed either by `onResume()` if the activity returns back to the front, or by `onStop()` if it becomes invisible to the user.	Yes	`onResume()` or `onStop()`
	`onStop()`		Called when the activity is no longer visible to the user. This may happen because it is being destroyed, or because another activity (either an existing one or a new one) has been resumed and is covering it.Followed either by `onRestart()` if the activity is coming back to interact with the user, or by`onDestroy()` if this activity is going away.	Yes	`onRestart()` or `onDestroy()`
`onDestroy()`			Called before the activity is destroyed. This is the final call that the activity will receive. It could be called either because the activity is finishing (someone called `finish()` on it), or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the`isFinishing()` method.	Yes	nothing

The column labeled "Killable after?" indicates whether or not the system can kill the process hosting the activity at any time after the method returns, without executing another line of the activity's code. Three methods are marked "yes": (onPause(), onStop(), and onDestroy()). Because onPause() is the first of the three, once the activity is created, onPause() is the last method that's guaranteed to be called before the process can be killed—if the system must recover memory in an emergency, then onStop() and onDestroy() might not be called. Therefore, you should use onPause() to write crucial persistent data (such as user edits) to storage. However, you should be selective about what information must be retained during onPause(), because any blocking procedures in this method block the transition to the next activity and slow the user experience.

Methods that are marked "No" in the Killable column protect the process hosting the activity from being killed from the moment they are called. Thus, an activity is killable from the time onPause() returns to the timeonResume() is called. It will not again be killable until onPause() is again called and returns.

Note: An activity that's not technically "killable" by this definition in table 1 might still be killed by the system—but that would happen only in extreme circumstances when there is no other recourse. When an activity might be killed is discussed more in the Processes and Threading document.

Saving activity state

The introduction to Managing the Activity Lifecycle briefly mentions that when an activity is paused or stopped, the state of the activity is retained. This is true because the Activity object is still held in memory when it is paused or stopped—all information about its members and current state is still alive. Thus, any changes the user made within the activity are retained so that when the activity returns to the foreground (when it "resumes"), those changes are still there.

However, when the system destroys an activity in order to recover memory, the Activity object is destroyed, so the system cannot simply resume it with its state intact. Instead, the system must recreate the Activity object if the user navigates back to it. Yet, the user is unaware that the system destroyed the activity and recreated it and, thus, probably expects the activity to be exactly as it was. In this situation, you can ensure that important information about the activity state is preserved by implementing an additional callback method that allows you to save information about the state of your activity: onSaveInstanceState().

The system calls onSaveInstanceState() before making the activity vulnerable to destruction. The system passes this method a Bundle in which you can save state information about the activity as name-value pairs, using methods such as putString() and putInt(). Then, if the system kills your application process and the user navigates back to your activity, the system recreates the activity and passes the Bundle to bothonCreate() and onRestoreInstanceState(). Using either of these methods, you can extract your saved state from the Bundle and restore the activity state. If there is no state information to restore, then the Bundlepassed to you is null (which is the case when the activity is created for the first time).

Figure 2. The two ways in which an activity returns to user focus with its state intact: either the activity is destroyed, then recreated and the activity must restore the previously saved state, or the activity is stopped, then resumed and the activity state remains intact.

Note: There's no guarantee that onSaveInstanceState() will be called before your activity is destroyed, because there are cases in which it won't be necessary to save the state (such as when the user leaves your activity using the Back button, because the user is explicitly closing the activity). If the system callsonSaveInstanceState(), it does so before onStop() and possibly before onPause().

However, even if you do nothing and do not implement onSaveInstanceState(), some of the activity state is restored by the Activity class's default implementation of onSaveInstanceState(). Specifically, the default implementation calls the corresponding onSaveInstanceState() method for every View in the layout, which allows each view to provide information about itself that should be saved. Almost every widget in the Android framework implements this method as appropriate, such that any visible changes to the UI are automatically saved and restored when your activity is recreated. For example, the EditText widget saves any text entered by the user and the CheckBox widget saves whether it's checked or not. The only work required by you is to provide a unique ID (with the android:id attribute) for each widget you want to save its state. If a widget does not have an ID, then the system cannot save its state.

You can also explicitly stop a view in your layout from saving its state by setting the android:saveEnabledattribute to "false" or by calling thesetSaveEnabled() method. Usually, you should not disable this, but you might if you want to restore the state of the activity UI differently.

Although the default implementation of onSaveInstanceState()saves useful information about your activity's UI, you still might need to override it to save additional information. For example, you might need to save member values that changed during the activity's life (which might correlate to values restored in the UI, but the members that hold those UI values are not restored, by default).

Because the default implementation of onSaveInstanceState()helps save the state of the UI, if you override the method in order to save additional state information, you should always call the superclass implementation of onSaveInstanceState() before doing any work. Likewise, you should also call the superclass implementation of onRestoreInstanceState()if you override it, so the default implementation can restore view states.

Note: Because onSaveInstanceState() is not guaranteed to be called, you should use it only to record the transient state of the activity (the state of the UI)—you should never use it to store persistent data. Instead, you should use onPause() to store persistent data (such as data that should be saved to a database) when the user leaves the activity.

A good way to test your application's ability to restore its state is to simply rotate the device so that the screen orientation changes. When the screen orientation changes, the system destroys and recreates the activity in order to apply alternative resources that might be available for the new screen configuration. For this reason alone, it's very important that your activity completely restores its state when it is recreated, because users regularly rotate the screen while using applications.

Handling configuration changes

Some device configurations can change during runtime (such as screen orientation, keyboard availability, and language). When such a change occurs, Android recreates the running activity (the system calls onDestroy(), then immediately calls onCreate()). This behavior is designed to help your application adapt to new configurations by automatically reloading your application with alternative resources that you've provided (such as different layouts for different screen orientations and sizes).

If you properly design your activity to handle a restart due to a screen orientation change and restore the activity state as described above, your application will be more resilient to other unexpected events in the activity lifecycle.

The best way to handle such a restart is to save and restore the state of your activity usingonSaveInstanceState() and onRestoreInstanceState() (or onCreate()), as discussed in the previous section.

For more information about configuration changes that happen at runtime and how you can handle them, read the guide to Handling Runtime Changes.

Coordinating activities

When one activity starts another, they both experience lifecycle transitions. The first activity pauses and stops (though, it won't stop if it's still visible in the background), while the other activity is created. In case these activities share data saved to disc or elsewhere, it's important to understand that the first activity is not completely stopped before the second one is created. Rather, the process of starting the second one overlaps with the process of stopping the first one.

The order of lifecycle callbacks is well defined, particularly when the two activities are in the same process and one is starting the other. Here's the order of operations that occur when Activity A starts Acivity B:

Activity A's onPause() method executes.
Activity B's onCreate(), onStart(), and onResume() methods execute in sequence. (Activity B now has user focus.)
Then, if Activity A is no longer visible on screen, its onStop() method executes.

This predictable sequence of lifecycle callbacks allows you to manage the transition of information from one activity to another. For example, if you must write to a database when the first activity stops so that the following activity can read it, then you should write to the database during `onPause()` instead of during`onStop()`.

Fragments

A Fragment represents a behavior or a portion of user interface in anActivity. You can combine multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities. You can think of a fragment as a modular section of an activity, which has its own lifecycle, receives its own input events, and which you can add or remove while the activity is running (sort of like a "sub activity" that you can reuse in different activities).

A fragment must always be embedded in an activity and the fragment's lifecycle is directly affected by the host activity's lifecycle. For example, when the activity is paused, so are all fragments in it, and when the activity is destroyed, so are all fragments. However, while an activity is running (it is in the resumed lifecycle state), you can manipulate each fragment independently, such as add or remove them. When you perform such a fragment transaction, you can also add it to a back stack that's managed by the activity—each back stack entry in the activity is a record of the fragment transaction that occurred. The back stack allows the user to reverse a fragment transaction (navigate backwards), by pressing the Back button.

When you add a fragment as a part of your activity layout, it lives in aViewGroup inside the activity's view hierarchy and the fragment defines its own view layout. You can insert a fragment into your activity layout by declaring the fragment in the activity's layout file, as a <fragment> element, or from your application code by adding it to an existing ViewGroup. However, a fragment is not required to be a part of the activity layout; you may also use a fragment without its own UI as an invisible worker for the activity.

This document describes how to build your application to use fragments, including how fragments can maintain their state when added to the activity's back stack, share events with the activity and other fragments in the activity, contribute to the activity's action bar, and more.

Design Philosophy

Android introduced fragments in Android 3.0 (API level 11), primarily to support more dynamic and flexible UI designs on large screens, such as tablets. Because a tablet's screen is much larger than that of a handset, there's more room to combine and interchange UI components. Fragments allow such designs without the need for you to manage complex changes to the view hierarchy. By dividing the layout of an activity into fragments, you become able to modify the activity's appearance at runtime and preserve those changes in a back stack that's managed by the activity.

For example, a news application can use one fragment to show a list of articles on the left and another fragment to display an article on the right—both fragments appear in one activity, side by side, and each fragment has its own set of lifecycle callback methods and handle their own user input events. Thus, instead of using one activity to select an article and another activity to read the article, the user can select an article and read it all within the same activity, as illustrated in the tablet layout in figure 1.

You should design each fragment as a modular and reusable activity component. That is, because each fragment defines its own layout and its own behavior with its own lifecycle callbacks, you can include one fragment in multiple activities, so you should design for reuse and avoid directly manipulating one fragment from another fragment. This is especially important because a modular fragment allows you to change your fragment combinations for different screen sizes. When designing your application to support both tablets and handsets, you can reuse your fragments in different layout configurations to optimize the user experience based on the available screen space. For example, on a handset, it might be necessary to separate fragments to provide a single-pane UI when more than one cannot fit within the same activity.

Figure 1. An example of how two UI modules defined by fragments can be combined into one activity for a tablet design, but separated for a handset design.

For example—to continue with the news application example—the application can embed two fragments inActivity A, when running on a tablet-sized device. However, on a handset-sized screen, there's not enough room for both fragments, so Activity A includes only the fragment for the list of articles, and when the user selects an article, it starts Activity B, which includes the second fragment to read the article. Thus, the application supports both tablets and handsets by reusing fragments in different combinations, as illustrated in figure 1.

For more information about designing your application with different fragment combinations for different screen configurations, see the guide to Supporting Tablets and Handsets.

Creating a Fragment

Figure 2. The lifecycle of a fragment (while its activity is running).

To create a fragment, you must create a subclass ofFragment (or an existing subclass of it). The Fragmentclass has code that looks a lot like an Activity. It contains callback methods similar to an activity, such as onCreate(), onStart(), onPause(), andonStop(). In fact, if you're converting an existing Android application to use fragments, you might simply move code from your activity's callback methods into the respective callback methods of your fragment.

Usually, you should implement at least the following lifecycle methods:

onCreate(): The system calls this when creating the fragment. Within your implementation, you should initialize essential components of the fragment that you want to retain when the fragment is paused or stopped, then resumed.
onCreateView(): The system calls this when it's time for the fragment to draw its user interface for the first time. To draw a UI for your fragment, you must return a View from this method that is the root of your fragment's layout. You can return null if the fragment does not provide a UI.
onPause(): The system calls this method as the first indication that the user is leaving the fragment (though it does not always mean the fragment is being destroyed). This is usually where you should commit any changes that should be persisted beyond the current user session (because the user might not come back).

Most applications should implement at least these three methods for every fragment, but there are several other callback methods you should also use to handle various stages of the fragment lifecycle. All the lifecycle callback methods are discussed in more detail in the section about Handling the Fragment Lifecycle.

There are also a few subclasses that you might want to extend, instead of the base Fragment class:

DialogFragment: Displays a floating dialog. Using this class to create a dialog is a good alternative to using the dialog helper methods in the Activity class, because you can incorporate a fragment dialog into the back stack of fragments managed by the activity, allowing the user to return to a dismissed fragment.
ListFragment: Displays a list of items that are managed by an adapter (such as a SimpleCursorAdapter), similar toListActivity. It provides several methods for managing a list view, such as the onListItemClick()callback to handle click events.
PreferenceFragment: Displays a hierarchy of Preference objects as a list, similar to PreferenceActivity. This is useful when creating a "settings" activity for your application.

Adding a user interface

A fragment is usually used as part of an activity's user interface and contributes its own layout to the activity.

To provide a layout for a fragment, you must implement the onCreateView() callback method, which the Android system calls when it's time for the fragment to draw its layout. Your implementation of this method must return a View that is the root of your fragment's layout.

Note: If your fragment is a subclass of ListFragment, the default implementation returns a ListView fromonCreateView(), so you don't need to implement it.

To return a layout from onCreateView(), you can inflate it from a layout resource defined in XML. To help you do so, onCreateView() provides a LayoutInflater object.

For example, here's a subclass of Fragment that loads a layout from the example_fragment.xml file:

public static class ExampleFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.example_fragment, container, false);
    }
}

Creating a layout

In the sample above,R.layout.example_fragment is a reference to a layout resource namedexample_fragment.xml saved in the application resources. For information about how to create a layout in XML, see the User Interfacedocumentation.

The container parameter passed to onCreateView() is the parent ViewGroup (from the activity's layout) in which your fragment layout will be inserted. The savedInstanceStateparameter is a Bundle that provides data about the previous instance of the fragment, if the fragment is being resumed (restoring state is discussed more in the section about Handling the Fragment Lifecycle).

The inflate() method takes three arguments:

The resource ID of the layout you want to inflate.
The ViewGroup to be the parent of the inflated layout. Passing the container is important in order for the system to apply layout parameters to the root view of the inflated layout, specified by the parent view in which it's going.
A boolean indicating whether the inflated layout should be attached to the ViewGroup (the second parameter) during inflation. (In this case, this is false because the system is already inserting the inflated layout into the container—passing true would create a redundant view group in the final layout.)

Now you've seen how to create a fragment that provides a layout. Next, you need to add the fragment to your activity.

Adding a fragment to an activity

Usually, a fragment contributes a portion of UI to the host activity, which is embedded as a part of the activity's overall view hierarchy. There are two ways you can add a fragment to the activity layout:

Declare the fragment inside the activity's layout file.
In this case, you can specify layout properties for the fragment as if it were a view. For example, here's the layout file for an activity with two fragments:
```
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <fragment android:name="com.example.news.ArticleListFragment"
            android:id="@+id/list"
            android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="match_parent" />
    <fragment android:name="com.example.news.ArticleReaderFragment"
            android:id="@+id/viewer"
            android:layout_weight="2"
            android:layout_width="0dp"
            android:layout_height="match_parent" />
</LinearLayout>
```
The android:name attribute in the <fragment> specifies the Fragment class to instantiate in the layout.

When the system creates this activity layout, it instantiates each fragment specified in the layout and calls the onCreateView() method for each one, to retrieve each fragment's layout. The system inserts the Viewreturned by the fragment directly in place of the <fragment> element.
Note: Each fragment requires a unique identifier that the system can use to restore the fragment if the activity is restarted (and which you can use to capture the fragment to perform transactions, such as remove it). There are three ways to provide an ID for a fragment:
- Supply the android:id attribute with a unique ID.
- Supply the android:tag attribute with a unique string.
- If you provide neither of the previous two, the system uses the ID of the container view.
Or, programmatically add the fragment to an existing ViewGroup.
At any time while your activity is running, you can add fragments to your activity layout. You simply need to specify a ViewGroup in which to place the fragment.

To make fragment transactions in your activity (such as add, remove, or replace a fragment), you must use APIs from FragmentTransaction. You can get an instance of FragmentTransaction from your Activitylike this:
```
FragmentManager fragmentManager = getFragmentManager()
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
```
You can then add a fragment using the add() method, specifying the fragment to add and the view in which to insert it. For example:
```
ExampleFragment fragment = new ExampleFragment();
fragmentTransaction.add(R.id.fragment_container, fragment);
fragmentTransaction.commit();
```
The first argument passed to add() is the ViewGroup in which the fragment should be placed, specified by resource ID, and the second parameter is the fragment to add.

Once you've made your changes with FragmentTransaction, you must call commit() for the changes to take effect.

Adding a fragment without a UI

The examples above show how to add a fragment to your activity in order to provide a UI. However, you can also use a fragment to provide a background behavior for the activity without presenting additional UI.

To add a fragment without a UI, add the fragment from the activity using add(Fragment, String) (supplying a unique string "tag" for the fragment, rather than a view ID). This adds the fragment, but, because it's not associated with a view in the activity layout, it does not receive a call to onCreateView(). So you don't need to implement that method.

Supplying a string tag for the fragment isn't strictly for non-UI fragments—you can also supply string tags to fragments that do have a UI—but if the fragment does not have a UI, then the string tag is the only way to identify it. If you want to get the fragment from the activity later, you need to use findFragmentByTag().

For an example activity that uses a fragment as a background worker, without a UI, see theFragmentRetainInstance.java sample.

Managing Fragments

To manage the fragments in your activity, you need to use FragmentManager. To get it, callgetFragmentManager() from your activity.

Some things that you can do with FragmentManager include:

Get fragments that exist in the activity, with findFragmentById() (for fragments that provide a UI in the activity layout) or findFragmentByTag() (for fragments that do or don't provide a UI).
Pop fragments off the back stack, with popBackStack() (simulating a Back command by the user).
Register a listener for changes to the back stack, with addOnBackStackChangedListener().

For more information about these methods and others, refer to the FragmentManager class documentation.

As demonstrated in the previous section, you can also use FragmentManager to open aFragmentTransaction, which allows you to perform transactions, such as add and remove fragments.

Performing Fragment Transactions

A great feature about using fragments in your activity is the ability to add, remove, replace, and perform other actions with them, in response to user interaction. Each set of changes that you commit to the activity is called a transaction and you can perform one using APIs in FragmentTransaction. You can also save each transaction to a back stack managed by the activity, allowing the user to navigate backward through the fragment changes (similar to navigating backward through activities).

You can acquire an instance of FragmentTransaction from the FragmentManager like this:

FragmentManager fragmentManager = getFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

Each transaction is a set of changes that you want to perform at the same time. You can set up all the changes you want to perform for a given transaction using methods such as add(), remove(), and replace(). Then, to apply the transaction to the activity, you must call commit().

Before you call commit(), however, you might want to call addToBackStack(), in order to add the transaction to a back stack of fragment transactions. This back stack is managed by the activity and allows the user to return to the previous fragment state, by pressing the Back button.

For example, here's how you can replace one fragment with another, and preserve the previous state in the back stack:

// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);
// Commit the transaction
transaction.commit();

In this example, newFragment replaces whatever fragment (if any) is currently in the layout container identified by the R.id.fragment_container ID. By calling addToBackStack(), the replace transaction is saved to the back stack so the user can reverse the transaction and bring back the previous fragment by pressing the Backbutton.

If you add multiple changes to the transaction (such as another add() or remove()) and calladdToBackStack(), then all changes applied before you call commit() are added to the back stack as a single transaction and the Back button will reverse them all together.

The order in which you add changes to a FragmentTransaction doesn't matter, except:

You must call commit() last
If you're adding multiple fragments to the same container, then the order in which you add them determines the order they appear in the view hierarchy

If you do not call addToBackStack() when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you do call addToBackStack() when removing a fragment, then the fragment is stopped and will be resumed if the user navigates back.

Tip: For each fragment transaction, you can apply a transition animation, by calling setTransition()before you commit.

Calling commit() does not perform the transaction immediately. Rather, it schedules it to run on the activity's UI thread (the "main" thread) as soon as the thread is able to do so. If necessary, however, you may callexecutePendingTransactions() from your UI thread to immediately execute transactions submitted bycommit(). Doing so is usually not necessary unless the transaction is a dependency for jobs in other threads.

Caution: You can commit a transaction using commit() only prior to the activity saving its state (when the user leaves the activity). If you attempt to commit after that point, an exception will be thrown. This is because the state after the commit can be lost if the activity needs to be restored. For situations in which its okay that you lose the commit, use commitAllowingStateLoss().

Communicating with the Activity

Although a Fragment is implemented as an object that's independent from an Activity and can be used inside multiple activities, a given instance of a fragment is directly tied to the activity that contains it.

Specifically, the fragment can access the Activity instance with getActivity() and easily perform tasks such as find a view in the activity layout:

View listView = getActivity().findViewById(R.id.list);

Likewise, your activity can call methods in the fragment by acquiring a reference to the Fragment fromFragmentManager, using findFragmentById() or findFragmentByTag(). For example:

ExampleFragment fragment = (ExampleFragment) getFragmentManager().findFragmentById(R.id.example_fragment);

Creating event callbacks to the activity

In some cases, you might need a fragment to share events with the activity. A good way to do that is to define a callback interface inside the fragment and require that the host activity implement it. When the activity receives a callback through the interface, it can share the information with other fragments in the layout as necessary.

For example, if a news application has two fragments in an activity—one to show a list of articles (fragment A) and another to display an article (fragment B)—then fragment A must tell the activity when a list item is selected so that it can tell fragment B to display the article. In this case, the OnArticleSelectedListener interface is declared inside fragment A:

public static class FragmentA extends ListFragment {
    ...
    // Container Activity must implement this interface
    public interface OnArticleSelectedListener {
        public void onArticleSelected(Uri articleUri);
    }
    ...
}

Then the activity that hosts the fragment implements the OnArticleSelectedListener interface and overrides onArticleSelected() to notify fragment B of the event from fragment A. To ensure that the host activity implements this interface, fragment A's onAttach() callback method (which the system calls when adding the fragment to the activity) instantiates an instance of OnArticleSelectedListener by casting theActivity that is passed into onAttach():

public static class FragmentA extends ListFragment {
    OnArticleSelectedListener mListener;
    ...
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnArticleSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
        }
    }
    ...
}

If the activity has not implemented the interface, then the fragment throws a ClassCastException. On success, the mListener member holds a reference to activity's implementation ofOnArticleSelectedListener, so that fragment A can share events with the activity by calling methods defined by the OnArticleSelectedListener interface. For example, if fragment A is an extension ofListFragment, each time the user clicks a list item, the system calls onListItemClick() in the fragment, which then calls onArticleSelected() to share the event with the activity:

public static class FragmentA extends ListFragment {
    OnArticleSelectedListener mListener;
    ...
    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Append the clicked item's row ID with the content provider Uri
        Uri noteUri = ContentUris.withAppendedId(ArticleColumns.CONTENT_URI, id);
        // Send the event and Uri to the host activity
        mListener.onArticleSelected(noteUri);
    }
    ...
}

The id parameter passed to onListItemClick() is the row ID of the clicked item, which the activity (or other fragment) uses to fetch the article from the application's ContentProvider.

More information about using a content provider is available in the Content Providers document.

Adding items to the Action Bar

Your fragments can contribute menu items to the activity's Options Menu (and, consequently, the Action Bar) by implementing onCreateOptionsMenu(). In order for this method to receive calls, however, you must callsetHasOptionsMenu() during onCreate(), to indicate that the fragment would like to add items to the Options Menu (otherwise, the fragment will not receive a call to onCreateOptionsMenu()).

Any items that you then add to the Options Menu from the fragment are appended to the existing menu items. The fragment also receives callbacks to onOptionsItemSelected() when a menu item is selected.

You can also register a view in your fragment layout to provide a context menu by callingregisterForContextMenu(). When the user opens the context menu, the fragment receives a call toonCreateContextMenu(). When the user selects an item, the fragment receives a call toonContextItemSelected().

Note: Although your fragment receives an on-item-selected callback for each menu item it adds, the activity is first to receive the respective callback when the user selects a menu item. If the activity's implementation of the on-item-selected callback does not handle the selected item, then the event is passed to the fragment's callback. This is true for the Options Menu and context menus.

For more information about menus, see the Menus and Action Bar developer guides.

Handling the Fragment Lifecycle

Figure 3. The effect of the activity lifecycle on the fragment lifecycle.

Managing the lifecycle of a fragment is a lot like managing the lifecycle of an activity. Like an activity, a fragment can exist in three states:

Resumed: The fragment is visible in the running activity.
Paused: Another activity is in the foreground and has focus, but the activity in which this fragment lives is still visible (the foreground activity is partially transparent or doesn't cover the entire screen).
Stopped: The fragment is not visible. Either the host activity has been stopped or the fragment has been removed from the activity but added to the back stack. A stopped fragment is still alive (all state and member information is retained by the system). However, it is no longer visible to the user and will be killed if the activity is killed.

Also like an activity, you can retain the state of a fragment using a Bundle, in case the activity's process is killed and you need to restore the fragment state when the activity is recreated. You can save the state during the fragment'sonSaveInstanceState() callback and restore it during either onCreate(), onCreateView(), oronActivityCreated(). For more information about saving state, see the Activities document.

The most significant difference in lifecycle between an activity and a fragment is how one is stored in its respective back stack. An activity is placed into a back stack of activities that's managed by the system when it's stopped, by default (so that the user can navigate back to it with the Back button, as discussed in Tasks and Back Stack). However, a fragment is placed into a back stack managed by the host activity only when you explicitly request that the instance be saved by calling addToBackStack() during a transaction that removes the fragment.

Otherwise, managing the fragment lifecycle is very similar to managing the activity lifecycle. So, the same practices for managing the activity lifecycle also apply to fragments. What you also need to understand, though, is how the life of the activity affects the life of the fragment.

Caution: If you need a Context object within your Fragment, you can call getActivity(). However, be careful to call getActivity() only when the fragment is attached to an activity. When the fragment is not yet attached, or was detached during the end of its lifecycle, getActivity() will return null.

Coordinating with the activity lifecycle

The lifecycle of the activity in which the fragment lives directly affects the lifecycle of the fragment, such that each lifecycle callback for the activity results in a similar callback for each fragment. For example, when the activity receives onPause(), each fragment in the activity receives onPause().

Fragments have a few extra lifecycle callbacks, however, that handle unique interaction with the activity in order to perform actions such as build and destroy the fragment's UI. These additional callback methods are:

onAttach(): Called when the fragment has been associated with the activity (the Activity is passed in here).
onCreateView(): Called to create the view hierarchy associated with the fragment.
onActivityCreated(): Called when the activity's onCreate() method has returned.
onDestroyView(): Called when the view hierarchy associated with the fragment is being removed.
onDetach(): Called when the fragment is being disassociated from the activity.

The flow of a fragment's lifecycle, as it is affected by its host activity, is illustrated by figure 3. In this figure, you can see how each successive state of the activity determines which callback methods a fragment may receive. For example, when the activity has received its onCreate() callback, a fragment in the activity receives no more than the onActivityCreated() callback.

Once the activity reaches the resumed state, you can freely add and remove fragments to the activity. Thus, only while the activity is in the resumed state can the lifecycle of a fragment change independently.

However, when the activity leaves the resumed state, the fragment again is pushed through its lifecycle by the activity.

Example

To bring everything discussed in this document together, here's an example of an activity using two fragments to create a two-pane layout. The activity below includes one fragment to show a list of Shakespeare play titles and another to show a summary of the play when selected from the list. It also demonstrates how to provide different configurations of the fragments, based on the screen configuration.

Note: The complete source code for this activity is available in FragmentLayout.java.

The main activity applies a layout in the usual way, during onCreate():

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.fragment_layout);
}

The layout applied is fragment_layout.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <fragment class="com.example.android.apis.app.FragmentLayout$TitlesFragment"
            android:id="@+id/titles" android:layout_weight="1"
            android:layout_width="0px" android:layout_height="match_parent" />

    <FrameLayout android:id="@+id/details" android:layout_weight="1"
            android:layout_width="0px" android:layout_height="match_parent"
            android:background="?android:attr/detailsElementBackground" />
</LinearLayout>

Using this layout, the system instantiates the TitlesFragment (which lists the play titles) as soon as the activity loads the layout, while the FrameLayout (where the fragment for showing the play summary will go) consumes space on the right side of the screen, but remains empty at first. As you'll see below, it's not until the user selects an item from the list that a fragment is placed into the FrameLayout.

However, not all screen configurations are wide enough to show both the list of plays and the summary, side by side. So, the layout above is used only for the landscape screen configuration, by saving it at res/layout-land/fragment_layout.xml.

Thus, when the screen is in portrait orientation, the system applies the following layout, which is saved atres/layout/fragment_layout.xml:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">
    <fragment class="com.example.android.apis.app.FragmentLayout$TitlesFragment"
            android:id="@+id/titles"
            android:layout_width="match_parent" android:layout_height="match_parent" />
</FrameLayout>

This layout includes only TitlesFragment. This means that, when the device is in portrait orientation, only the list of play titles is visible. So, when the user clicks a list item in this configuration, the application will start a new activity to show the summary, instead of loading a second fragment.

Next, you can see how this is accomplished in the fragment classes. First is TitlesFragment, which shows the list of Shakespeare play titles. This fragment extends ListFragment and relies on it to handle most of the list view work.

As you inspect this code, notice that there are two possible behaviors when the user clicks a list item: depending on which of the two layouts is active, it can either create and display a new fragment to show the details in the same activity (adding the fragment to the FrameLayout), or start a new activity (where the fragment can be shown).

public static class TitlesFragment extends ListFragment {
    boolean mDualPane;
    int mCurCheckPosition = 0;

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // Populate list with our static array of titles.
        setListAdapter(new ArrayAdapter<String>(getActivity(),
                android.R.layout.simple_list_item_activated_1, Shakespeare.TITLES));

        // Check to see if we have a frame in which to embed the details
        // fragment directly in the containing UI.
        View detailsFrame = getActivity().findViewById(R.id.details);
        mDualPane = detailsFrame != null && detailsFrame.getVisibility() == View.VISIBLE;

        if (savedInstanceState != null) {
            // Restore last state for checked position.
            mCurCheckPosition = savedInstanceState.getInt("curChoice", 0);
        }

        if (mDualPane) {
            // In dual-pane mode, the list view highlights the selected item.
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
            // Make sure our UI is in the correct state.
            showDetails(mCurCheckPosition);
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("curChoice", mCurCheckPosition);
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        showDetails(position);
    }

    /**
     * Helper function to show the details of a selected item, either by
     * displaying a fragment in-place in the current UI, or starting a
     * whole new activity in which it is displayed.
     */
    void showDetails(int index) {
        mCurCheckPosition = index;

        if (mDualPane) {
            // We can display everything in-place with fragments, so update
            // the list to highlight the selected item and show the data.
            getListView().setItemChecked(index, true);

            // Check what fragment is currently shown, replace if needed.
            DetailsFragment details = (DetailsFragment)
                    getFragmentManager().findFragmentById(R.id.details);
            if (details == null || details.getShownIndex() != index) {
                // Make new fragment to show this selection.
                details = DetailsFragment.newInstance(index);

                // Execute a transaction, replacing any existing fragment
                // with this one inside the frame.
                FragmentTransaction ft = getFragmentManager().beginTransaction();
                if (index == 0) {
                    ft.replace(R.id.details, details);
                } else {
                    ft.replace(R.id.a_item, details);
                }
                ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                ft.commit();
            }

        } else {
            // Otherwise we need to launch a new activity to display
            // the dialog fragment with selected text.
            Intent intent = new Intent();
            intent.setClass(getActivity(), DetailsActivity.class);
            intent.putExtra("index", index);
            startActivity(intent);
        }
    }
}

The second fragment, DetailsFragment shows the play summary for the item selected from the list fromTitlesFragment:

public static class DetailsFragment extends Fragment {
    /**
     * Create a new instance of DetailsFragment, initialized to
     * show the text at 'index'.
     */
    public static DetailsFragment newInstance(int index) {
        DetailsFragment f = new DetailsFragment();

        // Supply index input as an argument.
        Bundle args = new Bundle();
        args.putInt("index", index);
        f.setArguments(args);

        return f;
    }

    public int getShownIndex() {
        return getArguments().getInt("index", 0);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (container == null) {
            // We have different layouts, and in one of them this
            // fragment's containing frame doesn't exist.  The fragment
            // may still be created from its saved state, but there is
            // no reason to try to create its view hierarchy because it
            // won't be displayed.  Note this is not needed -- we could
            // just run the code below, where we would create and return
            // the view hierarchy; it would just never be used.
            return null;
        }

        ScrollView scroller = new ScrollView(getActivity());
        TextView text = new TextView(getActivity());
        int padding = (int)TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,
                4, getActivity().getResources().getDisplayMetrics());
        text.setPadding(padding, padding, padding, padding);
        scroller.addView(text);
        text.setText(Shakespeare.DIALOGUE[getShownIndex()]);
        return scroller;
    }
}

Recall from the TitlesFragment class, that, if the user clicks a list item and the current layout does not include the R.id.details view (which is where the DetailsFragment belongs), then the application starts theDetailsActivity activity to display the content of the item.

Here is the DetailsActivity, which simply embeds the DetailsFragment to display the selected play summary when the screen is in portrait orientation:

public static class DetailsActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (getResources().getConfiguration().orientation
                == Configuration.ORIENTATION_LANDSCAPE) {
            // If the screen is now in landscape mode, we can show the
            // dialog in-line with the list so we don't need this activity.
            finish();
            return;
        }

        if (savedInstanceState == null) {
            // During initial setup, plug in the details fragment.
            DetailsFragment details = new DetailsFragment();
            details.setArguments(getIntent().getExtras());
            getFragmentManager().beginTransaction().add(android.R.id.content, details).commit();
        }
    }
}

Notice that this activity finishes itself if the configuration is landscape, so that the main activity can take over and display the DetailsFragment alongside the TitlesFragment. This can happen if the user begins theDetailsActivity while in portrait orientation, but then rotates to landscape (which restarts the current activity).

For more samples using fragments (and complete source files for this example), see the API Demos sample app available in ApiDemos (available for download from the Samples SDK component).

Loaders

Introduced in Android 3.0, loaders make it easy to asynchronously load data in an activity or fragment. Loaders have these characteristics:

They are available to every Activity and Fragment.
They provide asynchronous loading of data.
They monitor the source of their data and deliver new results when the content changes.
They automatically reconnect to the last loader's cursor when being recreated after a configuration change. Thus, they don't need to re-query their data.

Loader API Summary

There are multiple classes and interfaces that may be involved in using loaders in an application. They are summarized in this table:

Class/Interface	Description
`LoaderManager`	An abstract class associated with an `Activity` or `Fragment` for managing one or more `Loader` instances. This helps an application manage longer-running operations in conjunction with the `Activity` or `Fragment` lifecycle; the most common use of this is with a `CursorLoader`, however applications are free to write their own loaders for loading other types of data. There is only one `LoaderManager` per activity or fragment. But a`LoaderManager` can have multiple loaders.
`LoaderManager.LoaderCallbacks`	A callback interface for a client to interact with the`LoaderManager`. For example, you use the `onCreateLoader()`callback method to create a new loader.
`Loader`	An abstract class that performs asynchronous loading of data. This is the base class for a loader. You would typically use`CursorLoader`, but you can implement your own subclass. While loaders are active they should monitor the source of their data and deliver new results when the contents change.
`AsyncTaskLoader`	Abstract loader that provides an `AsyncTask` to do the work.
`CursorLoader`	A subclass of `AsyncTaskLoader` that queries the`ContentResolver` and returns a `Cursor`. This class implements the `Loader` protocol in a standard way for querying cursors, building on `AsyncTaskLoader` to perform the cursor query on a background thread so that it does not block the application's UI. Using this loader is the best way to asynchronously load data from a `ContentProvider`, instead of performing a managed query through the fragment or activity's APIs.

The classes and interfaces in the above table are the essential components you'll use to implement a loader in your application. You won't need all of them for each loader you create, but you'll always need a reference to theLoaderManager in order to initialize a loader and an implementation of a Loader class such as CursorLoader. The following sections show you how to use these classes and interfaces in an application.

Using Loaders in an Application

This section describes how to use loaders in an Android application. An application that uses loaders typically includes the following:

An Activity or Fragment.
An instance of the LoaderManager.
A CursorLoader to load data backed by a ContentProvider. Alternatively, you can implement your own subclass of Loader or AsyncTaskLoader to load data from some other source.
An implementation for LoaderManager.LoaderCallbacks. This is where you create new loaders and manage your references to existing loaders.
A way of displaying the loader's data, such as a SimpleCursorAdapter.
A data source, such as a ContentProvider, when using a CursorLoader.

Starting a Loader

The LoaderManager manages one or more Loader instances within an Activity or Fragment. There is only one LoaderManager per activity or fragment.

You typically initialize a Loader within the activity's onCreate() method, or within the fragment'sonActivityCreated() method. You do this as follows:

// Prepare the loader.  Either re-connect with an existing one,
// or start a new one.
getLoaderManager().initLoader(0, null, this);

The initLoader() method takes the following parameters:

A unique ID that identifies the loader. In this example, the ID is 0.
Optional arguments to supply to the loader at construction (null in this example).
A LoaderManager.LoaderCallbacks implementation, which the LoaderManager calls to report loader events. In this example, the local class implements the LoaderManager.LoaderCallbacks interface, so it passes a reference to itself, this.

The initLoader() call ensures that a loader is initialized and active. It has two possible outcomes:

If the loader specified by the ID already exists, the last created loader is reused.
If the loader specified by the ID does not exist, initLoader() triggers theLoaderManager.LoaderCallbacks method onCreateLoader(). This is where you implement the code to instantiate and return a new loader. For more discussion, see the section onCreateLoader.

In either case, the given LoaderManager.LoaderCallbacks implementation is associated with the loader, and will be called when the loader state changes. If at the point of this call the caller is in its started state, and the requested loader already exists and has generated its data, then the system calls onLoadFinished()immediately (during initLoader()), so you must be prepared for this to happen. See onLoadFinished for more discussion of this callback

Note that the initLoader() method returns the Loader that is created, but you don't need to capture a reference to it. The LoaderManager manages the life of the loader automatically. The LoaderManager starts and stops loading when necessary, and maintains the state of the loader and its associated content. As this implies, you rarely interact with loaders directly (though for an example of using loader methods to fine-tune a loader's behavior, see the LoaderThrottle sample). You most commonly use theLoaderManager.LoaderCallbacks methods to intervene in the loading process when particular events occur. For more discussion of this topic, see Using the LoaderManager Callbacks.

Restarting a Loader

When you use initLoader(), as shown above, it uses an existing loader with the specified ID if there is one. If there isn't, it creates one. But sometimes you want to discard your old data and start over.

To discard your old data, you use restartLoader(). For example, this implementation ofSearchView.OnQueryTextListener restarts the loader when the user's query changes. The loader needs to be restarted so that it can use the revised search filter to do a new query:

public boolean onQueryTextChanged(String newText) {
    // Called when the action bar search text has changed.  Update
    // the search filter, and restart the loader to do a new query
    // with this filter.
    mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;
    getLoaderManager().restartLoader(0, null, this);
    return true;
}

Using the LoaderManager Callbacks

LoaderManager.LoaderCallbacks is a callback interface that lets a client interact with the LoaderManager.

Loaders, in particular CursorLoader, are expected to retain their data after being stopped. This allows applications to keep their data across the activity or fragment's onStop() and onStart() methods, so that when users return to an application, they don't have to wait for the data to reload. You use theLoaderManager.LoaderCallbacks methods when to know when to create a new loader, and to tell the application when it is time to stop using a loader's data.

LoaderManager.LoaderCallbacks includes these methods:

onCreateLoader() — Instantiate and return a new Loader for the given ID.

onLoadFinished() — Called when a previously created loader has finished its load.

onLoaderReset() — Called when a previously created loader is being reset, thus making its data unavailable.

These methods are described in more detail in the following sections.

onCreateLoader

When you attempt to access a loader (for example, through initLoader()), it checks to see whether the loader specified by the ID exists. If it doesn't, it triggers the LoaderManager.LoaderCallbacks methodonCreateLoader(). This is where you create a new loader. Typically this will be a CursorLoader, but you can implement your own Loader subclass.

In this example, the onCreateLoader() callback method creates a CursorLoader. You must build theCursorLoader using its constructor method, which requires the complete set of information needed to perform a query to the ContentProvider. Specifically, it needs:

uri — The URI for the content to retrieve.
projection — A list of which columns to return. Passing null will return all columns, which is inefficient.
selection — A filter declaring which rows to return, formatted as an SQL WHERE clause (excluding the WHERE itself). Passing null will return all rows for the given URI.
selectionArgs — You may include ?s in the selection, which will be replaced by the values from selectionArgs, in the order that they appear in the selection. The values will be bound as Strings.
sortOrder — How to order the rows, formatted as an SQL ORDER BY clause (excluding the ORDER BY itself). Passing null will use the default sort order, which may be unordered.

For example:

 // If non-null, this is the current filter the user has provided.
String mCurFilter;
...
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    // This is called when a new Loader needs to be created.  This
    // sample only has one Loader, so we don't care about the ID.
    // First, pick the base URI to use depending on whether we are
    // currently filtering.
    Uri baseUri;
    if (mCurFilter != null) {
        baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
                  Uri.encode(mCurFilter));
    } else {
        baseUri = Contacts.CONTENT_URI;
    }

    // Now create and return a CursorLoader that will take care of
    // creating a Cursor for the data being displayed.
    String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
            + Contacts.HAS_PHONE_NUMBER + "=1) AND ("
            + Contacts.DISPLAY_NAME + " != '' ))";
    return new CursorLoader(getActivity(), baseUri,
            CONTACTS_SUMMARY_PROJECTION, select, null,
            Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}

onLoadFinished

This method is called when a previously created loader has finished its load. This method is guaranteed to be called prior to the release of the last data that was supplied for this loader. At this point you should remove all use of the old data (since it will be released soon), but should not do your own release of the data since its loader owns it and will take care of that.

The loader will release the data once it knows the application is no longer using it. For example, if the data is a cursor from a CursorLoader, you should not call close() on it yourself. If the cursor is being placed in aCursorAdapter, you should use the swapCursor() method so that the old Cursor is not closed. For example:

// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
...
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    // Swap the new cursor in.  (The framework will take care of closing the
    // old cursor once we return.)
    mAdapter.swapCursor(data);
}

onLoaderReset

This method is called when a previously created loader is being reset, thus making its data unavailable. This callback lets you find out when the data is about to be released so you can remove your reference to it.

This implementation calls swapCursor() with a value of null:

// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
...
public void onLoaderReset(Loader<Cursor> loader) {
    // This is called when the last Cursor provided to onLoadFinished()
    // above is about to be closed.  We need to make sure we are no
    // longer using it.
    mAdapter.swapCursor(null);
}

Example

As an example, here is the full implementation of a Fragment that displays a ListView containing the results of a query against the contacts content provider. It uses a CursorLoader to manage the query on the provider.

For an application to access a user's contacts, as shown in this example, its manifest must include the permission READ_CONTACTS.

public static class CursorLoaderListFragment extends ListFragment
        implements OnQueryTextListener, LoaderManager.LoaderCallbacks<Cursor> {

    // This is the Adapter being used to display the list's data.
    SimpleCursorAdapter mAdapter;

    // If non-null, this is the current filter the user has provided.
    String mCurFilter;

    @Override public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // Give some text to display if there is no data.  In a real
        // application this would come from a resource.
        setEmptyText("No phone numbers");

        // We have a menu item to show in action bar.
        setHasOptionsMenu(true);

        // Create an empty adapter we will use to display the loaded data.
        mAdapter = new SimpleCursorAdapter(getActivity(),
                android.R.layout.simple_list_item_2, null,
                new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
                new int[] { android.R.id.text1, android.R.id.text2 }, 0);
        setListAdapter(mAdapter);

        // Prepare the loader.  Either re-connect with an existing one,
        // or start a new one.
        getLoaderManager().initLoader(0, null, this);
    }

    @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Place an action bar item for searching.
        MenuItem item = menu.add("Search");
        item.setIcon(android.R.drawable.ic_menu_search);
        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);
        SearchView sv = new SearchView(getActivity());
        sv.setOnQueryTextListener(this);
        item.setActionView(sv);
    }

    public boolean onQueryTextChange(String newText) {
        // Called when the action bar search text has changed.  Update
        // the search filter, and restart the loader to do a new query
        // with this filter.
        mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;
        getLoaderManager().restartLoader(0, null, this);
        return true;
    }

    @Override public boolean onQueryTextSubmit(String query) {
        // Don't care about this.
        return true;
    }

    @Override public void onListItemClick(ListView l, View v, int position, long id) {
        // Insert desired behavior here.
        Log.i("FragmentComplexList", "Item clicked: " + id);
    }

    // These are the Contacts rows that we will retrieve.
    static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
        Contacts._ID,
        Contacts.DISPLAY_NAME,
        Contacts.CONTACT_STATUS,
        Contacts.CONTACT_PRESENCE,
        Contacts.PHOTO_ID,
        Contacts.LOOKUP_KEY,
    };
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        // This is called when a new Loader needs to be created.  This
        // sample only has one Loader, so we don't care about the ID.
        // First, pick the base URI to use depending on whether we are
        // currently filtering.
        Uri baseUri;
        if (mCurFilter != null) {
            baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
                    Uri.encode(mCurFilter));
        } else {
            baseUri = Contacts.CONTENT_URI;
        }

        // Now create and return a CursorLoader that will take care of
        // creating a Cursor for the data being displayed.
        String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
                + Contacts.HAS_PHONE_NUMBER + "=1) AND ("
                + Contacts.DISPLAY_NAME + " != '' ))";
        return new CursorLoader(getActivity(), baseUri,
                CONTACTS_SUMMARY_PROJECTION, select, null,
                Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
    }

    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        // Swap the new cursor in.  (The framework will take care of closing the
        // old cursor once we return.)
        mAdapter.swapCursor(data);
    }

    public void onLoaderReset(Loader<Cursor> loader) {
        // This is called when the last Cursor provided to onLoadFinished()
        // above is about to be closed.  We need to make sure we are no
        // longer using it.
        mAdapter.swapCursor(null);
    }
}

More Examples

There are a few different samples in ApiDemos that illustrate how to use loaders:

LoaderCursor — A complete version of the snippet shown above.
LoaderThrottle — An example of how to use throttling to reduce the number of queries a content provider does when its data changes.

For information on downloading and installing the SDK samples, see Getting the Samples

Tasks and Back Stack

An application usually contains multiple activities. Each activity should be designed around a specific kind of action the user can perform and can start other activities. For example, an email application might have one activity to show a list of new email. When the user selects an email, a new activity opens to view that email.

An activity can even start activities that exist in other applications on the device. For example, if your application wants to send an email, you can define an intent to perform a "send" action and include some data, such as an email address and a message. An activity from another application that declares itself to handle this kind of intent then opens. In this case, the intent is to send an email, so an email application's "compose" activity starts (if multiple activities support the same intent, then the system lets the user select which one to use). When the email is sent, your activity resumes and it seems as if the email activity was part of your application. Even though the activities may be from different applications, Android maintains this seamless user experience by keeping both activities in the same task.

A task is a collection of activities that users interact with when performing a certain job. The activities are arranged in a stack (the "back stack"), in the order in which each activity is opened.

The device Home screen is the starting place for most tasks. When the user touches an icon in the application launcher (or a shortcut on the Home screen), that application's task comes to the foreground. If no task exists for the application (the application has not been used recently), then a new task is created and the "main" activity for that application opens as the root activity in the stack.

When the current activity starts another, the new activity is pushed on the top of the stack and takes focus. The previous activity remains in the stack, but is stopped. When an activity stops, the system retains the current state of its user interface. When the user presses the Back button, the current activity is popped from the top of the stack (the activity is destroyed) and the previous activity resumes (the previous state of its UI is restored). Activities in the stack are never rearranged, only pushed and popped from the stack—pushed onto the stack when started by the current activity and popped off when the user leaves it using the Back button. As such, the back stack operates as a "last in, first out" object structure. Figure 1 visualizes this behavior with a timeline showing the progress between activities along with the current back stack at each point in time.

Figure 1. A representation of how each new activity in a task adds an item to the back stack. When the user presses theBack button, the current activity is destroyed and the previous activity resumes.

If the user continues to press Back, then each activity in the stack is popped off to reveal the previous one, until the user returns to the Home screen (or to whichever activity was running when the task began). When all activities are removed from the stack, the task no longer exists.

Figure 2. Two tasks: Task B receives user interaction in the foreground, while Task A is in the background, waiting to be resumed.

Figure 3. A single activity is instantiated multiple times.

A task is a cohesive unit that can move to the "background" when users begin a new task or go to the Home screen, via theHome button. While in the background, all the activities in the task are stopped, but the back stack for the task remains intact—the task has simply lost focus while another task takes place, as shown in figure 2. A task can then return to the "foreground" so users can pick up where they left off. Suppose, for example, that the current task (Task A) has three activities in its stack—two under the current activity. The user presses the Homebutton, then starts a new application from the application launcher. When the Home screen appears, Task A goes into the background. When the new application starts, the system starts a task for that application (Task B) with its own stack of activities. After interacting with that application, the user returns Home again and selects the application that originally started Task A. Now, Task A comes to the foreground—all three activities in its stack are intact and the activity at the top of the stack resumes. At this point, the user can also switch back to Task B by going Home and selecting the application icon that started that task (or by touching and holding the Home button to reveal recent tasks and selecting one). This is an example of multitasking on Android.

Note: Multiple tasks can be held in the background at once. However, if the user is running many background tasks at the same time, the system might begin destroying background activities in order to recover memory, causing the activity states to be lost. See the following section about Activity state.

Because the activities in the back stack are never rearranged, if your application allows users to start a particular activity from more than one activity, a new instance of that activity is created and pushed onto the stack (rather than bringing any previous instance of the activity to the top). As such, one activity in your application might be instantiated multiple times (even from different tasks), as shown in figure 3. As such, if the user navigates backward using the Back button, each instance of the activity is revealed in the order they were opened (each with their own UI state). However, you can modify this behavior if you do not want an activity to be instantiated more than once. How to do so is discussed in the later section about Managing Tasks.

To summarize the default behavior for activities and tasks:

When Activity A starts Activity B, Activity A is stopped, but the system retains its state (such as scroll position and text entered into forms). If the user presses the Back button while in Activity B, Activity A resumes with its state restored.
When the user leaves a task by pressing the Home button, the current activity is stopped and its task goes into the background. The system retains the state of every activity in the task. If the user later resumes the task by selecting the launcher icon that began the task, the task comes to the foreground and resumes the activity at the top of the stack.
If the user presses the Back button, the current activity is popped from the stack and destroyed. The previous activity in the stack is resumed. When an activity is destroyed, the system does not retain the activity's state.
Activities can be instantiated multiple times, even from other tasks.

Navigation Design

For more about how app navigation works on Android, read Android Design's Navigation guide.

Saving Activity State

As discussed above, the system's default behavior preserves the state of an activity when it is stopped. This way, when users navigate back to a previous activity, its user interface appears the way they left it. However, you can—and should—proactively retain the state of your activities using callback methods, in case the activity is destroyed and must be recreated.

When the system stops one of your activities (such as when a new activity starts or the task moves to the background), the system might destroy that activity completely if it needs to recover system memory. When this happens, information about the activity state is lost. If this happens, the system still knows that the activity has a place in the back stack, but when the activity is brought to the top of the stack the system must recreate it (rather than resume it). In order to avoid losing the user's work, you should proactively retain it by implementing the onSaveInstanceState() callback methods in your activity.

For more information about how to save your activity state, see the Activities document.

Managing Tasks

The way Android manages tasks and the back stack, as described above—by placing all activities started in succession in the same task and in a "last in, first out" stack—works great for most applications and you shouldn't have to worry about how your activities are associated with tasks or how they exist in the back stack. However, you might decide that you want to interrupt the normal behavior. Perhaps you want an activity in your application to begin a new task when it is started (instead of being placed within the current task); or, when you start an activity, you want to bring forward an existing instance of it (instead of creating a new instance on top of the back stack); or, you want your back stack to be cleared of all activities except for the root activity when the user leaves the task.

You can do these things and more, with attributes in the <activity> manifest element and with flags in the intent that you pass to startActivity().

In this regard, the principal <activity> attributes you can use are:

And the principal intent flags you can use are:

In the following sections, you'll see how you can use these manifest attributes and intent flags to define how activities are associated with tasks and how the behave in the back stack.

Caution: Most applications should not interrupt the default behavior for activities and tasks. If you determine that it's necessary for your activity to modify the default behaviors, use caution and be sure to test the usability of the activity during launch and when navigating back to it from other activities and tasks with theBack button. Be sure to test for navigation behaviors that might conflict with the user's expected behavior.

Defining launch modes

Launch modes allow you to define how a new instance of an activity is associated with the current task. You can define different launch modes in two ways:

Using the manifest file
When you declare an activity in your manifest file, you can specify how the activity should associate with tasks when it starts.
Using Intent flags
When you call startActivity(), you can include a flag in the Intent that declares how (or whether) the new activity should associate with the current task.

As such, if Activity A starts Activity B, Activity B can define in its manifest how it should associate with the current task (if at all) and Activity A can also request how Activity B should associate with current task. If both activities define how Activity B should associate with a task, then Activity A's request (as defined in the intent) is honored over Activity B's request (as defined in its manifest).

Note: Some launch modes available for the manifest file are not available as flags for an intent and, likewise, some launch modes available as flags for an intent cannot be defined in the manifest.

Using the manifest file

When declaring an activity in your manifest file, you can specify how the activity should associate with a task using the <activity> element's launchMode attribute.

The launchMode attribute specifies an instruction on how the activity should be launched into a task. There are four different launch modes you can assign to the launchMode attribute:

"standard" (the default mode): Default. The system creates a new instance of the activity in the task from which it was started and routes the intent to it. The activity can be instantiated multiple times, each instance can belong to different tasks, and one task can have multiple instances.
"singleTop": If an instance of the activity already exists at the top of the current task, the system routes the intent to that instance through a call to its onNewIntent() method, rather than creating a new instance of the activity. The activity can be instantiated multiple times, each instance can belong to different tasks, and one task can have multiple instances (but only if the activity at the top of the back stack is not an existing instance of the activity).
For example, suppose a task's back stack consists of root activity A with activities B, C, and D on top (the stack is A-B-C-D; D is on top). An intent arrives for an activity of type D. If D has the default "standard"launch mode, a new instance of the class is launched and the stack becomes A-B-C-D-D. However, if D's launch mode is "singleTop", the existing instance of D receives the intent through onNewIntent(), because it's at the top of the stack—the stack remains A-B-C-D. However, if an intent arrives for an activity of type B, then a new instance of B is added to the stack, even if its launch mode is "singleTop".

Note: When a new instance of an activity is created, the user can press the Back button to return to the previous activity. But when an existing instance of an activity handles a new intent, the user cannot press the Back button to return to the state of the activity before the new intent arrived inonNewIntent().
"singleTask": The system creates a new task and instantiates the activity at the root of the new task. However, if an instance of the activity already exists in a separate task, the system routes the intent to the existing instance through a call to its onNewIntent() method, rather than creating a new instance. Only one instance of the activity can exist at a time.
Note: Although the activity starts in a new task, the Back button still returns the user to the previous activity.
"singleInstance".: Same as "singleTask", except that the system doesn't launch any other activities into the task holding the instance. The activity is always the single and only member of its task; any activities started by this one open in a separate task.

As another example, the Android Browser application declares that the web browser activity should always open in its own task—by specifying the singleTask launch mode in the <activity> element. This means that if your application issues an intent to open the Android Browser, its activity is not placed in the same task as your application. Instead, either a new task starts for the Browser or, if the Browser already has a task running in the background, that task is brought forward to handle the new intent.

Regardless of whether an activity starts in a new task or in the same task as the activity that started it, the Backbutton always takes the user to the previous activity. However, if you start an activity that specifies thesingleTask launch mode, then if an instance of that activity exists in a background task, that whole task is brought to the foreground. At this point, the back stack now includes all activities from the task brought forward, at the top of the stack. Figure 4 illustrates this type of scenario.

Figure 4. A representation of how an activity with launch mode "singleTask" is added to the back stack. If the activity is already a part of a background task with its own back stack, then the entire back stack also comes forward, on top of the current task.

For more information about using launch modes in the manifest file, see the <activity> element documentation, where the launchMode attribute and the accepted values are discussed more.

Note: The behaviors that you specify for your activity with the launchMode attribute can be overridden by flags included with the intent that start your activity, as discussed in the next section.

Using Intent flags

When starting an activity, you can modify the default association of an activity to its task by including flags in the intent that you deliver to startActivity(). The flags you can use to modify the default behavior are:

FLAG_ACTIVITY_NEW_TASK

Start the activity in a new task. If a task is already running for the activity you are now starting, that task is brought to the foreground with its last state restored and the activity receives the new intent inonNewIntent().

This produces the same behavior as the "singleTask" launchMode value, discussed in the previous section.

FLAG_ACTIVITY_SINGLE_TOP

If the activity being started is the current activity (at the top of the back stack), then the existing instance receives a call to onNewIntent(), instead of creating a new instance of the activity.

This produces the same behavior as the "singleTop" launchMode value, discussed in the previous section.

FLAG_ACTIVITY_CLEAR_TOP

If the activity being started is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it are destroyed and this intent is delivered to the resumed instance of the activity (now on top), through onNewIntent()).

There is no value for the launchMode attribute that produces this behavior.

FLAG_ACTIVITY_CLEAR_TOP is most often used in conjunction with FLAG_ACTIVITY_NEW_TASK. When used together, these flags are a way of locating an existing activity in another task and putting it in a position where it can respond to the intent.

Note: If the launch mode of the designated activity is "standard", it too is removed from the stack and a new instance is launched in its place to handle the incoming intent. That's because a new instance is always created for a new intent when the launch mode is "standard".

Handling affinities

The affinity indicates which task an activity prefers to belong to. By default, all the activities from the same application have an affinity for each other. So, by default, all activities in the same application prefer to be in the same task. However, you can modify the default affinity for an activity. Activities defined in different applications can share an affinity, or activities defined in the same application can be assigned different task affinities.

You can modify the affinity for any given activity with the taskAffinity attribute of the <activity> element.

The taskAffinity attribute takes a string value, which must be unique from the default package name declared in the <manifest> element, because the system uses that name to identify the default task affinity for the application.

The affinity comes into play in two circumstances:

When the intent that launches an activity contains the FLAG_ACTIVITY_NEW_TASK flag.
A new activity is, by default, launched into the task of the activity that called startActivity(). It's pushed onto the same back stack as the caller. However, if the intent passed to startActivity() contains theFLAG_ACTIVITY_NEW_TASK flag, the system looks for a different task to house the new activity. Often, it's a new task. However, it doesn't have to be. If there's already an existing task with the same affinity as the new activity, the activity is launched into that task. If not, it begins a new task.

If this flag causes an activity to begin a new task and the user presses the Home button to leave it, there must be some way for the user to navigate back to the task. Some entities (such as the notification manager) always start activities in an external task, never as part of their own, so they always putFLAG_ACTIVITY_NEW_TASK in the intents they pass to startActivity(). If you have an activity that can be invoked by an external entity that might use this flag, take care that the user has a independent way to get back to the task that's started, such as with a launcher icon (the root activity of the task has aCATEGORY_LAUNCHER intent filter; see the Starting a task section below).
When an activity has its allowTaskReparenting attribute set to "true".
In this case, the activity can move from the task it starts to the task it has an affinity for, when that task comes to the foreground.

For example, suppose that an activity that reports weather conditions in selected cities is defined as part of a travel application. It has the same affinity as other activities in the same application (the default application affinity) and it allows re-parenting with this attribute. When one of your activities starts the weather reporter activity, it initially belongs to the same task as your activity. However, when the travel application's task comes to the foreground, the weather reporter activity is reassigned to that task and displayed within it.

Tip: If an .apk file contains more than one "application" from the user's point of view, you probably want to use the taskAffinity attribute to assign different affinities to the activities associated with each "application".

Clearing the back stack

If the user leaves a task for a long time, the system clears the task of all activities except the root activity. When the user returns to the task again, only the root activity is restored. The system behaves this way, because, after an extended amount of time, users likely have abandoned what they were doing before and are returning to the task to begin something new.

There are some activity attributes that you can use to modify this behavior:

alwaysRetainTaskState: If this attribute is set to "true" in the root activity of a task, the default behavior just described does not happen. The task retains all activities in its stack even after a long period.
clearTaskOnLaunch: If this attribute is set to "true" in the root activity of a task, the stack is cleared down to the root activity whenever the user leaves the task and returns to it. In other words, it's the opposite ofalwaysRetainTaskState. The user always returns to the task in its initial state, even after a leaving the task for only a moment.
finishOnTaskLaunch: This attribute is like clearTaskOnLaunch, but it operates on a single activity, not an entire task. It can also cause any activity to go away, including the root activity. When it's set to "true", the activity remains part of the task only for the current session. If the user leaves and then returns to the task, it is no longer present.

Starting a task

You can set up an activity as the entry point for a task by giving it an intent filter with"android.intent.action.MAIN" as the specified action and "android.intent.category.LAUNCHER" as the specified category. For example:

<activity ... >
    <intent-filter ... >
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
    ...</activity>

An intent filter of this kind causes an icon and label for the activity to be displayed in the application launcher, giving users a way to launch the activity and to return to the task that it creates any time after it has been launched.

This second ability is important: Users must be able to leave a task and then come back to it later using this activity launcher. For this reason, the two launch modes that mark activities as always initiating a task,"singleTask" and ""singleInstance", should be used only when the activity has an ACTION_MAIN and aCATEGORY_LAUNCHER filter. Imagine, for example, what could happen if the filter is missing: An intent launches a "singleTask" activity, initiating a new task, and the user spends some time working in that task. The user then presses the Home button. The task is now sent to the background and is not visible. Now the user has no way to return to the task, because it is not represented in the application launcher.

For those cases where you don't want the user to be able to return to an activity, set the <activity> element'sfinishOnTaskLaunch to "true" (see Clearing the stack).

ANDROID

Android Fundamentals:

Activating Components

The Manifest File

Declaring components

Declaring component capabilities

Declaring application requirements

Application Resources

Activities

Creating an Activity

Implementing a user interface

Declaring the activity in the manifest

Using intent filters

Starting an Activity

Starting an activity for a result

Shutting Down an Activity

Managing the Activity Lifecycle

Implementing the lifecycle callbacks

Saving activity state

Handling configuration changes

Coordinating activities

Fragments

Design Philosophy

Creating a Fragment

Adding a user interface

Creating a layout

Adding a fragment to an activity

Adding a fragment without a UI

Managing Fragments

Performing Fragment Transactions

Communicating with the Activity

Creating event callbacks to the activity

Adding items to the Action Bar

Handling the Fragment Lifecycle

Coordinating with the activity lifecycle

Example

Loaders

Loader API Summary

Using Loaders in an Application

Starting a Loader

Restarting a Loader

Using the LoaderManager Callbacks

onCreateLoader

onLoadFinished

onLoaderReset

Example

More Examples

Tasks and Back Stack

Saving Activity State

Managing Tasks

Defining launch modes

Using the manifest file

Using Intent flags

Handling affinities

Clearing the back stack

Starting a task

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