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Building a media browser service

Your app must declare the MediaBrowserService with an intent-filter in its manifest. You can choose your own service name; in the following example, it is "MediaPlaybackService."

<service android:name=".MediaPlaybackService">
  <intent-filter>
    <action android:name="android.media.browse.MediaBrowserService" />
  </intent-filter>
</service>

Note: The recommended implementation of MediaBrowserService is MediaBrowserServiceCompat. which is defined in the media-compat support library. Throughout this page the term "MediaBrowserService" refers to an instance of of MediaBrowserServiceCompat.

Initialize the media session

When the service receives the onCreate() lifecycle callback method it should perform these steps:

The onCreate() code below demonstrates these steps:

Kotlin

private const val MY_MEDIA_ROOT_ID = "media_root_id"
private const val MY_EMPTY_MEDIA_ROOT_ID = "empty_root_id"

class MediaPlaybackService : MediaBrowserServiceCompat() {

    private var mMediaSession: MediaSessionCompat? = null
    private lateinit var mStateBuilder: PlaybackStateCompat.Builder

    override fun onCreate() {
        super.onCreate()

        // Create a MediaSessionCompat
        mMediaSession = MediaSessionCompat(baseContext, LOG_TAG).apply {

            // Enable callbacks from MediaButtons and TransportControls
            setFlags(MediaSessionCompat.FLAG_HANDLES_MEDIA_BUTTONS
                    or MediaSessionCompat.FLAG_HANDLES_TRANSPORT_CONTROLS
            )

            // Set an initial PlaybackState with ACTION_PLAY, so media buttons can start the player
            mStateBuilder = PlaybackStateCompat.Builder()
                    .setActions(PlaybackStateCompat.ACTION_PLAY
                                    or PlaybackStateCompat.ACTION_PLAY_PAUSE
                    )
            setPlaybackState(mStateBuilder.build())

            // MySessionCallback() has methods that handle callbacks from a media controller
            setCallback(MySessionCallback())

            // Set the session's token so that client activities can communicate with it.
            setSessionToken(sessionToken)
        }
    }
}

Java

public class MediaPlaybackService extends MediaBrowserServiceCompat {
    private static final String MY_MEDIA_ROOT_ID = "media_root_id";
    private static final String MY_EMPTY_MEDIA_ROOT_ID = "empty_root_id";

    private MediaSessionCompat mMediaSession;
    private PlaybackStateCompat.Builder mStateBuilder;

    @Override
    public void onCreate() {
        super.onCreate();

        // Create a MediaSessionCompat
        mMediaSession = new MediaSessionCompat(context, LOG_TAG);

        // Enable callbacks from MediaButtons and TransportControls
        mMediaSession.setFlags(
              MediaSessionCompat.FLAG_HANDLES_MEDIA_BUTTONS |
              MediaSessionCompat.FLAG_HANDLES_TRANSPORT_CONTROLS);

        // Set an initial PlaybackState with ACTION_PLAY, so media buttons can start the player
        mStateBuilder = new PlaybackStateCompat.Builder()
                            .setActions(
                                PlaybackStateCompat.ACTION_PLAY |
                                PlaybackStateCompat.ACTION_PLAY_PAUSE);
        mMediaSession.setPlaybackState(mStateBuilder.build());

        // MySessionCallback() has methods that handle callbacks from a media controller
        mMediaSession.setCallback(new MySessionCallback());

        // Set the session's token so that client activities can communicate with it.
        setSessionToken(mMediaSession.getSessionToken());
    }
}

Manage client connections

A MediaBrowserService has two methods that handle client connections: onGetRoot() controls access to the service, and onLoadChildren() provides the ability for a client to build and display a menu of the MediaBrowserService's content hierarchy.

Controlling client connections with onGetRoot()

The onGetRoot() method returns the root node of the content hierarchy. If the method returns null, the connection is refused.

To allow clients to connect to your service and browse its media content, onGetRoot() must return a non-null BrowserRoot which is a root ID that represents your content hierarchy.

To allow clients to connect to your MediaSession without browsing, onGetRoot() must still return a non-null BrowserRoot, but the root ID should represent an empty content hierarchy.

A typical implementation of onGetRoot() might look like this:

Kotlin

override fun onGetRoot(
        clientPackageName: String,
        clientUid: Int,
        rootHints: Bundle?
): MediaBrowserServiceCompat.BrowserRoot {

    // (Optional) Control the level of access for the specified package name.
    // You'll need to write your own logic to do this.
    return if (allowBrowsing(clientPackageName, clientUid)) {
        // Returns a root ID that clients can use with onLoadChildren() to retrieve
        // the content hierarchy.
        MediaBrowserServiceCompat.BrowserRoot(MY_MEDIA_ROOT_ID, null)
    } else {
        // Clients can connect, but this BrowserRoot is an empty hierachy
        // so onLoadChildren returns nothing. This disables the ability to browse for content.
        MediaBrowserServiceCompat.BrowserRoot(MY_EMPTY_MEDIA_ROOT_ID, null)
    }
}

Java

@Override
public BrowserRoot onGetRoot(String clientPackageName, int clientUid,
    Bundle rootHints) {

    // (Optional) Control the level of access for the specified package name.
    // You'll need to write your own logic to do this.
    if (allowBrowsing(clientPackageName, clientUid)) {
        // Returns a root ID that clients can use with onLoadChildren() to retrieve
        // the content hierarchy.
        return new BrowserRoot(MY_MEDIA_ROOT_ID, null);
    } else {
        // Clients can connect, but this BrowserRoot is an empty hierachy
        // so onLoadChildren returns nothing. This disables the ability to browse for content.
        return new BrowserRoot(MY_EMPTY_MEDIA_ROOT_ID, null);
    }
}

In some cases, you might want to implement a white/blacklist scheme to control connections. For an example of whitelisting, see the PackageValidator class in the Universal Android Music Player sample app.

Communicating content with onLoadChildren()

After the client connects, it can traverse the content hierarchy by making repeated calls to MediaBrowserCompat.subscribe() to build a local representation of the UI. The subscribe() method sends the callback onLoadChildren() to the service, which returns a list of MediaBrowser.MediaItem objects.

Each MediaItem has a unique ID string, which is an opaque token. When a client wants to open a submenu or play an item, it passes the ID. Your service is responsible for associating the ID with the appropriate menu node or content item.

A simple implementation of onLoadChildren() might look like this:

Kotlin

override fun onLoadChildren(
        parentMediaId: String,
        result: MediaBrowserServiceCompat.Result<List<MediaBrowserCompat.MediaItem>>
) {
    //  Browsing not allowed
    if (MY_EMPTY_MEDIA_ROOT_ID == parentMediaId) {
        result.sendResult(null)
        return
    }

    // Assume for example that the music catalog is already loaded/cached.

    val mediaItems = emptyList<MediaBrowserCompat.MediaItem>()

    // Check if this is the root menu:
    if (MY_MEDIA_ROOT_ID == parentMediaId) {
        // Build the MediaItem objects for the top level,
        // and put them in the mediaItems list...
    } else {
        // Examine the passed parentMediaId to see which submenu we're at,
        // and put the children of that menu in the mediaItems list...
    }
    result.sendResult(mediaItems)
}

Java

@Override
public void onLoadChildren(final String parentMediaId,
    final Result<List<MediaItem>> result) {

    //  Browsing not allowed
    if (TextUtils.equals(MY_EMPTY_MEDIA_ROOT_ID, parentMediaId)) {
        result.sendResult(null);
        return;
    }

    // Assume for example that the music catalog is already loaded/cached.

    List<MediaItem> mediaItems = new ArrayList<>();

    // Check if this is the root menu:
    if (MY_MEDIA_ROOT_ID.equals(parentMediaId)) {
        // Build the MediaItem objects for the top level,
        // and put them in the mediaItems list...
    } else {
        // Examine the passed parentMediaId to see which submenu we're at,
        // and put the children of that menu in the mediaItems list...
    }
    result.sendResult(mediaItems);
}

Note: MediaItem objects delivered by the MediaBrowserService should not contain icon bitmaps. Use a Uri instead by calling setIconUri() when you build the MediaDescription for each item.

For examples of how to implement onLoadChildren(), see the MediaBrowserService and Universal Android Music Player sample apps.

The media browser service lifecycle

The behavior of an Android service depends on whether it is started or bound to one or more clients. After a service is created, it can be started, bound, or both. In all of these states, it is fully functional and can perform the work it's designed to do. The difference is how long the service will exist. A bound service is not destroyed until all its bound clients unbind. A started service can be explicitly stopped and destroyed (assuming it is no longer bound to any clients).

When a MediaBrowser running in another activity connects to a MediaBrowserService, it binds the activity to the service, making the service bound (but not started). This default behavior is built into the MediaBrowserServiceCompat class.

A service that is only bound (and not started) is destroyed when all of its clients unbind. If your UI activity disconnects at this point, the service is destroyed. This isn't a problem if you haven't played any music yet. However, when playback starts, the user probably expects to continue listening even after switching apps. You don't want to destroy the player when you unbind the UI to work with another app.

For this reason, you need to be sure that the service is started when it begins to play by calling startService(). A started service must be explicitly stopped, whether or not it's bound. This ensures that your player continues to perform even if the controlling UI activity unbinds.

To stop a started service, call Context.stopService() or stopSelf(). The system stops and destroys the service as soon as possible. However, if one or more clients are still bound to the service, the call to stop the service is delayed until all its clients unbind.

The lifecycle of the MediaBrowserService is controlled by the way it is created, the number of clients that are bound to it, and the calls it receives from media session callbacks. To summarize:

  • The service is created when it is started in response to a media button or when an activity binds to it (after connecting via its MediaBrowser).
  • The media session onPlay() callback should include code that calls startService(). This ensures that the service starts and continues to run, even when all UI MediaBrowser activities that are bound to it unbind.
  • The onStop() callback should call stopSelf(). If the service was started, this stops it. In addition, the service is destroyed if there are no activities bound to it. Otherwise, the service remains bound until all its activities unbind. (If a subsequent startService() call is received before the service is destroyed, the pending stop is cancelled.)

The following flowchart demonstrates how the lifecycle of a service is managed. The variable counter tracks the number of bound clients:

Service Lifecycle

Using MediaStyle notifications with a foreground service

When a service is playing, it should be running in the foreground. This lets the system know that the service is performing a useful function and should not be killed if the system is low on memory. A foreground service must display a notification so the user knows about it and can optionally control it. The onPlay() callback should put the service in the foreground. (Note that this is a special meaning of "foreground." While Android considers the service in the foreground for purposes of process management, to the user the player is playing in the background while some other app is visible in the "foreground" on the screen.)

When a service runs in the foreground, it must display a notification, ideally with one or more transport controls. The notification should also include useful information from the session's metadata.

Build and display the notification when the player starts playing. The best place to do this is inside the MediaSessionCompat.Callback.onPlay() method.

The example below uses the NotificationCompat.MediaStyle, which is designed for media apps. It shows how to build a notification that displays metadata and transport controls. The convenience method getController() allows you to create a media controller directly from your media session.

Kotlin

// Given a media session and its context (usually the component containing the session)
// Create a NotificationCompat.Builder

// Get the session's metadata
val controller = mediaSession.controller
val mediaMetadata = controller.metadata
val description = mediaMetadata.description

val builder = NotificationCompat.Builder(context, channelId).apply {
    // Add the metadata for the currently playing track
    setContentTitle(description.title)
    setContentText(description.subtitle)
    setSubText(description.description)
    setLargeIcon(description.iconBitmap)

    // Enable launching the player by clicking the notification
    setContentIntent(controller.sessionActivity)

    // Stop the service when the notification is swiped away
    setDeleteIntent(
            MediaButtonReceiver.buildMediaButtonPendingIntent(
                    context,
                    PlaybackStateCompat.ACTION_STOP
            )
    )

    // Make the transport controls visible on the lockscreen
    setVisibility(NotificationCompat.VISIBILITY_PUBLIC)

    // Add an app icon and set its accent color
    // Be careful about the color
    setSmallIcon(R.drawable.notification_icon)
    color = ContextCompat.getColor(context, R.color.primaryDark)

    // Add a pause button
    addAction(
            NotificationCompat.Action(
                    R.drawable.pause,
                    getString(R.string.pause),
                    MediaButtonReceiver.buildMediaButtonPendingIntent(
                            context,
                            PlaybackStateCompat.ACTION_PLAY_PAUSE
                    )
            )
    )

    // Take advantage of MediaStyle features
    setStyle(android.support.v4.media.app.NotificationCompat.MediaStyle()
            .setMediaSession(mediaSession.sessionToken)
            .setShowActionsInCompactView(0)

            // Add a cancel button
            .setShowCancelButton(true)
            .setCancelButtonIntent(
                    MediaButtonReceiver.buildMediaButtonPendingIntent(
                            context,
                            PlaybackStateCompat.ACTION_STOP
                    )
            )
    )
}

// Display the notification and place the service in the foreground
startForeground(id, builder.build())

Java

// Given a media session and its context (usually the component containing the session)
// Create a NotificationCompat.Builder

// Get the session's metadata
MediaControllerCompat controller = mediaSession.getController();
MediaMetadataCompat mediaMetadata = controller.getMetadata();
MediaDescriptionCompat description = mediaMetadata.getDescription();

NotificationCompat.Builder builder = new NotificationCompat.Builder(context, channelId);

builder
    // Add the metadata for the currently playing track
    .setContentTitle(description.getTitle())
    .setContentText(description.getSubtitle())
    .setSubText(description.getDescription())
    .setLargeIcon(description.getIconBitmap())

    // Enable launching the player by clicking the notification
    .setContentIntent(controller.getSessionActivity())

    // Stop the service when the notification is swiped away
    .setDeleteIntent(MediaButtonReceiver.buildMediaButtonPendingIntent(context,
       PlaybackStateCompat.ACTION_STOP))

    // Make the transport controls visible on the lockscreen
    .setVisibility(NotificationCompat.VISIBILITY_PUBLIC)

    // Add an app icon and set its accent color
    // Be careful about the color
    .setSmallIcon(R.drawable.notification_icon)
    .setColor(ContextCompat.getColor(context, R.color.primaryDark))

    // Add a pause button
    .addAction(new NotificationCompat.Action(
        R.drawable.pause, getString(R.string.pause),
        MediaButtonReceiver.buildMediaButtonPendingIntent(context,
            PlaybackStateCompat.ACTION_PLAY_PAUSE)))

    // Take advantage of MediaStyle features
    .setStyle(new MediaStyle()
        .setMediaSession(mediaSession.getSessionToken())
        .setShowActionsInCompactView(0)

        // Add a cancel button
       .setShowCancelButton(true)
       .setCancelButtonIntent(MediaButtonReceiver.buildMediaButtonPendingIntent(context,
           PlaybackStateCompat.ACTION_STOP)));

// Display the notification and place the service in the foreground
startForeground(id, builder.build());

When using MediaStyle notifications, be aware of the behavior of these NotificationCompat settings:

  • When you use setContentIntent(), your service starts automatically when the notification is clicked, a handy feature.
  • In an "untrusted" situation like the lockscreen, the default visibility for notification contents is VISIBILITY_PRIVATE. You probably want to see the transport controls on the lockscreen, so VISIBILITY_PUBLIC is the way to go.
  • Be careful when you set the background color. In an ordinary notification in Android version 5.0 or later, the color is applied only to the background of the small app icon. But for MediaStyle notifications prior to Android 7.0, the color is used for the entire notification background. Test your background color. Go gentle on the eyes and avoid extremely bright or fluorescent colors.

These settings are available only when you are using NotificationCompat.MediaStyle:

  • Use setMediaSession() to associate the notification with your session. This allows third-party apps and companion devices to access and control the session.
  • Use setShowActionsInCompactView() to add up to 3 actions to be shown in the notification's standard-sized contentView. (Here the pause button is specified.)
  • In Android 5.0 (API level 21) and later you can swipe away a notification to stop the player once the service is no longer running in the foreground. You can't do this in earlier versions. To allow users to remove the notification and stop playback before Android 5.0 (API level 21), you can add a cancel button in the upper-right corner of the notification by calling setShowCancelButton(true) and setCancelButtonIntent().

When you add the pause and cancel buttons, you'll need a PendingIntent to attach to the playback action. The method MediaButtonReceiver.buildMediaButtonPendingIntent() does the job of converting a PlaybackState action into a PendingIntent.