Devbytes: Android Auto Audio
Drivers want to access their music and other audio content on the road. Audio books, podcasts, sports commentary, and recorded talks can make a long trip educational, inspirational, and enjoyable. The Android framework allows you to extend your audio app so users can listen to their favorite tunes and audio content using a simple, yet customizable user interface.
Apps running on mobile devices with Android 5.0 or higher can provide audio services for dashboard systems running Android Auto. By configuring your app with a few settings and implementing a service for accessing music tracks, you can enable Auto devices to discover your app and provide a browse and playback interface for your app's audio content.
This class assumes that you have built an app that plays audio through an Android device's integrated speakers or connected headphones. It describes how to extend your app to allow Auto devices to browse your content listings and play it through a car stereo system.
Audio apps do not directly control a car dashboard device that runs Android Auto. When the user connects an Android mobile device into a dashboard system, Android Auto discovers your app through manifest entries that indicate what audio services your app can provide. The dashboard system displays a launcher icon for your app as a music provider and the user can choose to use your app's services. If the user launches your app, the Auto device queries your app to see what content is available, displays your content items to the user, and sends requests to your app to control playback with actions such as play, pause, or skip track.
To enable your app to provide audio content for Auto devices, you need to:
When a user plugs an Android mobile device into a dashboard device running Auto, the system requests a list of installed apps that include app manifest entries to indicate they support services for Auto devices and how to access them. This section describes how to configure your app manifest to indicate your app supports audio services for Auto devices, and allow dashboard system to connect with your app.
You indicate that your app supports cars capabilities using the following manifest entry:
<application>
...
<meta-data android:name="com.google.android.gms.car.application"
android:resource="@xml/automotive_app_desc"/>
...
<application>
This manifest entry refers to a secondary XML file, where you declare what Auto capabilities your app supports. For an app that supports audio for cars, add an XML file to the {@code res/xml/} resources directory as {@code automotive_app_desc.xml}, with the following content:
<automotiveApp>
<uses name="media"/>
</automotiveApp>
For more information about declaring capabilities for Auto devices, see Getting Started with Auto.
Auto devices expect to connect to a service in order to browse audio track listings. You declare this service in your manifest to allow the dashboard system to discover this service and connect to your app.
The following code example shows how to declare this listing browser service in your manifest:
<application>
...
<service android:name=".MyMediaBrowserService"
android:exported="true">
<intent-filter>
<action android:name=
"android.media.browse.MediaBrowserService"/>
</intent-filter>
</service>
...
<application>
The service your app provides for browsing audio tracks must extend the {@link android.service.media.MediaBrowserService}. The implementation of this service is discussed in the Build a Browser Service section.
Note: Other clients can also contact your app's browser service aside from Auto devices. These media clients might be other apps on a user's mobile device, or they might be other remote clients.
The Auto user interface shows notifications about your audio app to the user during the course of operation. For example, if the user has a navigation app running, and one song finishes and a new song starts, the Auto device shows the user a notification to indicate the change with an icon from your app. You can specify an icon that is used to represent your app for these notifications using the following manifest declaration:
<application>
...
<meta-data android:name="com.google.android.gms.car.notification.SmallIcon"
android:resource="@drawable/ic_notification" />
...
<application>
Note: The icon you provide should have transparency enabled, so the icon's background gets filled in with the app's primary color.
It is possible to determine if your app is selected as the current media app.
Android Auto broadcasts an intent with com.google.android.gms.car.media.STATUS
action when a user connects or disconnects from a media app. The broadcast intent is
scoped to the package name of the media app selected. You can register a broadcast receiver in your
app, preferably in your
MediaBrowserService implementation and listen for this intent
and adjust advertisements, metadata, and UI buttons in your app to operate safely in a vehicle.
The broadcasted intent has a String extra media_connection_status, that
contains either media_connected or media_disconnected string that represents
the current connection status.
IntentFilter filter = new IntentFilter("com.google.android.gms.car.media.STATUS");
BroadcastReceiver receiver = new BroadcastReceiver() {
...
public void onReceive(Context context, Intent intent) {
String status = intent.getStringExtra("media_connection_status");
boolean isConnectedToCar = "media_connected".equals(status);
// adjust settings based on the connection status
}
};
registerReceiver(receiver, filter);
To prevent user distraction, Android Auto media apps must not start playing audio through the car speakers unless a user consciously starts playback (such as when the user presses play in your app). Even a user-scheduled alarm from the media app must not start playing music through the car speakers. If your media app has an alarm feature, the app should determine if the phone is in car mode before playing any audio. Your app can do this by calling UiModeManager.getCurrentModeType(), which checks whether the device is running in car mode.
If the device is in car mode, media apps that support alarms must do one of the following things:
public static boolean isCarUiMode(Context c) {
UiModeManager uiModeManager = (UiModeManager) c.getSystemService(Context.UI_MODE_SERVICE);
if (uiModeManager.getCurrentModeType() == Configuration.UI_MODE_TYPE_CAR) {
LogHelper.d(TAG, "Running in Car mode");
return true;
} else {
LogHelper.d(TAG, "Running on a non-Car mode");
return false;
}
}
By default, Android Auto displays a notification when the media metadata changes during an audio playback session. When a media app switches from playing music to running an advertisement, it is distracting (and unnecessary) to display a notification to the user. To prevent Android Auto from displaying a notification in this case, you must set the media metadata key {@code android.media.metadata.ADVERTISEMENT} to 1, as shown in the code snippet below:
@Override
public static final String EXTRA_METADATA_ADVERTISEMENT =
"android.media.metadata.ADVERTISEMENT";
public void onPlayFromMediaId(String mediaId, Bundle extras) {
MediaMetadata.Builder builder = new MediaMetadata.Builder();
// ...
if (isAd(mediaId)) {
builder.putLong(EXTRA_METADATA_ADVERTISEMENT, 1);
}
// ...
mediaSession.setMetadata(builder.build());
}
Auto devices interact with your app by contacting its implementation of a {@link android.service.media.MediaBrowserService}, which you declare in your app manifest. This service allows Auto devices to find out what content your app provides. Connected Auto devices can also query your app's media browser service to contact the {@link android.media.session.MediaSession} provided by your app, which handles content playback commands.
You create a media browser service by extending the {@link android.service.media.MediaBrowserService} class. Connected Auto devices can contact your service to do the following:
Auto devices acting as audio clients call your app's {@link android.service.media.MediaBrowserService} to find out what content you have available. You need to implement two methods in your browser service to support this: {@link android.service.media.MediaBrowserService#onGetRoot onGetRoot()} and {@link android.service.media.MediaBrowserService#onLoadChildren onLoadChildren()}.
Each node in your content hierarchy is represented by a {@link android.media.browse.MediaBrowser.MediaItem} object. Each of these objects is identified by a unique ID string. The client treats these ID strings as opaque tokens. When a client wants to browse to a submenu, or play a content item, it passes the ID token. Your app is responsible for associating the ID token with the appropriate menu node or content item.
Note: You should consider providing different content hierarchies depending on what client is making the query. In particular, Auto applications have strict limits on how large a menu they can display. This is intended to minimize distracting the driver, and to make it easy for the driver to operate the app via voice commands. For more information on the Auto user experience restrictions, see the Auto Audio Apps guidelines.
Your implementation of {@link android.service.media.MediaBrowserService#onGetRoot
onGetRoot()} returns information about the root node of the menu
hierarchy. This root node is the parent of the top items of your browse hierarchy.
The method is passed information about the calling client. You can use this
information to decide if the client should have access to your content at all.
For example, if you want to limit your app's content to a list of approved
clients, you can compare the passed {@code clientPackageName} to your whitelist
and verify the certificate used to sign the caller's APK.
If the caller can't be verified to be an approved package, return null to deny access to
your content. For an example of an app that validates that the caller is an
approved app, see the
PackageValidator class in the
Universal Android Music Player sample app.
A typical implementation of {@link android.service.media.MediaBrowserService#onGetRoot onGetRoot()} might look like this:
@Override
public BrowserRoot onGetRoot(String clientPackageName, int clientUid,
Bundle rootHints) {
// Verify that the specified package is allowed to access your
// content! You'll need to write your own logic to do this.
if (!isValid(clientPackageName, clientUid)) {
// If the request comes from an untrusted package, return null.
// No further calls will be made to other media browsing methods.
return null;
}
return new BrowserRoot(MY_MEDIA_ROOT_ID, null);
}
The Auto device client builds the top-level menu by calling {@link android.service.media.MediaBrowserService#onLoadChildren onLoadChildren()} with the root node object and getting its children. The client builds submenus by calling the same method with other child nodes. The following example code shows a simple implementation of {@link android.service.media.MediaBrowserService#onLoadChildren onLoadChildren()} method:
@Override
public void onLoadChildren(final String parentMediaId,
final Result<List<MediaItem>> result) {
// 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);
}
For examples of how to implement {@link android.service.media.MediaBrowserService#onLoadChildren onLoadChildren()}, see the MediaBrowserService and Universal Android Music Player sample apps.
Auto devices use {@link android.media.session.MediaSession} objects to pass playback control commands to an app that is providing audio services. Your audio app must create an instance of this object to pass to the dashboard device and implement callback methods to enable remote control of audio playback.
An Auto device using your app as audio service needs to obtain a {@link android.media.session.MediaSession} object from your app. The Auto device uses the session object to send playback commands requested by the Auto user back to your app.
When you initialize your browser service, you register that session object with your {@link android.service.media.MediaBrowserService} by calling the {@link android.service.media.MediaBrowserService#setSessionToken setSessionToken()} method. This step allows clients such as an Auto device to retrieve that object by calling your browser service's {@link android.service.media.MediaBrowserService#getSessionToken getSessionToken()} method.
In your browser service's {@link android.service.media.MediaBrowserService#onCreate() onCreate()} method, create a {@link android.media.session.MediaSession}. You can then query the {@link android.media.session.MediaSession} to get its token, and register the token with your browser service:
public void onCreate() {
super.onCreate();
...
// Start a new MediaSession
MediaSession mSession = new MediaSession(this, "session tag");
setSessionToken(mSession.getSessionToken());
// Set a callback object to handle play control requests, which
// implements MediaSession.Callback
mSession.setCallback(new MyMediaSessionCallback());
...
When you create the media session object, you set a callback object that is used to handle playback control requests. You create this callback object by providing an implementation of the {@link android.media.session.MediaSession.Callback} class for your app. The next section discusses how to implement this object.
When an Auto device requests playback of an audio track from your app, it uses the {@link android.media.session.MediaSession.Callback} class from your app's {@link android.media.session.MediaSession} object, which it obtained from your app's media browse service. When an Auto user wants to play content or control content playback, such as pausing play or skipping to the next track, Auto invokes one of the callback object's methods.
To handle content playback, your app must extend the abstract {@link android.media.session.MediaSession.Callback} class and implement the methods that your app supports. The most important callback methods are as follows:
Note: Google Play requires your app not to play music immediately when it launches. For more information on this and other requirements, see Auto App Quality.
Your app should override these methods to provide any desired functionality. In some cases you might not implement a method if it is not supported by your app. For example, if your app plays a live stream (such as a sports broadcast), the skip to next function might not make sense. In that case, you could simply use the default implementation of {@link android.media.session.MediaSession.Callback#onSkipToNext onSkipToNext()}.
When your app receives a request to play content, it should play audio the same way it would in a non-Auto situation (as if the user was listening through a device speaker or connected headphones). The audio content is automatically sent to the dashboard system to be played over the car's speakers.
For more information about playing audio content, see Media Playback, Managing Audio Playback, and ExoPlayer.
Devbytes: Android Auto Voice Actions
To reduce driver distractions, you can add voice actions in your audio playback app. With voice action support, users can launch your app and play audio by providing voice input on Auto screens. If your audio playback app is already active and the user says “Play a song”, the system starts playing music without requiring the user to look at or touch the screen.
Enable your audio app to launch with a voice command such as "Play [search query] on [your app name]" by adding the following entry in your manifest:
<activity>
<intent-filter>
<action android:name=
"android.media.action.MEDIA_PLAY_FROM_SEARCH" />
<category android:name=
"android.intent.category.DEFAULT" />
</intent-filter>
</activity>
When the user says “Play music on [your app name]” on an Auto screen, Auto
attempts to launch your app and play audio by calling your app’s
MediaSession.Callback.onPlayFromSearch()
method. If the user has not specified criteria such as a track name or music genre, the
MediaSession.Callback.onPlayFromSearch()
method receives an empty query parameter. Your app should respond by immediately playing audio, such
as a song from a random queue or the most recent playlist.
When a user searches for a specific criteria, such as “Play jazz on [your app name]”
or “Listen to [song title]”, the
onPlayFromSearch()
callback method receives the voice search results in the query parameter and an extras bundle. For
more information on how to handle search queries to play audio content, see
Play music
based on a search query.
To parse the voice search query to play back audio content in your app, follow these steps:
The
onPlayFromSearch()
method takes an extras parameter with more detailed information from the voice search.
These extras help you find the audio content in your app for playback. If the search results are
unable to provide this data, you can implement logic to parse the raw search query and play the
appropriate tracks based on the query.
The following extras are supported in Android Auto:
android.intent.extra.albumandroid.intent.extra.artistandroid.intent.extra.genreandroid.intent.extra.playlistandroid.intent.extra.titleThe following snippet shows how to override the
onPlayFromSearch()
method in your
MediaSession.Callback
implementation to handle the search query and extras for playing audio content in your app:
@Override
public void onPlayFromSearch(String query, Bundle extras) {
if (TextUtils.isEmpty(query)) {
// The user provided generic string e.g. 'Play music'
// Build appropriate playlist queue
} else {
// Build a queue based on songs that match "query" or "extras" param
String mediaFocus = extras.getString(MediaStore.EXTRA_MEDIA_FOCUS);
if (TextUtils.equals(mediaFocus,
MediaStore.Audio.Artists.ENTRY_CONTENT_TYPE)) {
isArtistFocus = true;
artist = extras.getString(MediaStore.EXTRA_MEDIA_ARTIST);
} else if (TextUtils.equals(mediaFocus,
MediaStore.Audio.Albums.ENTRY_CONTENT_TYPE)) {
isAlbumFocus = true;
album = extras.getString(MediaStore.EXTRA_MEDIA_ALBUM);
}
// Implement additional "extras" param filtering
}
// Implement your logic to retrieve the queue
if (isArtistFocus) {
result = searchMusicByArtist(artist);
} else if (isAlbumFocus) {
result = searchMusicByAlbum(album);
}
if (result == null) {
// No focus found, search by query for song title
result = searchMusicBySongTitle(query);
}
if (result != null && !result.isEmpty()) {
// Immediately start playing from the beginning of the search results
// Implement your logic to start playing music
playMusic(result);
} else {
// Handle no queue found. Stop playing if the app
// is currently playing a song
}
}
Note: To minimize driver distractions, immediately initiate audio content
playback in the
onPlayFromSearch()
method when you have generated the audio content queue based on the user's request.
For a more detailed example on how to implement voice search to play audio content in your app, see the Universal Media Player sample.
To provide a hands-free experience while users drive and listen to audio content in Android Auto, your app should allow users to control audio content playback with voice actions. When users speak commands such as “Next song”, “Pause music”, or “Resume music”, the system triggers the corresponding callback method where you implement the playback control action.
To provide voice-enabled playback controls, first enable the hardware controls by setting these
flags in your app’s
MediaSession
object:
mSession.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS |
MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS);
Then, implement the callback methods with the playback controls that you support in your app. Here’s a list of voice-enabled playback controls supported by Android Auto:
| Example phrase | Callback method |
|---|---|
| "Next song" | onSkipToNext() |
| "Previous song" | onSkipToPrevious() |
| "Pause music" | onPause() |
| "Stop music" | onStop() |
| "Resume music" | onPlay() |
For a more detailed example on how to implement voice-enabled playback actions in your app, see the Universal Media Player sample.