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 simpler, safer 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.
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 prevent 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 Media 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 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. If the caller isn't an approved package, you can return null to deny access to your content.
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) {
// To ensure you are not allowing any arbitrary app to browse your app's
// contents, you need to check the origin:
if (!PackageValidator.isCallerAllowed(this, clientPackageName, clientUid)) {
// If the request comes from an untrusted package, return null.
// No further calls will be made to other media browsing methods.
LogHelper.w(TAG, "OnGetRoot: IGNORING request from untrusted package "
+ clientPackageName);
return null;
}
if (ANDROID_AUTO_PACKAGE_NAME.equals(clientPackageName)) {
// Optional: if your app needs to adapt ads, music library or anything
// else that needs to run differently when connected to the car, this
// is where you should handle it.
}
return new BrowserRoot(MEDIA_ID_ROOT, 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 it's 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<MediaBrowser.MediaItem> mediaItems = new ArrayList<>();
// Check if this is the root menu:
if (MEDIA_BROWSER_ROOT.equals(parentMediaId)) {
// build the MediaItem objects for the top level,
// and put them in the <result> list
} else {
// examine the passed parentMediaId to see which submenu we're at,
// and put the children of that menu in the <result> list
}
}
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:
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. (for example, by using a {@link android.media.MediaPlayer} or ExoPlayer). If the phone is connected to an Auto device, .