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Providing Audio Playback for Auto

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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 Android Auto. By configuring your app with a few settings and implementing a service for accessing music tracks, you can enable Android Auto 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 Android Auto to browse your content listings and play them through a car stereo system.

Provide Audio Services

Audio apps do not directly control a car dashboard device or a phone that runs Android Auto. When the user connects an Android mobile device into a dashboard system or launches Android Auto on a phone, Android Auto discovers your app through manifest entries that indicate what audio services your app can provide. The Android Auto user interface 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, Android Auto 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 Android Auto, you need to:

Configure Your Manifest

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 Android Auto and how to access them. This section describes how to configure your app manifest to indicate your app supports audio services for Android Auto, and allow Android Auto to connect with your app.

Declare Auto audio support

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 Android Auto, add an XML file to the res/xml/ resources directory as automotive_app_desc.xml, with the following content: 

<automotiveApp>
    <uses name="media"/>
</automotiveApp>

For more information about declaring capabilities for Android Auto, see Getting Started with Auto.

Declare your media browser service

Android Auto expects 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 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 Android Auto. These media clients might be other apps on a user's mobile device, or they might be other remote clients.

Specify a notification icon

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, Android Auto 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.

Determine if Your App is Connected

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);

Handle Alarms

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:

The following code snippet checks whether an app is running in car mode: 
 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;
        }
  }

Handle Media Advertisements

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 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());
}

Build a Browser Service

Android Auto interact with your app by contacting its implementation of a MediaBrowserService, which you declare in your app manifest. This service allows Android Auto to find out what content your app provides. Android Auto can also query your app's media browser service to contact the MediaSession provided by your app, which handles content playback commands.

You create a media browser service by extending the MediaBrowserService class. Android Auto can contact your service to do the following:

Media browser service workflow

  1. When your app's audio services are requested by a user through Android Auto, Android Auto contacts your app's media browser service. In your implementation of the onCreate() method, you must create and register a MediaSession object and its callback object.
  2. Android Auto calls the browser service's onGetRoot() method to get the top node of your content hierarchy. The node retrieved by this call is not used as a menu item. It is only used to retrieve its child nodes, which are subsequently displayed as the top menu items.
  3. Auto invokes the onLoadChildren() method to get the children of the root node, and uses this information to present a menu to the user.
  4. If the user selects a submenu, Auto invokes onLoadChildren() again to retrieve the child nodes of the selected menu item.
  5. If the user begins playback, Auto invokes the appropriate media session callback method to perform that action. For more information, see the section about how to Implement Playback Controls.

To help users quickly browse your app's content, Android Auto includes a browsing capability that lets users select a letter from an on-screen keyboard. The user is then presented with a list of items beginning with that letter in the current drawer list. This works on both sorted and unsorted content, and is currently available only in English.

Figure 1. Alpha picker on car screen.

Figure 2. Alpha picker on phone screen.

Figure 3. Alpha keyboard on phone screen.

Building your content hierarchy

Android Auto, when acting as an audio client, calls your app's MediaBrowserService to find out what content you have available. You need to implement two methods in your browser service to support this: onGetRoot() and onLoadChildren().

Each node in your content hierarchy is represented by a 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. To encourage drivers to focus on driving, Android Auto triggers a speed bump notification on the phone screen (a temporary lock on content browsing), if the user exceeds the expected threshold for situationally aware driving. For more information on the Auto user experience restrictions, see the Auto Audio Apps guidelines.

Your implementation of onGetRoot() returns information about the root node of the menunhierarchy. 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 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 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 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 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 onLoadChildren(), see the MediaBrowserService and Universal Android Music Player sample apps.

Enable Playback Control

Android Auto sends playback control commands through your MediaSession. You must register a session and implement the associated callback methods.

Note: You can inspect the media session at any time with the command adb shell dumpsys media_session.

Register a media session

In your browser service's onCreate() method, create a MediaSession. Register the media session by calling setSessionToken().

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 MediaSession.Callback class for your app. The next section discusses how to implement this object.

Implement play commands

When an Android Auto user requests playback of an audio track from your app, Android Auto uses the MediaSession.Callback class from your app's 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 MediaSession.Callback class and implement the methods that your app supports. The most important callback methods are as follows:

onPlay()
Invoked if the user chooses play without choosing a specific item. Your app should play its default content. If playback was paused with onPause(), your app should resume playback.

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.

onPlayFromMediaId()
Invoked when the user chooses to play a specific item. The method is passed the item's media ID, which you assigned to the item in the content hierarchy.
onPlayFromSearch()
Invoked when the user chooses to play from a search query. The app should make an appropriate choice based on the passed search string.
onPause()
Pause playback.
onSkipToNext()
Skip to the next item.
onSkipToPrevious()
Skip to the previous item.
onStop()
Stop playback.

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 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 were listening through a device speaker or through connected headphones). If the app is running on an Android Auto supported car screen, 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.

Providing custom actions

Android Auto displays playback controls on the user interface based on the PlaybackState. Apps should set the supported playback actions, such as play, pause, or skip track. Based on the supported actions, Android Auto will display the appropriate buttons on the screen.

Android Auto apps must also support ACTION_PLAY_FROM_MEDIA_ID and ACTION_PLAY_FROM_SEARCH.

Developers can use the PlaybackState.Builder to add additional playback custom actions. The order in which the custom actions are added determines the order in which they appear to the user.

Each custom action requires an icon resource. Since apps that work with Auto are designed to run in cars with different screen sizes and densities, it is important that you provide your app’s custom icons for different screen densities. This will help avoid blurring or other scaling artifacts. Here are some tips that you might find useful as you develop custom icons for your application.

Use vector format where possible

Use the vector format for custom icons whenever possible. A vector drawable allows you to scale assets without losing the detail. A vector drawable also makes it easy to align edges and corners to pixel boundaries at smaller resolutions.

Provide drawables in multiple densities

If you must provide icons as bitmap drawables (.png, .jpg, and .gif files) and Nine-Patch drawables (.9.png files), then as a minimum, supply a version of each icon that's optimized for the following common car screen densities:

It is preferred to have your custom icons in the following densities as well:

For more information about designing for different screens, see the Supporting Multiple Screens developer guide.

Provide off icon style for disabled actions

For cases when a custom action is unavailable for the current context, swap the custom action icon with a corresponding off icon style resource.

Sample off style custom action icons.

Video

Devbytes: Android Auto Voice Actions

Support Voice Actions

To reduce driver distractions, you must 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 app to handle audio playback requests

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.

Parse the voice query to build the playback queue

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:

  1. Use the extras bundle and search query string returned from the voice search to filter results.
  2. Build the audio content queue based on these results.
  3. Play the audio content.

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:

The 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.

Implement playback control actions

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.

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