docs/html/training/auto/audio/index.jd

page.title=Providing Audio Playback for Auto
page.tags="auto", "car", "automotive", "audio"
page.article=true

page.metaDescription=Learn how to extend your audio apps for use in Android Auto devices.
page.image=auto/images/assets/icons/media_app_playback.png

@jd:body

<div id="tb-wrapper">
<div id="tb">
  <h2>Dependencies and Prerequisites</h2>
  <ul>
    <li>Android 5.0 (API level 21) or higher</li>
  </ul>

    <h2>This class teaches you how to</h2>

    <ol>
      <li><a href="#overview">Provide Audio Services</a></li>
      <li><a href="#config_manifest">Configure Your Manifest</a></li>
      <li><a href="#implement_browser">Build a Browser Service</a></li>
      <li><a href="#implement_callback">Implement Play Controls</a></li>
    </ol>

    <h2>Related Samples</h2>

    <ul>
      <li><a href="{@docRoot}samples/MediaBrowserService/index.html">
        MediaBrowserService</a></li>
    </ul>

    <h2>See Also</h2>

    <ul>
      <li>
        <a href="{@docRoot}shareables/auto/AndroidAuto-audio-apps.pdf">
              User Experience Guidelines: Audio Apps</a>
      </li>
      <li><a href="{@docRoot}training/managing-audio/index.html">Managing Audio
        Playback</a></li>
      <li><a href="{@docRoot}guide/topics/media/exoplayer.html">ExoPlayer</a>
      </li>
    </ul>

</div>
</div>

<a class="notice-developers-video wide"
href="https://www.youtube.com/watch?v=Q96Sw6v4ULg">
<div>
    <h3>Video</h3>
    <p>Devbytes: Android Auto Audio</p>
</div>
</a>

<p>
  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.
</p>

<p>
  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.
</p>

<p>
  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.
</p>


<h2 id="overview">Provide Audio Services</h2>

<p>
  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.
</p>

<p>To enable your app to provide audio content for Auto devices, you need to:
</p>

<ul>
  <li>Configure your app manifest to do the following:</li>
    <ul>
      <li>Declare that your app can provide audio content for Auto devices.</li>
      <li>Define a service that provides a browsable list of your audio tracks.</li>
    </ul>
  </li>
  <li>Build a service that provides audio track listing information extending
    {@link android.service.media.MediaBrowserService}.</li>
  <li>Register a {@link android.media.session.MediaSession} object and implement the
    {@link android.media.session.MediaSession.Callback} object to enable playback controls.</li>
</ul>


<h2 id="config_manifest">Configure Your Manifest</h2>

<p>
  When a user plugs an Android mobile device into a dashboard device running Auto, the system
  requests a list of installed apps that include <a href=
  "{@docRoot}guide/topics/manifest/manifest-intro.html">app manifest</a> 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.
</p>


<h3 id="manifest-car-app">Declare Auto audio support</h3>

<p>
  You indicate that your app supports cars capabilities using the following manifest entry:
</p>

<pre>
&lt;application&gt;
    ...
    &lt;meta-data android:name="com.google.android.gms.car.application"
        android:resource="&#64;xml/automotive_app_desc"/&gt;
    ...
&lt;application&gt;
</pre>

<p>
  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:
</p>

<pre>
&lt;automotiveApp&gt;
    &lt;uses name="media"/&gt;
&lt;/automotiveApp&gt;
</pre>

<p>
  For more information about declaring capabilities for Auto devices, see <a href=
  "{@docRoot}training/auto/start/index.html#auto-metadata">Getting Started with Auto</a>.
</p>


<h3 id="manifest-service">Declare your media browser service</h3>

<p>
  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.
</p>

<p>The following code example shows how to declare this listing browser service in your manifest:</p>

<pre>
&lt;application&gt;
    ...
    &lt;service android:name="<em>.MyMediaBrowserService</em>"
                android:exported="true"&gt;
        &lt;intent-filter&gt;
            <strong>&lt;action android:name=</strong>
                <strong>"android.media.browse.MediaBrowserService"/&gt;</strong>
        &lt;/intent-filter&gt;
    &lt;/service&gt;
    ...
&lt;application&gt;
</pre>

<p>
  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 <a href="#implement_browser">Build a Browser Service</a> section.
</p>

<p class="note">
  <strong>Note:</strong> 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.
</p>

<h3 id="manifest-icon">Specify a notification icon</h3>

<p>
  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:
</p>

<pre>
&lt;application&gt;
    ...
    &lt;meta-data android:name="com.google.android.gms.car.notification.SmallIcon"
        android:resource="&#64;drawable/ic_notification" /&gt;
    ...
&lt;application&gt;
</pre>

<p class="note"><strong>Note:</strong> The icon you provide should have transparency enabled, so the
icon's background gets filled in with the app's primary color.</p>


<h2 id="implement_browser">Build a Browser Service</h2>

<p>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.</p>

<p>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:</p>

<ul>
  <li>Browse your app's content hierarchy, in order to present a menu to the
    user</li>
  <li>Get the token for your app's {@link android.media.session.MediaSession}
    object, in order to control audio playback</li>
</ul>


<h3 id="browser_workflow">Media browser service workflow</h3>

<ol>

<li>When your app's audio services are requested by a user through a connected Auto device, the
dashboard system contacts your app's media browser service.
In your implementation of the {@link android.service.media.MediaBrowserService#onCreate()
onCreate()} method, you must create and register a {@link
android.media.session.MediaSession} object and its callback object.</li>

<li>The Auto device calls the browser service's {@link
android.service.media.MediaBrowserService#onGetRoot 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.
</li>

<li>Auto invokes the {@link android.service.media.MediaBrowserService#onLoadChildren
onLoadChildren()} method to get the children of the root node, and uses this information to
present a menu to the user.</li>

<li>If the user selects a submenu, Auto invokes
{@link android.service.media.MediaBrowserService#onLoadChildren
onLoadChildren()} again to retrieve the child nodes of the selected menu item.</li>

<li>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
<a href="#implement_callback">Implement Playback Controls</a>. </li>

</ol>


<h3 id="build_hierarchy">Building your content hierarchy</h3>

<p>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()}.</p>

<p>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.</p>

<p class="note"><strong>Note:</strong> 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 <a href="{@docRoot}shareables/auto/AndroidAuto-audio-apps.pdf">
Auto Audio Apps</a> guidelines.</p>

<p>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.</p>

<p>A typical implementation of {@link
android.service.media.MediaBrowserService#onGetRoot onGetRoot()} might
look like this:</p>

<pre>
&#64;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);
}
</pre>

<p>
  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:
</p>

<pre>
&#64;Override
public void onLoadChildren(final String parentMediaId,
    final Result&lt;List&lt;MediaItem&gt;&gt; result) {

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

    List&lt;MediaBrowser.MediaItem&gt; mediaItems = new ArrayList&lt;&gt;();

    // 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 &lt;result&gt; list
    } else {

        // examine the passed parentMediaId to see which submenu we're at,
        // and put the children of that menu in the &lt;result&gt; list
    }
}
</pre>


<h2 id="implement_callback">Enable Playback Control</h2>

<p>
  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.
</p>

<h3 id="registering_mediasession">Register a media session</h3>

<p>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.</p>

<p>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.</p>

<p>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:</p>

<pre>
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());

    ...
</pre>

<p>
  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.
</p>


<h3 id="playback-commands">Implement play commands</h3>

<p>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.</p>

<p>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:</p>

<dl>

<dt>{@link android.media.session.MediaSession.Callback#onPlay onPlay()}</dt>
<dd>Invoked if the user chooses play without choosing a specific item. Your
app should play its default content. If playback was paused with
{@link android.media.session.MediaSession.Callback#onPause onPause()}, your
app should resume playback.</dd>

<dt>{@link android.media.session.MediaSession.Callback#onPlayFromMediaId
onPlayFromMediaId()}</dt>
<dd>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.</dd>

<dt>{@link android.media.session.MediaSession.Callback#onPlayFromSearch
onPlayFromSearch()}</dt>
<dd>Invoked when the user chooses to play from a search query. The app should
make an appropriate choice based on the passed search string.</dd>

<dt>{@link android.media.session.MediaSession.Callback#onPause onPause()}</dt>
<dd>Pause playback.</dd>

<dt>{@link android.media.session.MediaSession.Callback#onSkipToNext
onSkipToNext()}</dt>
<dd>Skip to the next item.</dd>

<dt>{@link android.media.session.MediaSession.Callback#onSkipToPrevious
onSkipToPrevious()}</dt>
<dd>Skip to the previous item.</dd>

<dt>{@link android.media.session.MediaSession.Callback#onStop onStop()}</dt>
<dd>Stop playback.</dd>

</dl>

<p>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()}.</p>

<p>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.</p>

<p>For more information about playing audio content, see
<a href="{@docRoot}guide/topics/media/mediaplayer.html">Media Playback</a>,
<a href="{@docRoot}training/managing-audio/index.html">Managing Audio Playback</a>, and
<a href="{@docRoot}guide/topics/media/exoplayer.html">ExoPlayer</a>.</p>