android_frameworks_base/docs/html/training/managing-audio/audio-focus.jd

page.title=Managing Audio Focus
parent.title=Managing Audio Playback
parent.link=index.html

trainingnavtop=true
previous.title=Controlling Your App's Volume and Playback
previous.link=volume-playback.html
next.title=Dealing with Audio Output Hardware
next.link=audio-output.html

@jd:body


<div id="tb-wrapper"> 
<div id="tb">

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#RequestFocus">Request the Audio Focus</a></li>
  <li><a href="#HandleFocusLoss">Handle the Loss of Audio Focus</a></li>
  <li><a href="#DUCK">Duck!</a></li>
</ol>


<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/topics/media/mediaplayer.html">Media Playback</a></li>
</ul>

</div> 
</div>
 

<p>With multiple apps potentially playing audio it's important to think about how they should
interact. To avoid every music app playing at the same time, Android uses audio focus to moderate
audio playback&mdash;only apps that hold the audio focus should play audio.</p>

<p>Before your app starts playing audio it should request&mdash;and receive&mdash;the audio focus. 
Likewise, it should know how to listen for a loss of audio focus and respond appropriately when that
happens.</p>

 
<h2 id="RequestFocus">Request the Audio Focus</h2> 
 
<p>Before your app starts playing any audio, it should hold the audio focus for the stream
it will be using. This is done with a call to {@link android.media.AudioManager#requestAudioFocus
requestAudioFocus()} which returns
{@link android.media.AudioManager#AUDIOFOCUS_REQUEST_GRANTED} if your request is successful.</p>

<p>You must specify which stream you're using and whether you expect to require transient or
permanent audio focus. Request transient focus when you expect to play audio for only a short time
(for example when playing navigation instructions). Request permanent audio focus when you
plan to play audio for the foreseeable future (for example, when playing music).</p>

<p>The following snippet requests permanent audio focus on the music audio stream. You should
request the audio focus immediately before you begin playback, such as when the user presses
play or the background music for the next game level begins.</p>
  
<pre>
AudioManager am = mContext.getSystemService(Context.AUDIO_SERVICE);
...

// Request audio focus for playback
int result = am.requestAudioFocus(afChangeListener,
                                 // Use the music stream.
                                 AudioManager.STREAM_MUSIC,
                                 // Request permanent focus.
                                 AudioManager.AUDIOFOCUS_GAIN);
   
if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
    am.unregisterMediaButtonEventReceiver(RemoteControlReceiver);
    // Start playback.
}
</pre>

<p>Once you've finished playback be sure to call {@link
android.media.AudioManager#abandonAudioFocus abandonAudioFocus()}. This notifies
the system that you no longer require focus and unregisters the associated {@link
android.media.AudioManager.OnAudioFocusChangeListener}. In the case of abandoning transient focus,
this allows any interupted app to continue playback.</p>

<pre>
// Abandon audio focus when playback complete    
am.abandonAudioFocus(afChangeListener);
</pre>

<p>When requesting transient audio focus you have an additional option: whether or not you want to
enable "ducking." Normally, when a well-behaved audio app loses audio focus it immediately
silences its playback. By requesting a transient audio focus that allows ducking you tell other
audio apps that it’s acceptable for them to keep playing, provided they lower their volume until the
focus returns to them.</p>

<pre>
// Request audio focus for playback
int result = am.requestAudioFocus(afChangeListener,
                             // Use the music stream.
                             AudioManager.STREAM_MUSIC,
                             // Request permanent focus.
                             AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK);
   
if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
    // Start playback.
}
</pre>

<p>Ducking is particularly suitable for apps that use the audio stream intermittently, such as for
audible driving directions.</p>

<p>Whenever another app requests audio focus as described above, its choice between permanent and
transient (with or without support for ducking) audio focus is received by the listener you
registered when requesting focus.</p>


<h2 id="HandleFocusLoss">Handle the Loss of Audio Focus</h2> 

<p>If your app can request audio focus, it follows that it will in turn lose that focus when another
app requests it. How your app responds to a loss of audio focus depends on the manner of that
loss.</p>

<p>The {@link android.media.AudioManager.OnAudioFocusChangeListener#onAudioFocusChange
onAudioFocusChange()} callback method of they audio focus change listener you registered when
requesting audio focus receives a parameter that describes the focus change event. Specifically,
the possible focus loss events mirror the focus request types from the previous
section&mdash;permanent loss, transient loss, and transient with ducking permitted.</p>

<p>Generally speaking, a transient (temporary) loss of audio focus should result in your app
silencing it’s audio stream, but otherwise maintaining the same state. You should continue to
monitor changes in audio focus and be prepared to resume playback where it was paused once you’ve
regained the focus.</p>

<p>If the audio focus loss is permanent, it’s assumed that another application is now being used to
listen to audio and your app should effectively end itself. In practical terms, that means stopping
playback, removing media button listeners&mdash;allowing the new audio player to exclusively handle
those events&mdash;and abandoning your audio focus. At that point, you would expect a user action
(pressing play in your app) to be required before you resume playing audio.</p>

<p>In the following code snippet, we pause the playback or our media player object if the audio
loss is transient and resume it when we have regained the focus. If the loss is permanent, it
unregisters our media button event receiver and stops monitoring audio focus changes.<p>

<pre>
OnAudioFocusChangeListener afChangeListener = new OnAudioFocusChangeListener() {
    public void onAudioFocusChange(int focusChange) {
        if (focusChange == AUDIOFOCUS_LOSS_TRANSIENT
            // Pause playback
        } else if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {
            // Resume playback 
        } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS) {
            am.unregisterMediaButtonEventReceiver(RemoteControlReceiver);
            am.abandonAudioFocus(afChangeListener);
            // Stop playback
        }
    }
};
</pre>
 
<p>In the case of a transient loss of audio focus where ducking is permitted, rather than pausing
playback, you can "duck" instead.</p>


<h2 id="DUCK">Duck!</h2> 

<p>Ducking is the process of lowering your audio stream output volume to make transient audio from
another app easier to hear without totally disrupting the audio from your own application.</p>

<p>In the following code snippet lowers the volume on our media player object when we temporarily
lose focus, then returns it to its previous level when we regain focus.</p>

<pre>
OnAudioFocusChangeListener afChangeListener = new OnAudioFocusChangeListener() {
    public void onAudioFocusChange(int focusChange) {
        if (focusChange == AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK) {
            // Lower the volume
        } else if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {
            // Raise it back to normal
        }
    }
};
</pre>

<p>A loss of audio focus is the most important broadcast to react to, but not the only one. The
system broadcasts a number of intents to alert you to changes in user’s audio experience.
The next lesson demonstrates how to monitor them to improve the user’s overall experience.</p>