Android - 音频焦点学习

在做蓝牙带屏音箱项目的时候，碰到一个问题，具体是在连接蓝牙音箱回连的时候，播放音乐会在手机端点击播放过一会就回被暂停。

调试了一下问题后，大概问题是这样的，一般情况下，我们在播放音乐时会请求系统音频焦点，然后在不播放的时候释放焦点。

我也是按照这种逻辑处理的，但和一般情况不同的是，我应用所在的底层系统是经过定制的，而在音频焦点的处理上，除了我上层应用会在蓝牙播放变化的时候请求焦点，底层的系统也会做同样的操作，这就造成了在两者来回请求的过程中，焦点丢失了。

这种问题还是比较少见的，所以记录一下，而最终的解决办法是让提供定制系统的硬件方解决。不过这里也记录一下对于音频焦点请求和监听的学习。

完整的代码

public class MusicService extends Service {
   private AudioManager mAm;
   private boolean isPlaymusic;
   private String url;
   private MediaPlayer mediaPlayer;
   @Override
   public void onCreate() {
       super.onCreate();
       mAm = (AudioManager) getSystemService(AUDIO_SERVICE);
   }
   @Override
   public void onStart(Intent intent, int startId) {
       if (intent != null) {
           Bundle bundle = intent.getExtras();
           if (bundle != null) {
               isPlaymusic = bundle.getBoolean("isPlay", true);
               url = bundle.getString("url");
               if (isPlaymusic)
                   play();
               else
                   stop();
           }
       }
   }
   OnAudioFocusChangeListener afChangeListener = new OnAudioFocusChangeListener() {
       public void onAudioFocusChange(int focusChange) {
           if (focusChange == AudioManager.AUDIOFOCUS_LOSS_TRANSIENT) {
               // Pause playback
               pause();
           } else if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {
               // Resume playback
               resume();
           } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS) {
               // mAm.unregisterMediaButtonEventReceiver(RemoteControlReceiver);
               mAm.abandonAudioFocus(afChangeListener);
               // Stop playback
               stop();
           }
       }
   };
   private boolean requestFocus() {
       // Request audio focus for playback
       int result = mAm.requestAudioFocus(afChangeListener,
       // Use the music stream.
               AudioManager.STREAM_MUSIC,
               // Request permanent focus.
               AudioManager.AUDIOFOCUS_GAIN);
       return result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED;
   }
   private void resume() {
       if (mediaPlayer != null) {
           mediaPlayer.start();
       }
   }
   private void pause() {
       if (mediaPlayer != null && mediaPlayer.isPlaying()) {
           mediaPlayer.pause();
       }
   }
   OnCompletionListener completionListener = new OnCompletionListener() {
       @Override
       public void onCompletion(MediaPlayer player) {
           if(!player.isLooping()){
               mAm.abandonAudioFocus(afChangeListener);
           }
       }
   };
   private void play() {
       if (requestFocus()) {
           if (mediaPlayer == null) {
               try {
                   mediaPlayer = new MediaPlayer();
                   mediaPlayer.setDataSource(url);
                   mediaPlayer.prepare();
                   mediaPlayer.setOnCompletionListener(completionListener);
               } catch (IOException e) {
                   e.printStackTrace();
               }
           }
           if (!mediaPlayer.isPlaying()) {
               mediaPlayer.start();
           }
       }
   }
   @Override
   public void onDestroy() {
       super.onDestroy();
       if (mediaPlayer != null)
           mediaPlayer.release();
   }
   private void stop() {
       if (mediaPlayer != null) {
           mediaPlayer.stop();
       }
   }
   @Override
   public IBinder onBind(Intent arg0) {
       // TODO Auto-generated method stub
       return null;
   }
}

需要解释一下的是，在 onAudioFocusChange 方法里的返回值代表了当前焦点变化的状态

常量	说明
AUDIOFOCUS_GAIN	持有焦点
AUDIOFOCUS_LOSS	失去焦点
AUDIOFOCUS_LOSS_TRANSIENT	临时失去焦点
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK	临时失去焦点，允许小声播放