Managing Audio Playback之Managing Audio Focus

同一时间可能要好多个app在播放audio，Android用audio focus 来避免app之前互相影响.只有持有audio focus的app才可以播放audio.
可以用requestAudioFocus（）来获取对应audio stream的audio focus.如果返回值是AUDIOFOCUS_REQUEST_GRANTED的话，即说明成功获得audio focus.
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.registerMediaButtonEventReceiver(RemoteControlReceiver);
    // Start playback.
}
当播放完成时，应该调用abandonAudioFocus（）来告诉系统不再需要focus,其参数是AudioManager。OnAudioFocusChangeListner().
// Abandon audio focus when playback complete
am.abandonAudioFocus(afChangeListener);
可以通过下面的code来获得短暂的focus，和上面的差别在requestAudioFocus的第三个参数为AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK。
/ 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.
}
在OnAudioFocusChangeListener 中我们可以检测audio focus的loss/gain 等事件.
AudioManager.OnAudioFocusChangeListener afChangeListener =
    new AudioManager.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
            }
        }
    };
当发生duck是，需要降低音量.
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
        }
    }
};