Android EasyPlayer聲音自動停止、恢復，一鍵靜音等功能

int getPlayState ()
Returns the playback state of the AudioTrack instance.
獲取當前的播放狀態。這個接口會返回PLAYSTATE_STOPPED、PLAYSTATE_PAUSED、PLAYSTATE_PLAYING
三種狀態，分別表示未播放、暫停中、正在播放

void pause ()
Pauses the playback of the audio data. Data that has not been played back will not be discarded. Subsequent calls to play() will play this data back. See flush() to discard this data.
暫停播放音頻數據。已經在緩沖區中的未播放數據將不會被丟棄，在下次play的時候繼續播放。調用flush則會丟棄緩沖數據。

void play ()
Starts playing an AudioTrack.
開始播放

int setStereoVolume (float leftGain, 
        float rightGain)

Sets the specified left and right output gain values on the AudioTrack.
設置左右聲道的音量增益。

mAudioEnable = mAudioTrack!=null &&　mAudioTrack.getPlayState()==PLAYSTATE_PLAYING;

public void setAudioEnable(boolean enable) {
   mAudioEnable = enable;
   AudioTrack at = mAudioTrack;
   if (at != null) {
     synchronized (at) {
       if (!enable) {
         at.pause();
         at.flush();
       } else {
         at.flush();
         at.play();
       }
     }
   }
 }

int requestAudioFocus (AudioManager.OnAudioFocusChangeListener l, 
        int streamType, 
        int durationHint)
Request audio focus. Send a request to obtain the audio focus
請求獲取音頻焦點。
第一個參數為音頻焦點更改時的回調；
第二個參數為音頻類型，在我們調節音量時可以看到有若干種音量，就對應的這里的streamType，這里我們基本用MUSIC，表示“媒體”。
第三個參數表示獲取焦點的“時長”，有如下幾種情況：
AUDIOFOCUS_GAIN_TRANSIENT
表示僅僅為臨時獲取焦點。比如播放導航語音、通知聲音等，屬于時間很短暫的情況；
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK
表示為DUCK模式，表示當獲取焦點后，允許先前獲取過焦點的程序在降低輸出音量的前提下繼續播放。
AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE
痛第一種情況類似，只是不允許系統再播放其他聲音。通常應用在語音備忘、語音識別等情況；
AUDIOFOCUS_GAIN
表示要獲取焦點的時長未知。比如播放音樂等等。

當獲取到焦點時，函數放回AUDIOFOCUS_REQUEST_GRANTED，當獲取失敗時，返回AUDIOFOCUS_REQUEST_FAILED

// 獲取AudioManager實例
final AudioManager am = (AudioManager) mContext.getSystemService(Context.AUDIO_SERVICE);
AudioManager.OnAudioFocusChangeListener l = new AudioManager.OnAudioFocusChangeListener() {
  @Override
  public void onAudioFocusChange(int focusChange) {
    if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {// 焦點獲取到了，那繼續播放，并恢復音量。
      AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        audioTrack.setStereoVolume(1.0f, 1.0f);
        if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PAUSED) {
          audioTrack.flush();
          audioTrack.play();
        }
      }
    } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS) {// 焦點丟失了，暫停播放。
       AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {
          audioTrack.pause();
        }
      }
    } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK) { // 焦點丟失了，但是允許在降低音量的前提下繼續播放，那么降低聲音。
      AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        audioTrack.setStereoVolume(0.5f, 0.5f);
      }
    }
  }
};
// 因為這里要獲得的焦點無法預知時長，因此用AUDIOFOCUS_GAIN模式。
int requestCode = am.requestAudioFocus(l, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);
if (requestCode == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
  // 成功獲取到了焦點。那啟動播放
  AudioTrack audioTrack = mAudioTrack;
  if (audioTrack != null) {
    audioTrack.setStereoVolume(1.0f, 1.0f);
    if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PAUSED) {
      audioTrack.flush();
      audioTrack.play();
    }
  }
}else{ // 沒有獲取到音頻焦點。那不播放聲音
  AudioTrack audioTrack = mAudioTrack;
  if (audioTrack != null) {
    if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {
      audioTrack.pause();
    }
  }
}