int getPlayState ()
Returns the playback state of the AudioTrack instance.
獲取當(dāng)前的播放狀態(tài)。這個(gè)接口會(huì)返回PLAYSTATE_STOPPED、PLAYSTATE_PAUSED、PLAYSTATE_PLAYING
三種狀態(tài)，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(shù)據(jù)。已經(jīng)在緩沖區(qū)中的未播放數(shù)據(jù)將不會(huì)被丟棄，在下次play的時(shí)候繼續(xù)播放。調(diào)用flush則會(huì)丟棄緩沖數(shù)據(jù)。

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(shè)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(qǐng)求獲取音頻焦點(diǎn)。
第一個(gè)參數(shù)為音頻焦點(diǎn)更改時(shí)的回調(diào)；
第二個(gè)參數(shù)為音頻類型，在我們調(diào)節(jié)音量時(shí)可以看到有若干種音量，就對(duì)應(yīng)的這里的streamType，這里我們基本用MUSIC，表示“媒體”。
第三個(gè)參數(shù)表示獲取焦點(diǎn)的“時(shí)長(zhǎng)”，有如下幾種情況：
AUDIOFOCUS_GAIN_TRANSIENT
表示僅僅為臨時(shí)獲取焦點(diǎn)。比如播放導(dǎo)航語音、通知聲音等，屬于時(shí)間很短暫的情況；
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK
表示為DUCK模式，表示當(dāng)獲取焦點(diǎn)后，允許先前獲取過焦點(diǎn)的程序在降低輸出音量的前提下繼續(xù)播放。
AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE
痛第一種情況類似，只是不允許系統(tǒng)再播放其他聲音。通常應(yīng)用在語音備忘、語音識(shí)別等情況；
AUDIOFOCUS_GAIN
表示要獲取焦點(diǎn)的時(shí)長(zhǎng)未知。比如播放音樂等等。

當(dāng)獲取到焦點(diǎn)時(shí)，函數(shù)放回AUDIOFOCUS_REQUEST_GRANTED，當(dāng)獲取失敗時(shí)，返回AUDIOFOCUS_REQUEST_FAILED

// 獲取AudioManager實(shí)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(diǎn)獲取到了，那繼續(xù)播放，并恢復(fù)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(diǎn)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(diǎn)丟失了，但是允許在降低音量的前提下繼續(xù)播放，那么降低聲音。
      AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        audioTrack.setStereoVolume(0.5f, 0.5f);
      }
    }
  }
};
// 因?yàn)檫@里要獲得的焦點(diǎn)無法預(yù)知時(shí)長(zhǎng)，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(diǎn)。那啟動(dòng)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(diǎn)。那不播放聲音
  AudioTrack audioTrack = mAudioTrack;
  if (audioTrack != null) {
    if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {
      audioTrack.pause();
    }
  }
}