android 音頻處理mediaplay介紹
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處理音頻焦點
本文引用地址：http://www.104case.com/article/201609/305045.htm盡管某個時刻只有一個activity可以運行，Android卻是一個多任務環境.這對使用音頻的應用帶來了特殊的挑戰，因為只有一個音頻輸出而可能多個媒體都想用它.在Android2.2之前，沒有內建的機制來處理這個問題，所以可能在某些情況下導致壞的用戶體驗.例如，當一個用戶正在聽音樂而另一個應用需要通知用戶一些重要的事情時，用戶可能由于音樂聲音大而不能聽的通知.從Android2.2開始，平臺為應用提供了一個協商它們如何使用設備音頻輸出的途徑，這個機制叫做音頻焦點.
當你的應用需要輸出像樂音和通知之類的音頻時，你應該總是請求音頻焦點.一旦應用具有了焦點，它就可以自由的使用音頻輸出.但它總是應該監聽焦點的變化.如果被通知丟失焦點，它應該立即殺死聲音或降低到靜音水平(有一個標志表明應選擇哪一個)并且僅當重新獲得焦點后才恢復大聲播放.
將來的音頻焦點是合作的.所以，應用被希望(并被強列鼓勵)遵守音頻焦點的方針，但是卻不是被系統強制的.如果一個應用在丟失音頻焦點后依然想大聲播放音樂，系統不會去阻止它.然而用戶卻體驗很壞并且很想把這鳥應用卸載.
要請求音頻焦點，你必須從AudioManager調用requestAudioFocus()，如下所示：
[java]
AudioManager audioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
int result = audioManager.requestAudioFocus(this, AudioManager.STREAM_MUSIC,
AudioManager.AUDIOFOCUS_GAIN);
if (result != AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
// 不能獲得音頻焦點
}
AudioManager audioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
int result = audioManager.requestAudioFocus(this, AudioManager.STREAM_MUSIC,
AudioManager.AUDIOFOCUS_GAIN);
if (result != AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
// 不能獲得音頻焦點
}
requestAudioFocus()的第一個參數是一個AudioManager.OnAudioFocusChangeListener，它的onAudioFocusChange()方法在音頻焦點發改變時被調用.因此，你也應該在你的service和activity上實現此接口.例如：
[java]
class MyService extends Service
implements AudioManager.OnAudioFocusChangeListener {
// ....
public void onAudioFocusChange(int focusChange) {
// Do something based on focus change...
}
}
class MyService extends Service
implements AudioManager.OnAudioFocusChangeListener {
// ....
public void onAudioFocusChange(int focusChange) {
// Do something based on focus change...
}
}
參數focusChange告訴你音頻焦點如何發生了變化，它可以是以上幾種值(它們都是定義在AudioManager中的常量)：
AUDIOFOCUS_GAIN:你已獲得了音頻焦點.
AUDIOFOCUS_LOSS:你已經丟失了音頻焦點比較長的時間了.你必須停止所有的音頻播放.因為預料到你可能很長時間也不能再獲音頻焦點，所以這里是清理你的資源的好地方.比如，你必須釋放MediaPlayer.
AUDIOFOCUS_LOSS_TRANSIENT:你臨時性的丟掉了音頻焦點，很快就會重新獲得.你必須停止所有的音頻播放，但是可以保留你的資源，因為你可能很快就能重新獲得焦點.
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:你臨時性的丟掉了音頻焦點，但是你被允許繼續以低音量播放，而不是完全停止.
下面是一個例子：
[java]
public void onAudioFocusChange(int focusChange) {
switch (focusChange) {
case AudioManager.AUDIOFOCUS_GAIN:
// resume playback
if (mMediaPlayer == null) initMediaPlayer();
else if (!mMediaPlayer.isPlaying()) mMediaPlayer.start();
mMediaPlayer.setVolume(1.0f, 1.0f);
break;
case AudioManager.AUDIOFOCUS_LOSS:
// Lost focus for an unbounded amount of time: stop playback and release media player
if (mMediaPlayer.isPlaying()) mMediaPlayer.stop();
mMediaPlayer.release();
mMediaPlayer = null;
break;
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
// Lost focus for a short time, but we have to stop
// playback. We don't release the media player because playback
// is likely to resume
if (mMediaPlayer.isPlaying()) mMediaPlayer.pause();
break;
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
// Lost focus for a short time, but it's ok to keep playing
// at an attenuated level
if (mMediaPlayer.isPlaying()) mMediaPlayer.setVolume(0.1f, 0.1f);
break;
}
}
public void onAudioFocusChange(int focusChange) {
switch (focusChange) {
case AudioManager.AUDIOFOCUS_GAIN:
// resume playback
if (mMediaPlayer == null) initMediaPlayer();
else if (!mMediaPlayer.isPlaying()) mMediaPlayer.start();
mMediaPlayer.setVolume(1.0f, 1.0f);
break;
case AudioManager.AUDIOFOCUS_LOSS:
// Lost focus for an unbounded amount of time: stop playback and release media player
if (mMediaPlayer.isPlaying()) mMediaPlayer.stop();
mMediaPlayer.release();
mMediaPlayer = null;
break;
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
// Lost focus for a short time, but we have to stop
// playback. We don't release the media player because playback
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