android 音頻處理mediaplay介紹
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處理音頻焦點(diǎn)
本文引用地址：http://www.ex-cimer.com/article/201609/305045.htm盡管某個(gè)時(shí)刻只有一個(gè)activity可以運(yùn)行，Android卻是一個(gè)多任務(wù)環(huán)境.這對使用音頻的應(yīng)用帶來了特殊的挑戰(zhàn)，因?yàn)橹挥幸粋€(gè)音頻輸出而可能多個(gè)媒體都想用它.在Android2.2之前，沒有內(nèi)建的機(jī)制來處理這個(gè)問題，所以可能在某些情況下導(dǎo)致壞的用戶體驗(yàn).例如，當(dāng)一個(gè)用戶正在聽音樂而另一個(gè)應(yīng)用需要通知用戶一些重要的事情時(shí)，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(tái)為應(yīng)用提供了一個(gè)協(xié)商它們?nèi)绾问褂迷O(shè)備音頻輸出的途徑，這個(gè)機(jī)制叫做音頻焦點(diǎn).
當(dāng)你的應(yīng)用需要輸出像樂音和通知之類的音頻時(shí)，你應(yīng)該總是請求音頻焦點(diǎn).一旦應(yīng)用具有了焦點(diǎn)，它就可以自由的使用音頻輸出.但它總是應(yīng)該監(jiān)聽焦點(diǎn)的變化.如果被通知丟失焦點(diǎn)，它應(yīng)該立即殺死聲音或降低到靜音水平(有一個(gè)標(biāo)志表明應(yīng)選擇哪一個(gè))并且僅當(dāng)重新獲得焦點(diǎn)后才恢復(fù)大聲播放.
將來的音頻焦點(diǎn)是合作的.所以，應(yīng)用被希望(并被強(qiáng)列鼓勵(lì))遵守音頻焦點(diǎn)的方針，但是卻不是被系統(tǒng)強(qiáng)制的.如果一個(gè)應(yīng)用在丟失音頻焦點(diǎn)后依然想大聲播放音樂，系統(tǒng)不會(huì)去阻止它.然而用戶卻體驗(yàn)很壞并且很想把這鳥應(yīng)用卸載.
要請求音頻焦點(diǎn)，你必須從AudioManager調(diào)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(diǎn)
}
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(diǎn)
}
requestAudioFocus()的第一個(gè)參數(shù)是一個(gè)AudioManager.OnAudioFocusChangeListener，它的onAudioFocusChange()方法在音頻焦點(diǎn)發(fā)改變時(shí)被調(diào)用.因此，你也應(yīng)該在你的service和activity上實(shí)現(xiàn)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(shù)focusChange告訴你音頻焦點(diǎn)如何發(fā)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(diǎn).
AUDIOFOCUS_LOSS:你已經(jīng)丟失了音頻焦點(diǎn)比較長的時(shí)間了.你必須停止所有的音頻播放.因?yàn)轭A(yù)料到你可能很長時(shí)間也不能再獲音頻焦點(diǎn)，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(shí)性的丟掉了音頻焦點(diǎn)，很快就會(huì)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?yàn)槟憧赡芎芸炀湍苤匦芦@得焦點(diǎn).
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:你臨時(shí)性的丟掉了音頻焦點(diǎn)，但是你被允許繼續(xù)以低音量播放，而不是完全停止.
下面是一個(gè)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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