android 音頻處理mediaplay介紹
處理音頻焦點
盡管某個時刻只有一個activity可以運行，Android卻是一個多任務(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(nèi)建的機(jī)制來處理這個問題，所以可能在某些情況下導(dǎo)致壞的用戶體驗.例如，當(dāng)一個用戶正在聽音樂而另一個應(yīng)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(yīng)用提供了一個協(xié)商它們?nèi)绾问褂迷O(shè)備音頻輸出的途徑，這個機(jī)制叫做音頻焦點.
當(dāng)你的應(yīng)用需要輸出像樂音和通知之類的音頻時，你應(yīng)該總是請求音頻焦點.一旦應(yīng)用具有了焦點，它就可以自由的使用音頻輸出.但它總是應(yīng)該監(jiān)聽焦點的變化.如果被通知丟失焦點，它應(yīng)該立即殺死聲音或降低到靜音水平(有一個標(biāo)志表明應(yīng)選擇哪一個)并且僅當(dāng)重新獲得焦點后才恢復(fù)大聲播放.
將來的音頻焦點是合作的.所以，應(yīng)用被希望(并被強(qiáng)列鼓勵)遵守音頻焦點的方針，但是卻不是被系統(tǒng)強(qiáng)制的.如果一個應(yīng)用在丟失音頻焦點后依然想大聲播放音樂，系統(tǒng)不會去阻止它.然而用戶卻體驗很壞并且很想把這鳥應(yīng)用卸載.
要請求音頻焦點，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
}
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(shù)是一個AudioManager.OnAudioFocusChangeListener，它的onAudioFocusChange()方法在音頻焦點發(fā)改變時被調(diào)用.因此，你也應(yīng)該在你的service和activity上實現(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.
AUDIOFOCUS_LOSS:你已經(jīng)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(yù)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(xù)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
[!--empirenews.page--]// 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;
}
}
記住音頻焦點API僅在APIlevel 8 (Android2.2)及更高版本上可以，所以如果你想支持更早的Android版本，你必須在可能時采取兼容性的策略使用特性，如果不可能,you should adopt a backward compatibility strategy that allows you touse this feature if available, and fall back seamlessly if not.
你可以用反射的方式調(diào)用音頻焦點方法或自己在一個單獨的類(叫做AudioFocusHelper)中實現(xiàn)所有的音頻焦點功能來達(dá)到向前兼容.下面是一個這樣的類：
[java]
public class AudioFocusHelper implements AudioManager.OnAudioFocusChangeListener {
AudioManager mAudioManager;
// 這里是其它的字段，你可能要保存一個接口的引用，這個接口
// 被用于與你的service通訊以報告焦點的變化.
public AudioFocusHelper(Context ctx, ) {
mAudioManager = (AudioManager) mContext.getSystemService(Context.AUDIO_SERVICE);
// ...
}
public boolean requestFocus() {
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED ==
mAudioManager.requestAudioFocus(mContext, AudioManager.STREAM_MUSIC,
AudioManager.AUDIOFOCUS_GAIN);
}
public boolean abandonFocus() {
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED ==
mAudioManager.abandonAudioFocus(this);
}
@Override
public void onAudioFocusChange(int focusChange) {
// 讓你的service知道焦點變化了
}
}
你可以僅在檢測到系統(tǒng)運行的是API level 8 或更早的版本時才創(chuàng)建AudioFocusHelper類的實例.例如：
if (android.os.Build.VERSION.SDK_INT >= 8) {
mAudioFocusHelper = new AudioFocusHelper(getApplicationContext(), this);
} else {
mAudioFocusHelper = null;
}
public class AudioFocusHelper implements AudioManager.OnAudioFocusChangeListener {
AudioManager mAudioManager;
// 這里是其它的字段，你可能要保存一個接口的引用，這個接口
// 被用于與你的service通訊以報告焦點的變化.
public AudioFocusHelper(Context ctx, ) {
mAudioManager = (AudioManager) mContext.getSystemService(Context.AUDIO_SERVICE);
// ...
}
public boolean requestFocus() {
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED ==
mAudioManager.requestAudioFocus(mContext, AudioManager.STREAM_MUSIC,
AudioManager.AUDIOFOCUS_GAIN);
}
public boolean abandonFocus() {
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED ==
mAudioManager.abandonAudioFocus(this);
}
@Override
public void onAudioFocusChange(int focusChange) {
// 讓你的service知道焦點變化了
}
}
你可以僅在檢測到系統(tǒng)運行的是API level 8 或更早的版本時才創(chuàng)建AudioFocusHelper類的實例.例如：
if (android.os.Build.VERSION.SDK_INT >= 8) {
mAudioFocusHelper = new AudioFocusHelper(getApplicationContext(), this);
} else {
mAudioFocusHelper = null;
}
清理
前面提到過，一個MediaPlayer對象可以消耗掉大量的系統(tǒng)資源，所以你應(yīng)該僅在需要它時保持它并在用完時立即釋放.明確的調(diào)用清理方法而不是依靠系統(tǒng)的垃圾收集機(jī)制是很重要的，因為在被收集之前MediaPlayer可能會存在很長時間，雖然此時它只是占用內(nèi)存而不影響其它的媒體相關(guān)的資源.所以，當(dāng)你使用一個service時，你應(yīng)該總四重寫onDestroy()方法來保證釋放MediaPlayer：
[java]
public class MyService extends Service {
MediaPlayer mMediaPlayer;
// ...
@Override
public void onDestroy() {
if (mMediaPlayer != null) mMediaPlayer.release();
}
}
public class MyService extends Service {
MediaPlayer mMediaPlayer;
// ...
@Override
public void onDestroy() {
if (mMediaPlayer != null) mMediaPlayer.release();
}www.2cto.com
}
你也應(yīng)該尋找其它需要釋放你的MediaPlayer的時機(jī).例如，如果你預(yù)料到長時間不能播放媒體(比如丟掉音頻焦點以后)，你應(yīng)該明確地釋放你的MediaPlayer，然后在后面重新創(chuàng)建它.反過來，如果你預(yù)測到只會短時間停止播放，你應(yīng)該保持你的MediaPlayer來避免過多的創(chuàng)建，而不是重新"準(zhǔn)備"它.
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