Android Audio 焦点管理详解
1.概述
- 音频焦点：Android 系统中用于管理多个应用同时播放音频的机制。
- 核心目标：确保用户在不同应用之间切换时，音频播放行为合理且有序。
2.音频焦点的基本概念
2.1 什么是音频焦点？
- 定义：音频焦点是系统为应用分配的一种资源，用于控制音频播放的优先级。
- 作用：避免多个应用同时播放音频，导致声音混乱。
2.2音频焦点的类型
| 焦点类型 | 使用场景 |
|---|---|
AUDIOFOCUS_NONE |
无焦点状态，通常用于初始化或未请求焦点时。 |
AUDIOFOCUS_GAIN |
长时间占用音频焦点，如音乐播放器、视频播放器等。 |
AUDIOFOCUS_GAIN_TRANSIENT |
短时获取焦点，失去焦点时暂停播放，如语音提示、通知音等。 |
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK |
短时获取焦点，失去焦点时降低音量，如导航提示音等。 |
AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE |
短时获取焦点，失去焦点时完全停止播放，如电话铃声、紧急提示音等。 |
3.获取与释放音频焦点
3.1获取音频焦点
-
方法 ：使用
AudioManager.requestAudioFocus。 -
示例代码 ：
javaAudioManager audioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE); int result = audioManager.requestAudioFocus( focusChangeListener, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN ); if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) { // 成功获取焦点 }
3.2释放音频焦点
- 方法 ：使用
AudioManager.abandonAudioFocus。 - 示例代码：
java
audioManager.abandonAudioFocus(focusChangeListener);4.处理音频焦点变化
4.1焦点变化事件
| 焦点类型 | 使用场景 |
|---|---|
AUDIOFOCUS_LOSS |
永久失去焦点，停止播放并释放资源。 |
AUDIOFOCUS_LOSS_TRANSIENT |
暂时失去焦点，暂停播放。 |
AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK |
暂时失去焦点，降低音量。 |
4.2实现 OnAudioFocusChangeListener
- 示例代码：
java
AudioManager.OnAudioFocusChangeListener focusChangeListener = new AudioManager.OnAudioFocusChangeListener() {
@Override
public void onAudioFocusChange(int focusChange) {
switch (focusChange) {
case AudioManager.AUDIOFOCUS_GAIN:
// 恢复播放
break;
case AudioManager.AUDIOFOCUS_LOSS:
// 停止播放
break;
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
// 暂停播放
break;
case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
// 降低音量
break;
}
}
};5.代码分析
5.1requestAudioFocus分析
AudioManager.java AudioServer.java MediaFocusControl.java FocusRequester.java
在AudioManager中做了如下工作：
- 检查传入的 AudioManager.OnAudioFocusChangeListener 是否为空。
- 检查音频流类型（如 AudioManager.STREAM_MUSIC）是否有效。
- 检查焦点请求类型（如 AUDIOFOCUS_GAIN）是否合法。
- 将传参打包成
AudioFocusRequest对象，并且生成FocusRequestInfo存储到mAudioFocusIdListenerMap中便于后续管理和触发焦点变化事件。 - 通过binder调用
AudioService.java接口requestAudioFocus
在AudioService.java中做了如下工作： - 初始化MediaMetrics,这说明这个方法有日志记录的功能，帮助开发者跟踪焦点请求的情况。
- 完成权限和参数校验
java
protected int requestAudioFocus(@NonNull AudioAttributes aa, int focusChangeHint, IBinder cb,
IAudioFocusDispatcher fd, @NonNull String clientId, @NonNull String callingPackageName,
int flags, int sdk, boolean forceDuck, int testUid) {
// we need a valid binder callback for clients
if (!cb.pingBinder()) {
Log.e(TAG, " AudioFocus DOA client for requestAudioFocus(), aborting.");
return AudioManager.AUDIOFOCUS_REQUEST_FAILED;
}
synchronized(mAudioFocusLock) {
if (mFocusStack.size() > MAX_STACK_SIZE) {
Log.e(TAG, "Max AudioFocus stack size reached, failing requestAudioFocus()");
return AudioManager.AUDIOFOCUS_REQUEST_FAILED;
}
boolean enteringRingOrCall = !mRingOrCallActive
& (AudioSystem.IN_VOICE_COMM_FOCUS_ID.compareTo(clientId) == 0);
if (enteringRingOrCall) { mRingOrCallActive = true; }
final AudioFocusInfo afiForExtPolicy;
if (mFocusPolicy != null) {
// construct AudioFocusInfo as it will be communicated to audio focus policy
afiForExtPolicy = new AudioFocusInfo(aa, uid,
clientId, callingPackageName, focusChangeHint, 0 /*lossReceived*/,
flags, sdk);
} else {
afiForExtPolicy = null;
}
// handle delayed focus
boolean focusGrantDelayed = false;
if (!canReassignAudioFocus()) {
if ((flags & AudioManager.AUDIOFOCUS_FLAG_DELAY_OK) == 0) {
return AudioManager.AUDIOFOCUS_REQUEST_FAILED;
} else {
// request has AUDIOFOCUS_FLAG_DELAY_OK: focus can't be
// granted right now, so the requester will be inserted in the focus stack
// to receive focus later
focusGrantDelayed = true;
}
}
// external focus policy?
if (mFocusPolicy != null) {
if (notifyExtFocusPolicyFocusRequest_syncAf(afiForExtPolicy, fd, cb)) {
// stop handling focus request here as it is handled by external audio
// focus policy (return code will be handled in AudioManager)
return AudioManager.AUDIOFOCUS_REQUEST_WAITING_FOR_EXT_POLICY;
} else {
// an error occured, client already dead, bail early
return AudioManager.AUDIOFOCUS_REQUEST_FAILED;
}
}
AudioFocusDeathHandler afdh = new AudioFocusDeathHandler(cb);
try {
cb.linkToDeath(afdh, 0);
} catch (RemoteException e) {
// client has already died!
Log.w(TAG, "AudioFocus requestAudioFocus() could not link to "+cb+" binder death");
return AudioManager.AUDIOFOCUS_REQUEST_FAILED;
}
if (!mFocusStack.empty() && mFocusStack.peek().hasSameClient(clientId)) {
// if focus is already owned by this client and the reason for acquiring the focus
// hasn't changed, don't do anything
final FocusRequester fr = mFocusStack.peek();
if (fr.getGainRequest() == focusChangeHint && fr.getGrantFlags() == flags) {
// unlink death handler so it can be gc'ed.
// linkToDeath() creates a JNI global reference preventing collection.
cb.unlinkToDeath(afdh, 0);
notifyExtPolicyFocusGrant_syncAf(fr.toAudioFocusInfo(),
AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED;
}
// the reason for the audio focus request has changed: remove the current top of
// stack and respond as if we had a new focus owner
if (!focusGrantDelayed) {
mFocusStack.pop();
// the entry that was "popped" is the same that was "peeked" above
fr.release();
}
}
// focus requester might already be somewhere below in the stack, remove it
removeFocusStackEntry(clientId, false /* signal */, false /*notifyFocusFollowers*/);
final FocusRequester nfr = new FocusRequester(aa, focusChangeHint, flags, fd, cb,
clientId, afdh, callingPackageName, uid, this, sdk);
if (mMultiAudioFocusEnabled
&& (focusChangeHint == AudioManager.AUDIOFOCUS_GAIN)) {
if (enteringRingOrCall) {
if (!mMultiAudioFocusList.isEmpty()) {
for (FocusRequester multifr : mMultiAudioFocusList) {
multifr.handleFocusLossFromGain(focusChangeHint, nfr, forceDuck);
}
}
} else {
boolean needAdd = true;
if (!mMultiAudioFocusList.isEmpty()) {
for (FocusRequester multifr : mMultiAudioFocusList) {
if (multifr.getClientUid() == Binder.getCallingUid()) {
needAdd = false;
break;
}
}
}
if (needAdd) {
mMultiAudioFocusList.add(nfr);
}
nfr.handleFocusGainFromRequest(AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
notifyExtPolicyFocusGrant_syncAf(nfr.toAudioFocusInfo(),
AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED;
}
}
if (focusGrantDelayed) {
//找到最后一个被锁定的焦点所有者（exclusive focus owner）的位置，然后将nfr插入到该位置之下
final int requestResult = pushBelowLockedFocusOwners(nfr);
if (requestResult != AudioManager.AUDIOFOCUS_REQUEST_FAILED) {
notifyExtPolicyFocusGrant_syncAf(nfr.toAudioFocusInfo(), requestResult);
}
return requestResult;
} else {
// propagate the focus change through the stack
propagateFocusLossFromGain_syncAf(focusChangeHint, nfr, forceDuck);
// push focus requester at the top of the audio focus stack
mFocusStack.push(nfr);
nfr.handleFocusGainFromRequest(AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
}
notifyExtPolicyFocusGrant_syncAf(nfr.toAudioFocusInfo(),
AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
if (ENFORCE_MUTING_FOR_RING_OR_CALL & enteringRingOrCall) {
runAudioCheckerForRingOrCallAsync(true/*enteringRingOrCall*/);
}
}//synchronized(mAudioFocusLock)
return AudioManager.AUDIOFOCUS_REQUEST_GRANTED;
}在MediaFocusControl.java中做了如下工作：
- 检查Binder的存活状态来确保客户端有效，如果客户端已经死亡，则直接返回失败
- 电话或铃声场景的特殊情况，设置相关标志位 enteringRingOrCall
- 构建AudioFocusInfo
- 当存在外部焦点策略（mFocusPolicy）时，方法将焦点请求转发给外部策略处理，并根据返回结果决定是否继续内部处理 比如CarAudio
- 注册死亡通知，确保在客户端死亡时能正确释放焦点，避免资源泄漏。
- 如果当前堆栈顶部的焦点请求者与当前客户端相同且参数未变，则通知外部焦点策略然后直接返回已授予状态，避免重复处理，
- 移除旧的焦点请求从堆栈中
- 生成FocusRequester对象
- 根据是否启用多音频焦点（mMultiAudioFocusEnabled）来处理不同的场景
- 对于延迟授予焦点的情况（AUDIOFOCUS_FLAG_DELAY_OK），方法将新请求插入堆栈中的适当位置，等待后续处理。否则，立即传播焦点变化，并更新焦点堆栈。
- 最后，方法处理进入电话或铃声场景时的音频检查，确保系统音频状态正确更新，并返回焦点请求的结果。
是 否 是 否 是 否 开始 日志记录 权限检查 外部策略? 外部处理 从堆栈移除焦点 多焦点场景? 多焦点处理 延迟焦点场景? 找到合适的堆栈位置插入 发送通知 堆栈更新 恢复音量
5.2.propagateFocusLossFromGain_syncAf分析
这个函数的作用就是给注册的焦点发送焦点loss的消息。
handleFocusLossFromGain dispatchAudioFocusChange MediaFocusControl.java FocusRequester.java AudioManager.java
直接来看FocusRequester.java中的代码
java
void handleFocusLossFromGain(int focusChange, FocusRequester requester, boolean forceDuck) {
if (mAudioFocusDispatcher != null) {
mAudioFocusDispatcher.dispatchAudioFocusChange(focusChange, requester);
} else {
dispatchAudioFocusChange(focusChange, requester);
}
}
- 1、2、3分别是A、B和C申请焦点，分别在栈中的位置以及其丢失焦点类型
3步骤C申请AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK时，会根据MediaFocusControl的ENFORCE_DUCKING 来决定谁负责降音，true由framework进行降音，false将会以AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK类型返回给B，由B自己负责降音 - 4步骤是再次申请焦点时，
1）如果申请者A再次申请，已经存在栈中但又不在栈顶，则先移除栈中的A，在从新push到栈顶
2）如果申请者C再次申请，已经存在且在栈顶，并且申请的焦点类型与之前相同就直接返回给C获得焦点；反之申请焦点类型不同，则会移除栈顶C，然后在重新push到栈顶 - 4步骤释放abandon焦点；何时释放焦点呢？
1）焦点使用者播放完音频资源后主动释放焦点
2）焦点使用者收到了LOSS_GAIN永久丢失焦点类型，主动释放焦点
释放规则如上图，自行查看
5.3frameworkHandleFocusLoss
frameworkHandleFocusLoss来负责framework的降音逻辑，如果返回值是true那么代表framework已经处理过了不需要通知app，如果是false那么就是需要通知app处理
注意
焦点切换发生在同一个应用内时，framework不会处理，而是直接返回false，由应用自己处理
框架处理焦点丢失的主要场景包括：
- 降低音量：当焦点丢失类型为 AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK 且满足特定条件时，由框架强制处理 ducking。
- 淡出播放：当焦点丢失类型为 AUDIOFOCUS_LOSS 且满足特定条件时，由框架处理淡出逻辑。
其他情况：通常由应用自己处理焦点丢失事件。
5.4 dispatchAudioFocusChange
在AudioManager.java中dispatchAudioFocusChange会从mAudioFocusIdListenerMap找到指定的app,然后把焦点改变的消息通知给app。app在自身做相应的处理。
6.总结
音频焦点框架主要就是对各个APP的焦点进行注册、通知、释放。焦点释放可能会在framework完成那么app也就收不到相应的焦点通知了。