android媒体焦点音量压低/暂停逻辑源码简析

客户端通过AudioManager的调用requestAudioFocus请求焦点的过程中，其他媒体客户端会因媒体焦点丢失进行媒体压低或暂停，下面从framework层简单分析其过程。
首先调用AudioManager的requestAudioFocus方法：

    //AudioManager.java@SystemApi@RequiresPermission(android.Manifest.permission.MODIFY_AUDIO_ROUTING)public int requestAudioFocus(@NonNull AudioFocusRequest afr, @Nullable AudioPolicy ap) {...registerAudioFocusRequest(afr);final IAudioService service = getService();final int status;...synchronized (mFocusRequestsLock) {try {// TODO status contains result and generation counter for ext policystatus = service.requestAudioFocus(afr.getAudioAttributes(),afr.getFocusGain(), mICallBack,mAudioFocusDispatcher,clientId,getContext().getOpPackageName() /* package name */, afr.getFlags(),ap != null ? ap.cb() : null,sdk);} catch (RemoteException e) {throw e.rethrowFromSystemServer();}...}return focusReceiver.requestResult();}

AudioManager与系统服务AudioService通信，调用AudioService的requestAudioFocus：

    //AudioService.javapublic int requestAudioFocus(AudioAttributes aa, int durationHint, IBinder cb,IAudioFocusDispatcher fd, String clientId, String callingPackageName, int flags,IAudioPolicyCallback pcb, int sdk) {...return mMediaFocusControl.requestAudioFocus(aa, durationHint, cb, fd,clientId, callingPackageName, flags, sdk,forceFocusDuckingForAccessibility(aa, durationHint, Binder.getCallingUid()));}

接着调用MediaFocusControl的方法：

  //MediaFocusControl.javaprotected int requestAudioFocus(AudioAttributes aa, int focusChangeHint, IBinder cb,IAudioFocusDispatcher fd, String clientId, String callingPackageName, int flags,int sdk, boolean forceDuck) {...// propagate the focus change through the stackif (!mFocusStack.empty()) {propagateFocusLossFromGain_syncAf(focusChangeHint, nfr, forceDuck);}...}

MediaFocusControl通过propagateFocusLossFromGain_syncAf方法同步处理焦点丢失：

    @GuardedBy("mAudioFocusLock")private void propagateFocusLossFromGain_syncAf(int focusGain, final FocusRequester fr,boolean forceDuck) {final List<String> clientsToRemove = new LinkedList<String>();// going through the audio focus stack to signal new focus, traversing order doesn't// matter as all entries respond to the same external focus gainfor (FocusRequester focusLoser : mFocusStack) {final boolean isDefinitiveLoss =focusLoser.handleFocusLossFromGain(focusGain, fr, forceDuck);if (isDefinitiveLoss) {clientsToRemove.add(focusLoser.getClientId());}}for (String clientToRemove : clientsToRemove) {removeFocusStackEntry(clientToRemove, false /*signal*/,true /*notifyFocusFollowers*/);}}

接着调用FocusRequester的handleFocusLossFromGain方法再到handleFocusLoss方法

    //FocusRequester.java@GuardedBy("MediaFocusControl.mAudioFocusLock")void handleFocusLoss(int focusLoss, @Nullable final FocusRequester frWinner, boolean forceDuck){try {if (focusLoss != mFocusLossReceived) {...handled = mFocusController.duckPlayers(frWinner, this, forceDuck);...}

其中mFocusController即MediaFocusControl
MediaFocusControl的requestAudioFocus方法中创建FocusRequester传入

            final FocusRequester nfr = new FocusRequester(aa, focusChangeHint, flags, fd, cb,clientId, afdh, callingPackageName, Binder.getCallingUid(), this, sdk);

其中this即MediaFocusControl对象。

回到MediaFocusControl的duckPlayers方法：

    //MediaFocusControl.java@Overridepublic boolean duckPlayers(FocusRequester winner, FocusRequester loser, boolean forceDuck) {return mFocusEnforcer.duckPlayers(winner, loser, forceDuck);}

其中mFocusEnforcer即PlaybackActivityMonitor，在创建MediaFocusControl对象时传入。

    private final PlaybackActivityMonitor mPlaybackMonitor;mMediaFocusControl = new MediaFocusControl(mContext, mPlaybackMonitor);

来到PlaybackActivityMonitor执行duckPlayers方法，通过apcsToDuck.add(apc)添加到压低媒体apc（AudioPlaybackConfiguration）列表。

    //PlaybackActivityMonitor.java@Overridepublic boolean duckPlayers(FocusRequester winner, FocusRequester loser, boolean forceDuck) {...apcsToDuck.add(apc);...mDuckingManager.duckUid(loser.getClientUid(), apcsToDuck);}return true;}

其中mDuckingManager.duckUid(loser.getClientUid(), apcsToDuck)为内部类DuckingManager根据mDuckers获取内部类DuckedApp，遍历apcsToDuck去addDuck音量压低逻辑。

    //PlaybackActivityMonitor.javaprivate static final class DuckingManager {private final HashMap<Integer, DuckedApp> mDuckers = new HashMap<Integer, DuckedApp>();synchronized void duckUid(int uid, ArrayList<AudioPlaybackConfiguration> apcsToDuck) {if (DEBUG) {  Log.v(TAG, "DuckingManager: duckUid() uid:"+ uid); }if (!mDuckers.containsKey(uid)) {mDuckers.put(uid, new DuckedApp(uid));}final DuckedApp da = mDuckers.get(uid);for (AudioPlaybackConfiguration apc : apcsToDuck) {da.addDuck(apc, false /*skipRamp*/);}}

内部类DuckedApp，遍历apcsToDuck去addDuck音量压低逻辑:

    //PlaybackActivityMonitor.javaprivate static final class DuckedApp {private final int mUid;private final ArrayList<Integer> mDuckedPlayers = new ArrayList<Integer>();DuckedApp(int uid) {mUid = uid;}...void addDuck(@NonNull AudioPlaybackConfiguration apc, boolean skipRamp) {...try {apc.getPlayerProxy().applyVolumeShaper(FLY_DUCK_VSHAPE,VolumeShaper.Operation.PLAY);mDuckedPlayers.add(piid);} catch (Exception e) {Log.e(TAG, "Error ducking player piid:" + piid + " uid:" + mUid, e);}}

DuckedApp 中执行apc.getPlayerProxy().applyVolumeShaper进行音量压低，apc即AudioPlaybackConfiguration 。
存在AudioPlaybackConfiguration 的Map，在trackPlayer时添加：

    //PlaybackActivityMonitor.java private final HashMap<Integer, AudioPlaybackConfiguration> mPlayers =new HashMap<Integer, AudioPlaybackConfiguration>();public int trackPlayer(PlayerBase.PlayerIdCard pic) {final AudioPlaybackConfiguration apc =new AudioPlaybackConfiguration(pic, newPiid,Binder.getCallingUid(), Binder.getCallingPid());apc.init();synchronized(mPlayerLock) {mPlayers.put(newPiid, apc);}return newPiid;}

trackPlayer方法在PlayerBase的baseRegisterPlayer方法中调用。

   //PlayerBase.java/*** Call from derived class when instantiation / initialization is successful*/protected void baseRegisterPlayer() {...try {newPiid = getService().trackPlayer(new PlayerIdCard(mImplType, mAttributes, new IPlayerWrapper(this)));} catch (RemoteException e) {Log.e(TAG, "Error talking to audio service, player will not be tracked", e);}mPlayerIId = newPiid;}

AudioPlaybackConfiguration中getPlayerProxy获取IPlayerShell的getIPlayer()得到的是IPlayer实例,内部类IPlayerShell获取的IPlayer即为PlayerBase.PlayerIdCard pic的参数mIPlayer。

回到AudioService中通过PlayerBase的方法baseRegisterPlayer调用getService().trackPlayer( new PlayerIdCard(mImplType, mAttributes, new IPlayerWrapper(this)))
IPlayerWrapper即为IPlayer的实现类执行applyVolumeShaper调用PlayerBase实现类的playerApplyVolumeShaper进行压低
MediaPlayer、SoundPool、AudioTrack实现了PlayerBase, 构造函数中调用baseRegisterPlayer进行媒体注册，方法playerApplyVolumeShaper调用native_applyVolumeShaper进行压低。

VolumeShaper实现音频音量控制，调用mediaPlayer的playerApplyVolumeShaper方法。

以上为媒体焦点音量压低的简单逻辑分析。

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android媒体焦点音量压低/暂停逻辑源码简析

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