关键类：

/packages/apps/Bluetooth/src/com/android/bluetooth/avrcpcontroller/AvrcpControllerStateMachine.java

/packages/apps/Bluetooth/src/com/android/bluetooth/avrcpcontroller/AvrcpControllerService.java

一、音乐播放状态

CPP中通过JNI接口将接从手机端接收来的播放状态回调到AvrcpControllerService的onPlayStatusChanged方法，然后通过状态机AvrcpControllerStateMachine处理。

// Called by JNI on changes of play statusprivate synchronized void onPlayStatusChanged(byte[] address, byte playStatus) {if (DBG) {Log.d(TAG, "onPlayStatusChanged " + playStatus);}int playbackState = PlaybackStateCompat.STATE_NONE;switch (playStatus) {case JNI_PLAY_STATUS_STOPPED:playbackState = PlaybackStateCompat.STATE_STOPPED;break;case JNI_PLAY_STATUS_PLAYING:playbackState = PlaybackStateCompat.STATE_PLAYING;break;case JNI_PLAY_STATUS_PAUSED:playbackState = PlaybackStateCompat.STATE_PAUSED;break;case JNI_PLAY_STATUS_FWD_SEEK:playbackState = PlaybackStateCompat.STATE_FAST_FORWARDING;break;case JNI_PLAY_STATUS_REV_SEEK:playbackState = PlaybackStateCompat.STATE_REWINDING;break;default:playbackState = PlaybackStateCompat.STATE_NONE;}BluetoothDevice device = mAdapter.getRemoteDevice(address);AvrcpControllerStateMachine stateMachine = getStateMachine(device);if (stateMachine != null) {stateMachine.sendMessage(AvrcpControllerStateMachine.MESSAGE_PROCESS_PLAY_STATUS_CHANGED, playbackState);}}

AvrcpControllerStateMachine.java中Connected内部类的processMessage方法中，调BluetoothMediaBrowserService类的方法将播放状态通知到UI应用，如果是播放状态，要申请音频焦点。

case MESSAGE_PROCESS_PLAY_STATUS_CHANGED:mAddressedPlayer.setPlayStatus(msg.arg1);if (!isActive()) {sendMessage(MSG_AVRCP_PASSTHRU,AvrcpControllerService.PASS_THRU_CMD_ID_PAUSE);return true;}PlaybackStateCompat playbackState = mAddressedPlayer.getPlaybackState();// 将播放状态通过mediasession通知给UIBluetoothMediaBrowserService.notifyChanged(playbackState);// 获取当前的音频焦点状态int focusState = AudioManager.ERROR;A2dpSinkService a2dpSinkService = A2dpSinkService.getA2dpSinkService();if (a2dpSinkService != null) {focusState = a2dpSinkService.getFocusState();}if (focusState == AudioManager.ERROR) {sendMessage(MSG_AVRCP_PASSTHRU,AvrcpControllerService.PASS_THRU_CMD_ID_PAUSE);return true;}if (playbackState.getState() == PlaybackStateCompat.STATE_PLAYING&& focusState == AudioManager.AUDIOFOCUS_NONE) {// 申请音频焦点if (shouldRequestFocus()) {requestAudioFocus();} else {sendMessage(MSG_AVRCP_PASSTHRU,AvrcpControllerService.PASS_THRU_CMD_ID_PAUSE);}}return true;

BluetoothMediaBrowserService是MediaSession框架的一个服务，UI端通过MediaSession框架绑定该服务就可以监听蓝牙音频的播放状态，控制蓝牙音频的播放。

BluetoothMediaBrowserService的notifyChanged方法中通过MediaSession框架将播放状态通知到UI端。

static synchronized void notifyChanged(PlaybackStateCompat playbackState) {Log.d(TAG, "notifyChanged PlaybackState" + playbackState);if (sBluetoothMediaBrowserService != null) {sBluetoothMediaBrowserService.mSession.setPlaybackState(playbackState);} else {Log.w(TAG, "notifyChanged Unavailable");}}

二、音频焦点申请

上面播放状态处理的逻辑中会调到A2dpSinkService的requestAudioFocus方法

 /*** Request audio focus such that the designated device can stream audio*/public void requestAudioFocus(BluetoothDevice device, boolean request) {synchronized (mStreamHandlerLock) {if (mA2dpSinkStreamHandler == null) return;mA2dpSinkStreamHandler.requestAudioFocus(request);}}

A2dpSinkStreamHandler

private synchronized int requestAudioFocus() {if (DBG) Log.d(TAG, "requestAudioFocus()");// Bluetooth A2DP may carry Music, Audio Books, Navigation, or other sounds so mark content// type unknown.AudioAttributes streamAttributes =new AudioAttributes.Builder().setUsage(AudioAttributes.USAGE_MEDIA).setContentType(AudioAttributes.CONTENT_TYPE_UNKNOWN).build();// Bluetooth ducking is handled at the native layer at the request of AudioManager.AudioFocusRequest focusRequest =new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN).setAudioAttributes(streamAttributes).setOnAudioFocusChangeListener(mAudioFocusListener, this).build();int focusRequestStatus = mAudioManager.requestAudioFocus(focusRequest);// If the request is granted begin streaming immediately and schedule an upgrade.if (focusRequestStatus == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {startFluorideStreaming();mAudioFocus = AudioManager.AUDIOFOCUS_GAIN;}return focusRequestStatus;}

三、播放控制

UI通过MediaSession框架绑定到BluetoothMediaBrowserService服务，BluetoothMediaBrowserService中的MediaSessionCompat对象是关键，MediaSessionCompat.CallBack是专门接收MediaSession客户端的指令并处理的。AvrcpControllerStateMachine的setActive方法中，会将MediaSessionCompat.CallBack对象传给BluetoothMediaBrowserService的MediaSessionCompat。

BluetoothMediaBrowserService.addressedPlayerChanged(mSessionCallbacks);

MediaSessionCompat.Callback mSessionCallbacks = new MediaSessionCompat.Callback() {@Overridepublic void onPlay() {logD("onPlay");requestAudioFocus();sendMessage(MSG_AVRCP_PASSTHRU, AvrcpControllerService.PASS_THRU_CMD_ID_PLAY);}@Overridepublic void onPause() {logD("onPause");sendMessage(MSG_AVRCP_PASSTHRU, AvrcpControllerService.PASS_THRU_CMD_ID_PAUSE);}};

public boolean processMessage(Message msg) {logD(STATE_TAG + " processMessage " + msg.what);switch (msg.what) {case MSG_AVRCP_PASSTHRU:passThru(msg.arg1);return true;}
}

而后调AvrcpControllerService类的sendPassThroughCommandNative方法将指令通过JNI接口发给CPP层。