Android 实现了对Headset 和Handsfree 两种profile 的支持。其实现核心是BluetoothHeadsetService，在PhoneApp 创建的时候会启动它。
if (getSystemService(Context.BLUETOOTH_SERVICE) != null) { mBtHandsfree = new BluetoothHandsfree(this, phone); startService(new Intent(this, BluetoothHeadsetService.class)); } else { // Device is not bluetooth capable mBtHandsfree = null; }BluetoothHeadsetService 通过接收ENABLED_ACTION、BONDING_CREATED_ACTION 、DISABLED_ACTION 和REMOTE_DEVICE_DISCONNECT_REQUESTEDACTION 来改变状态，它也会监听Phone 的状态变化。
IntentFilter filter = new IntentFilter(BluetoothIntent.BONDING_CREATED_ACTION);filter.addAction(BluetoothIntent.REMOTE_DEVICE_DISCONNECT_REQUESTED_ACTION);filter.addAction(BluetoothIntent.ENABLED_ACTION);filter.addAction(BluetoothIntent.DISABLED_ACTION);registerReceiver(mBluetoothIntentReceiver, filter);mPhone.registerForPhoneStateChanged(mStateChangeHandler,PHONE_STATE_CHANGED, null);BluetoothHeadsetService 收到ENABLED_ACTION时，会先向BlueZ注册Headset 和Handsfree 两种profile(通过执行sdptool 来实现的，均作为Audio Gateway)，然后让BluetoothAudioGateway 接收RFCOMM 连接，让BluetoothHandsfree 接收SCO连接(这些操作都是为了让蓝牙耳机能主动连上Android)。
if (action.equals(BluetoothIntent.ENABLED_ACTION)) { // SDP server may not be ready, so wait 3 seconds before // registering records. // TODO: Use a different mechanism to register SDP records, // that actually ACK’s on success, so that we can retry rather // than hardcoding a 3 second guess. mHandler.sendMessageDelayed(mHandler.obtainMessage(REGISTER_SDP_RECORDS),3000); mAg.start(mIncomingConnectionHandler); mBtHandsfree.onBluetoothEnabled(); } BluetoothHeadsetService 收到DISABLED_ACTION 时，会停止BluetoothAudioGateway 和BluetoothHandsfree。
if (action.equals(BluetoothIntent.DISABLED_ACTION)) { mBtHandsfree.onBluetoothDisabled(); mAg.stop(); }
Android 跟蓝牙耳机建立连接有两种方式。
1. Android 主动跟蓝牙耳机连BluetoothSettings 中和蓝牙耳机配对上之后， BluetoothHeadsetService 会收到BONDING_CREATED_ACTION，这个时候BluetoothHeadsetService 会主动去和蓝牙耳机建立RFCOMM 连接。
if (action.equals(BluetoothIntent.BONDING_CREATED_ACTION)) { if (mState == BluetoothHeadset.STATE_DISCONNECTED) { // Lets try and initiate an RFCOMM connection try { mBinder.connectHeadset(address, null); } catch (RemoteException e) {} } }RFCOMM 连接的真正实现是在ConnectionThread 中，它分两步，第一步先通过SDPClient 查询蓝牙设备时候支持Headset 和Handsfree profile。
// 1) SDP query SDPClient client = SDPClient.getSDPClient(address); if (DBG) log(”Connecting to SDP server (” + address + “)…”); if (!client.connectSDPAsync()) { Log.e(TAG, “Failed to start SDP connection to ” + address); mConnectingStatusHandler.obtainMessage(SDP_ERROR).sendToTarget(); client.disconnectSDP(); return; } if (isInterrupted()) { client.disconnectSDP(); return; }
if (!client.waitForSDPAsyncConnect(20000)) { // 20 secs if (DBG) log(”Failed to make SDP connection to ” + address); mConnectingStatusHandler.obtainMessage(SDP_ERROR).sendToTarget(); client.disconnectSDP(); return; }
if (DBG) log(”SDP server connected (” + address + “)”); int headsetChannel = client.isHeadset(); if (DBG) log(”headset channel = ” + headsetChannel); int handsfreeChannel = client.isHandsfree(); if (DBG) log(”handsfree channel = ” + handsfreeChannel); client.disconnectSDP(); 第2步才是去真正建立RFCOMM 连接。// 2) RFCOMM connect
mHeadset = new HeadsetBase(mBluetooth, address, channel); if (isInterrupted()) { return; } int result = mHeadset.waitForAsyncConnect(20000, // 20 secs mConnectedStatusHandler); if (DBG) log(”Headset RFCOMM connection attempt took ” +(System.currentTimeMillis() – timestamp) + ” ms”); if (isInterrupted()) { return; } if (result < 0) { Log.e(TAG, “mHeadset.waitForAsyncConnect() error: ” + result); mConnectingStatusHandler.obtainMessage(RFCOMM_ERROR).sendToTarget(); return; } else if (result == 0) { Log.e(TAG, “mHeadset.waitForAsyncConnect() error: ” + result +”(timeout)”); mConnectingStatusHandler.obtainMessage(RFCOMM_ERROR).sendToTarget(); return; } else { if (DBG) log(”mHeadset.waitForAsyncConnect() success”); mConnectingStatusHandler.obtainMessage(RFCOMM_CONNECTED).sendToTarget(); }当RFCOMM连接成功建立后，BluetoothHeadsetDevice 会收到RFCOMM_CONNECTED消息，它会调用BluetoothHandsfree 来建立SCO 连接，广播通知Headset状态变化的Intent(PhoneApp 和BluetoothSettings 会接收这个Intent)。 case RFCOMM_CONNECTED: // success if (DBG) log(”Rfcomm connected”); if (mConnectThread != null) { try { mConnectThread.join(); } catch (InterruptedException e) { Log.w(TAG, “Connect attempt cancelled, ignoring RFCOMM_CONNECTED”, e); return; } mConnectThread = null; } setState(BluetoothHeadset.STATE_CONNECTED,BluetoothHeadset.RESULT_SUCCESS); mBtHandsfree.connectHeadset(mHeadset, mHeadsetType); break;
BluetoothHandsfree 会先做一些初始化工作，比如根据是Headset 还是Handsfree 初始化不同的ATParser，并且启动一个接收线程从已建立的RFCOMM上接收蓝牙耳机过来的控制命令(也就是AT 命令)，接着判断如果是在打电话过程中，才去建立SCO 连接来打通数据通道。
void connectHeadset(HeadsetBase headset, int headsetType) { mHeadset = headset; mHeadsetType = headsetType; if (mHeadsetType == TYPE_HEADSET) { initializeHeadsetAtParser(); } else { initializeHandsfreeAtParser(); } headset.startEventThread(); configAudioParameters(); if (inDebug()) { startDebug(); } if (isIncallAudio()) { audioOn(); } }
建立SCO 连接是通过SCOSocket 实现的
synchronized boolean audioOn() { mOutgoingSco = createScoSocket(); if (!mOutgoingSco.connect(mHeadset.getAddress())) { mOutgoingSco = null; } return true; }当SCO 连接成功建立后，BluetoothHandsfree 会收到SCO_CONNECTED 消息，它就会去调用AudioManager 的setBluetoothScoOn函数，从而通知音频系统有个蓝牙耳机可用了。到此，Android 完成了和蓝牙耳机的全部连接。
case SCO_CONNECTED: if (msg.arg1 == ScoSocket.STATE_CONNECTED && isHeadsetConnected()&&mConnectedSco == null) { if (DBG) log(”Routing audio for outgoing SCO conection”); mConnectedSco = (ScoSocket)msg.obj; mAudioManager.setBluetoothScoOn(true); } else if (msg.arg1 == ScoSocket.STATE_CONNECTED) { if (DBG) log(”Rejecting new connected outgoing SCO socket”); ((ScoSocket)msg.obj).close(); mOutgoingSco.close(); } mOutgoingSco = null; break;
2. 蓝牙耳机主动跟Android 连首先BluetoothAudioGateway 会在一个线程中收到来自蓝牙耳机的RFCOMM 连接，然后发送消息给BluetoothHeadsetService。
mConnectingHeadsetRfcommChannel = -1; mConnectingHandsfreeRfcommChannel = -1; if(waitForHandsfreeConnectNative(SELECT_WAIT_TIMEOUT) == false) { if (mTimeoutRemainingMs > 0) { try { Log.i(tag, “select thread timed out, but ” + mTimeoutRemainingMs + “ms of waiting remain.”); Thread.sleep(mTimeoutRemainingMs); } catch (InterruptedException e) { Log.i(tag, “select thread was interrupted (2), exiting”); mInterrupted = true; } } } BluetoothHeadsetService 会根据当前的状态来处理消息，分3 种情况，第一是当前状态是非连接状态，会发送RFCOMM_CONNECTED 消息，后续处理请参见前面的分析。 case BluetoothHeadset.STATE_DISCONNECTED: // headset connecting us, lets join setState(BluetoothHeadset.STATE_CONNECTING); mHeadsetAddress = info.mAddress; mHeadset = new HeadsetBase(mBluetooth, mHeadsetAddress,info.mSocketFd,info.mRfcommChan,mConnectedStatusHandler); mHeadsetType = type; mConnectingStatusHandler.obtainMessage(RFCOMM_CONNECTED).sendToTarget(); break;如果当前是正在连接状态， 则先停掉已经存在的ConnectThread，并直接调用BluetoothHandsfree 去建立SCO 连接。 case BluetoothHeadset.STATE_CONNECTING: // If we are here, we are in danger of a race condition // incoming rfcomm connection, but we are also attempting an // outgoing connection. Lets try and interrupt the outgoing // connection. mConnectThread.interrupt(); // Now continue with new connection, including calling callback mHeadset = new HeadsetBase(mBluetooth,mHeadsetAddress,info.mSocketFd,info.mRfcommChan,mConnectedStatusHandler); mHeadsetType = type; setState(BluetoothHeadset.STATE_CONNECTED,BluetoothHeadset.RESULT_SUCCESS); mBtHandsfree.connectHeadset(mHeadset,mHeadsetType); // Make sure that old outgoing connect thread is dead. break; 如果当前是已连接的状态，这种情况是一种错误case，所以直接断掉所有连接。 case BluetoothHeadset.STATE_CONNECTED: if (DBG) log(”Already connected to ” + mHeadsetAddress + “,disconnecting” +info.mAddress); mBluetooth.disconnectRemoteDeviceAcl(info.mAddress); break;蓝牙耳机也可能会主动发起SCO 连接， BluetoothHandsfree 会接收到一个SCO_ACCEPTED消息，它会去调用AudioManager 的setBluetoothScoOn 函数，从而通知音频系统有个蓝牙耳机可用了。到此，蓝牙耳机完成了和Android 的全部连接。
case SCO_ACCEPTED: if (msg.arg1 == ScoSocket.STATE_CONNECTED) { if (isHeadsetConnected() && mAudioPossible && mConnectedSco ==null) { Log.i(TAG, “Routing audio for incoming SCO connection”); mConnectedSco = (ScoSocket)msg.obj; mAudioManager.setBluetoothScoOn(true); } else { Log.i(TAG, “Rejecting incoming SCO connection”); ((ScoSocket)msg.obj).close(); } } // else error trying to accept, try again mIncomingSco = createScoSocket(); mIncomingSco.accept(); break;