蓝牙耳机简单的流程分析

最近在关注蓝牙耳机方面的问题，做下简单的流程分析。
解码后，在AudioFlinger里把音频数据写到设备里。这里主要看看AudioFlinger，AudioPolicyManager和external/bluetooth/bluez/audio里面的android_audio_hw.c和liba2dp.c。

在AudioPolicyManager里有设备连接判断。
status_t AudioPolicyManagerBase::setDeviceConnectionState(AudioSystem::audio_devices device,
                                                  AudioSystem::device_connection_state state,
                                                  const char *device_address)
{
#ifdef WITH_A2DP
            // handle A2DP device connection
            if (AudioSystem::isA2dpDevice(device)) {
                status_t status = handleA2dpConnection(device, device_address);//这里是执行蓝牙连接
                if (status != NO_ERROR) {
                    mAvailableOutputDevices &= ~device;
                    return status;
                }
            } else
#endif
}
。。。。。
status_t AudioPolicyManagerBase::handleA2dpConnection(AudioSystem::audio_devices device,
                                                 const char *device_address)
{
    ...............
       mA2dpOutput = mpClientInterface->openOutput(&outputDesc->mDevice,
                                            &outputDesc->mSamplingRate,
                                            &outputDesc->mFormat,
                                            &outputDesc->mChannels,
                                            &outputDesc->mLatency,
                                            outputDesc->mFlags);
    if (mA2dpOutput) {
        // add A2DP output descriptor
        addOutput(mA2dpOutput, outputDesc);

//TODO: configure audio effect output stage here

        // set initial stream volume for A2DP device
        applyStreamVolumes(mA2dpOutput, device);
        if (a2dpUsedForSonification()) {
            mDuplicatedOutput = mpClientInterface->openDuplicateOutput(mA2dpOutput, mHardwareOutput);
        }
        if (mDuplicatedOutput != 0 ||
            !a2dpUsedForSonification()) {
            // If both A2DP and duplicated outputs are open, send device address to A2DP hardware
            // interface
            AudioParameter param;
            param.add(String8("a2dp_sink_address"), String8(device_address));
            mpClientInterface->setParameters(mA2dpOutput, param.toString());
            mA2dpDeviceAddress = String8(device_address, MAX_DEVICE_ADDRESS_LEN);

            if (a2dpUsedForSonification()) {
                // add duplicated output descriptor
                AudioOutputDescriptor *dupOutputDesc = new AudioOutputDescriptor();
                dupOutputDesc->mOutput1 = mOutputs.valueFor(mHardwareOutput);
                dupOutputDesc->mOutput2 = mOutputs.valueFor(mA2dpOutput);
                dupOutputDesc->mSamplingRate = outputDesc->mSamplingRate;
                dupOutputDesc->mFormat = outputDesc->mFormat;
                dupOutputDesc->mChannels = outputDesc->mChannels;
                dupOutputDesc->mLatency = outputDesc->mLatency;
                addOutput(mDuplicatedOutput, dupOutputDesc);
                applyStreamVolumes(mDuplicatedOutput, device);
            }
        } else {
    .........
}
如果只是蓝牙播放，那么mDuplicatedOutput和a2dpUsedForSonification都为0，仅执行 addOutput(mA2dpOutput, outputDesc);，走类似speaker和麦克风的流程。
如果是蓝牙和speaker或麦克风同时播放声音，走mpClientInterface->openDuplicateOutput(mA2dpOutput, mHardwareOutput);注意

补充：移动开发 , Android ,