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Android 系统播放声音，需要创建AudioTrack来和AudioFlinger通信，其创建过程如下

1. 根据传入的声音属性得到output
2. 通过得到的output，找到播放线程
3. AudioFlinger在播放线程内，创建Track，和AudioTrack对应。后续通过它们进行通信

接下来分别看下其过程

找到output
在getOutputForAttr函数内，直接调用getOutputForAttrInt函数

//frameworks/av/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
status_t AudioPolicyManager::getOutputForAttrInt(audio_attributes_t *resultAttr,audio_io_handle_t *output,audio_session_t session,const audio_attributes_t *attr,audio_stream_type_t *stream,uid_t uid,const audio_config_t *config,audio_output_flags_t *flags,audio_port_handle_t *selectedDeviceId,bool *isRequestedDeviceForExclusiveUse,std::vector<sp<AudioPolicyMix>> *secondaryMixes,output_type_t *outputType)
{DeviceVector outputDevices;const audio_port_handle_t requestedPortId = *selectedDeviceId;DeviceVector msdDevices = getMsdAudioOutDevices();const sp<DeviceDescriptor> requestedDevice =mAvailableOutputDevices.getDeviceFromId(requestedPortId);*outputType = API_OUTPUT_INVALID;status_t status = getAudioAttributes(resultAttr, attr, *stream);//将attr赋值给resultAttr//省略outputDevices = mEngine->getOutputDevicesForAttributes(*resultAttr, requestedDevice, false);//根据resultAttr，找到device//省略if (*output == AUDIO_IO_HANDLE_NONE) {*output = getOutputForDevices(outputDevices, session, *stream, config,flags, resultAttr->flags & AUDIO_FLAG_MUTE_HAPTIC);//根据device，找到output}//省略
}

1，找到device

//frameworks/av/services/audiopolicy/enginedefault/src/Engine.cpp
DeviceVector Engine::getOutputDevicesForAttributes(const audio_attributes_t &attributes,const sp<DeviceDescriptor> &preferredDevice,bool fromCache) const
{product_strategy_t strategy = getProductStrategyForAttributes(attributes);//省略return fromCache? mDevicesForStrategies.at(strategy) : getDevicesForProductStrategy(strategy);
}

首先，通过声音属性，从mProductStrategies中找到strategy，然后根据strategy，找到对应的device。getDevicesForProductStrategy这个函数由厂商自定义，修改声音的音频策略，大多数情况下可以修改这个函数。

2，根据device，找到output

//frameworks/av/services/audiopolicy/managerdefault/AudioPolicyManager.cpp
audio_io_handle_t AudioPolicyManager::getOutputForDevices(const DeviceVector &devices,audio_session_t session,audio_stream_type_t stream,const audio_config_t *config,audio_output_flags_t *flags,bool forceMutingHaptic)
{//省略if (audio_is_linear_pcm(config->format)) {SortedVector<audio_io_handle_t> outputs = getOutputsForDevices(devices, mOutputs);*flags = (audio_output_flags_t)(*flags & ~AUDIO_OUTPUT_FLAG_DIRECT);output = selectOutput(outputs, *flags, config->format, channelMask, config->sample_rate);}return output;}

mOutputs中，支持该device的output可能有多个，通过getOutputsForDevices把它们找出来，然后调用selectOutput从中选择一个最符合的。怎么从中找到最合适的output，可以看其注释

	// select one output among several that provide a path to a particular device or set of// devices (the list was previously build by getOutputsForDevices()).// The priority is as follows:// 1: the output supporting haptic playback when requesting haptic playback// 2: the output with the highest number of requested functional flags// 3: the output supporting the exact channel mask// 4: the output with a higher channel count than requested// 5: the output with a higher sampling rate than requested// 6: the output with the highest number of requested performance flags// 7: the output with the bit depth the closest to the requested one// 8: the primary output// 9: the first output in the list

综上，找到output的流程为： audio_attributes_t >>> strategy >>> device >>> output

找到播放线程

AudioFlinger::PlaybackThread *AudioFlinger::checkPlaybackThread_l(audio_io_handle_t output) const
{return mPlaybackThreads.valueFor(output).get();
}

根据output，从mPlaybackThreads中找到对应的播放线程

创建Track

//frameworks/av/services/audioflinger/Threads.cpp
sp<AudioFlinger::PlaybackThread::Track> AudioFlinger::PlaybackThread::createTrack_l(/*省略*/)
{//省略track = new Track(this, client, streamType, attr, sampleRate, format,channelMask, frameCount,nullptr /* buffer */, (size_t)0 /* bufferSize */, sharedBuffer,sessionId, creatorPid, uid, *flags, TrackBase::TYPE_DEFAULT, portId,SIZE_MAX /*frameCountToBeReady*/, opPackageName);//省略mTracks.add(track);//省略
}

AudioFlinger的PlaybackThread中有一个mTracks数组，先创建Track，然后将该Track保存到mTracks中。

总结
应用程序创建AudioTrack，导致在对应的播放线程内，创建Track和其对应。如何找到对应的播放线程？

1. 根据传入的声音属性，找到strategy
2. 根据strategy ，找到device
3. 根据device找到output
4. 根据output，从mPlaybackThreads中找到对应的播放线程