前言概念

WindowsCore的构造函数

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STA与MTA

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STA: Single-Thread Apartment， 中文叫单线程套间。就是在COM库初始化的时候创建一个内存结构，然后让它和调用CoInitialize的线程相关联。这个内存结构针对每个线程都会有一个。支持STA的COM对象只能在创建它的线程里被使用，其它线程如果再创建它就会失败。

MTA: Mutil-Thread Apartment，中文叫多线程套间。COM库在进程中创建一个内存结构，这个内存结构在整个进程中只能有一个，然后让它和调用CoInitializeEx的线程相关联。支持MTA的COM对象可以在任意线程里被使用。多有针对它的调用都会被封装成为消息。

其实STA和MTA是COM规定的一套线程模型，用于保障多线程情况下你的组件代码的同步。比如说有一个COM对象它内部有一个静态变量 gHello，那么这个对象无论生成多少实例对于gHello在内存中只能有一份，那么如果有两个不同的实例在两个线程里面同时去读写它，就有可能出错，所以就要就要有种机制进行同步保护，STA或者MTA就是这种机制。

AVRT库

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DMO

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源码分析

WebRtcVoiceEngine::Init

直接打断点
H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\media\engine\webrtc_voice_engine.cc
WebRtcVoiceEngine::Init
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调用堆栈

  1. cricket::ChannelManager::Init
  2. ->
  3. cricket::CompositeMediaEngine::Init()
  4. ->
  5. cricket::WebRtcVoiceEngine::Init()

AudioDeviceWindowsCore::AudioDeviceWindowsCore

打断点
H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\modules\audio_device\win\audio_device_core_win.cc 构造函数的398行
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调用堆栈
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  1. AudioDeviceWindowsCore::AudioDeviceWindowsCore()
  2.     : _avrtLibrary(NULL),
  3.       _winSupportAvrt(false),
  4.       _comInit(ScopedCOMInitializer::kMTA),
  5.       _ptrAudioBuffer(NULL),
  6.       _ptrEnumerator(NULL),
  7.       _ptrRenderCollection(NULL),
  8.       _ptrCaptureCollection(NULL),
  9.       _ptrDeviceOut(NULL),
  10.       _ptrDeviceIn(NULL),
  11.       _ptrClientOut(NULL),
  12.       _ptrClientIn(NULL),
  13.       _ptrRenderClient(NULL),
  14.       _ptrCaptureClient(NULL),
  15.       _ptrCaptureVolume(NULL),
  16.       _ptrRenderSimpleVolume(NULL),
  17.       _dmo(NULL),
  18.       _mediaBuffer(NULL),
  19.       _builtInAecEnabled(false),
  20.       _hRenderSamplesReadyEvent(NULL),
  21.       _hPlayThread(NULL),
  22.       _hRenderStartedEvent(NULL),
  23.       _hShutdownRenderEvent(NULL),
  24.       _hCaptureSamplesReadyEvent(NULL),
  25.       _hRecThread(NULL),
  26.       _hCaptureStartedEvent(NULL),
  27.       _hShutdownCaptureEvent(NULL),
  28.       _hMmTask(NULL),
  29.       _playAudioFrameSize(0),
  30.       _playSampleRate(0),
  31.       _playBlockSize(0),
  32.       _playChannels(2),
  33.       _sndCardPlayDelay(0),
  34.       _writtenSamples(0),
  35.       _readSamples(0),
  36.       _recAudioFrameSize(0),
  37.       _recSampleRate(0),
  38.       _recBlockSize(0),
  39.       _recChannels(2),
  40.       _initialized(false),
  41.       _recording(false),
  42.       _playing(false),
  43.       _recIsInitialized(false),
  44.       _playIsInitialized(false),
  45.       _speakerIsInitialized(false),
  46.       _microphoneIsInitialized(false),
  47.       _usingInputDeviceIndex(false),
  48.       _usingOutputDeviceIndex(false),
  49.       _inputDevice(AudioDeviceModule::kDefaultCommunicationDevice),
  50.       _outputDevice(AudioDeviceModule::kDefaultCommunicationDevice),
  51.       _inputDeviceIndex(0),
  52.       _outputDeviceIndex(0) {
  53.   RTC_DLOG(LS_INFO) << __FUNCTION__ << " created";
  54.   RTC_DCHECK(_comInit.Succeeded());
  56.   // Try to load the Avrt DLL
  57.   if (!_avrtLibrary) {
  58.     // Get handle to the Avrt DLL module.
  59.     _avrtLibrary = LoadLibrary(TEXT("Avrt.dll"));
  60.     if (_avrtLibrary) {
  61.       // Handle is valid (should only happen if OS larger than vista & win7).
  62.       // Try to get the function addresses.
  63.       RTC_LOG(LS_VERBOSE) << "AudioDeviceWindowsCore::AudioDeviceWindowsCore()"
  64.                              " The Avrt DLL module is now loaded";
  66.         // 获取三个方法
  67.       _PAvRevertMmThreadCharacteristics =
  68.           (PAvRevertMmThreadCharacteristics)GetProcAddress(
  69.               _avrtLibrary, "AvRevertMmThreadCharacteristics");
  70.       _PAvSetMmThreadCharacteristicsA =
  71.           (PAvSetMmThreadCharacteristicsA)GetProcAddress(
  72.               _avrtLibrary, "AvSetMmThreadCharacteristicsA");
  73.       _PAvSetMmThreadPriority = (PAvSetMmThreadPriority)GetProcAddress(
  74.           _avrtLibrary, "AvSetMmThreadPriority");
  76.       if (_PAvRevertMmThreadCharacteristics &&
  77.           _PAvSetMmThreadCharacteristicsA && _PAvSetMmThreadPriority) {
  78.         RTC_LOG(LS_VERBOSE)
  79.             << "AudioDeviceWindowsCore::AudioDeviceWindowsCore()"
  80.                " AvRevertMmThreadCharacteristics() is OK";
  81.         RTC_LOG(LS_VERBOSE)
  82.             << "AudioDeviceWindowsCore::AudioDeviceWindowsCore()"
  83.                " AvSetMmThreadCharacteristicsA() is OK";
  84.         RTC_LOG(LS_VERBOSE)
  85.             << "AudioDeviceWindowsCore::AudioDeviceWindowsCore()"
  86.                " AvSetMmThreadPriority() is OK";
  87.           // 设置标志
  88.         _winSupportAvrt = true;
  89.       }
  90.     }
  91.   }
  92.           // Create our samples ready events - we want auto reset events that start in
  93.   // the not-signaled state. The state of an auto-reset event object remains
  94.   // signaled until a single waiting thread is released, at which time the
  95.   // system automatically sets the state to nonsignaled. If no threads are
  96.   // waiting, the event object's state remains signaled. (Except for
  97.   // _hShutdownCaptureEvent, which is used to shutdown multiple threads).
  98.   _hRenderSamplesReadyEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
  99.   _hCaptureSamplesReadyEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
  100.   _hShutdownRenderEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
  101.   _hShutdownCaptureEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
  102.   _hRenderStartedEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
  103.   _hCaptureStartedEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
  105.   _perfCounterFreq.QuadPart = 1;
  106.   _perfCounterFactor = 0.0;
  108.   // list of number of channels to use on recording side
  109.   _recChannelsPrioList[0] = 2;  // stereo is prio 1
  110.   _recChannelsPrioList[1] = 1;  // mono is prio 2
  111.   _recChannelsPrioList[2] = 4;  // quad is prio 3
  113.   // list of number of channels to use on playout side
  114.   _playChannelsPrioList[0] = 2;  // stereo is prio 1
  115.   _playChannelsPrioList[1] = 1;  // mono is prio 2
  117.   HRESULT hr;
  119.   // We know that this API will work since it has already been verified in
  120.   // CoreAudioIsSupported, hence no need to check for errors here as well.
  122.   // Retrive the IMMDeviceEnumerator API (should load the MMDevAPI.dll)
  123.   // TODO(henrika): we should probably move this allocation to Init() instead
  124.   // and deallocate in Terminate() to make the implementation more symmetric.
  125.   CoCreateInstance(__uuidof(MMDeviceEnumerator), NULL, CLSCTX_ALL,
  126.                    __uuidof(IMMDeviceEnumerator),
  127.                    reinterpret_cast<void**>(&_ptrEnumerator));
  128.   assert(NULL != _ptrEnumerator);
  130.   // DMO初始化，主要用于回音消除，硬件层次。如果是软件层次，就是WebRTC的算法了。
  131.   // 苹果手机，硬件。。Windows一般使用软件。
  132.   // DMO initialization for built-in WASAPI AEC.
  133.   {
  134.     IMediaObject* ptrDMO = NULL;
  135.     hr = CoCreateInstance(CLSID_CWMAudioAEC, NULL, CLSCTX_INPROC_SERVER,
  136.                           IID_IMediaObject, reinterpret_cast<void**>(&ptrDMO));
  137.     if (FAILED(hr) || ptrDMO == NULL) {
  138.       // Since we check that _dmo is non-NULL in EnableBuiltInAEC(), the
  139.       // feature is prevented from being enabled.
  140.       _builtInAecEnabled = false;
  141.       _TraceCOMError(hr);
  142.     }
  143.     _dmo = ptrDMO;
  144.     SAFE_RELEASE(ptrDMO);
  145.   }
  146. }

其中ScopedCOMInitializer _comInit;
去到该类的定义去，可以发现，选择的是MTA 多线程套间。
class ScopedCOMInitializer {

public:
enum SelectMTA { kMTA };
explicit ScopedCOMInitializer(SelectMTA mta);

}

学习资料

STA和MTA线程模式的区别