前言

代码分析

VideoCaptureDS::Init

int32_t VideoCaptureDS::Init(const char* deviceUniqueIdUTF8) {
 ****
  _outputCapturePin = GetOutputPin(_captureFilter, PIN_CATEGORY_CAPTURE);
  if (!_outputCapturePin) {
    RTC_LOG(LS_INFO) << "Failed to get output capture pin";
    return -1;
  }
  // Create the sink filte used for receiving Captured frames.
  sink_filter_ = new ComRefCount<CaptureSinkFilter>(this);
  hr = _graphBuilder->AddFilter(sink_filter_, SINK_FILTER_NAME);
  if (FAILED(hr)) {
    RTC_LOG(LS_INFO) << "Failed to add the send filter to the graph.";
    return -1;
  }
  _inputSendPin = GetInputPin(sink_filter_);
  if (!_inputSendPin) {
    RTC_LOG(LS_INFO) << "Failed to get input send pin";
    return -1;
  }
****
}

实际上在调用graphBuilder->AddFilter(sink_filter, SINK_FILTER_NAME);之后，后面还会去调用CaptureSinkFilter::JoinFilterGraph，去告诉上层已经加入到FilterGraphl .

STDMETHODIMP CaptureSinkFilter::JoinFilterGraph(IFilterGraph* graph,
                                                LPCWSTR name) {
  RTC_DCHECK_RUN_ON(&main_checker_);
  RTC_DCHECK(IsStopped());
  // Note, since a reference to the filter is held by the graph manager,
  // filters must not hold a reference to the graph. If they would, we'd have
  // a circular reference. Instead, a pointer to the graph can be held without
  // reference. See documentation for IBaseFilter::JoinFilterGraph for more.
  info_.pGraph = graph;  // No AddRef().
  sink_ = nullptr;
  if (info_.pGraph) {
    // make sure we don't hold on to the reference we may receive.
    // Note that this assumes the same object identity, but so be it.
    // 当filter有事件发生时，通过sink来通知filtergraph  
    rtc::scoped_refptr<IMediaEventSink> sink;
    GetComInterface(info_.pGraph, &sink);
    sink_ = sink.get();
  }
  info_.achName[0] = L'\0';
  if (name)
    lstrcpynW(info_.achName, name, arraysize(info_.achName));
  return S_OK;
}

—》 inputSendPin = GetInputPin(sink_filter);

GetInputPin

IPin* GetInputPin(IBaseFilter* filter) {
  HRESULT hr;
  IPin* pin = NULL;
  IEnumPins* pPinEnum = NULL;
  filter->EnumPins(&pPinEnum);
  if (pPinEnum == NULL) {
    return NULL;
  }
  // get first unconnected pin
  // 引用计数设置为0，从第一个pin开始
  hr = pPinEnum->Reset();  // set to first pin
  while (S_OK == pPinEnum->Next(1, &pin, NULL)) {
    PIN_DIRECTION pPinDir;
    pin->QueryDirection(&pPinDir);
    if (PINDIR_INPUT == pPinDir)  // This is an input pin
    {
      IPin* tempPin = NULL;
      if (S_OK != pin->ConnectedTo(&tempPin))  // The pint is not connected
      {
        pPinEnum->Release();
        return pin;
      }
    }
    pin->Release();
  }
  pPinEnum->Release();
  return NULL;
}

1、 filter->EnumPins(&pPinEnum); 的定义如下

STDMETHODIMP CaptureSinkFilter::EnumPins(IEnumPins** pins) {
  RTC_DCHECK_RUN_ON(&main_checker_);
  *pins = new ComRefCount<class EnumPins>(input_pin_.get());
  (*pins)->AddRef();
  return S_OK;
}

2、 hr = pPinEnum->Reset();

  STDMETHOD(Reset)() {
    pos_ = 0;
    return S_OK;
  }

3、while (S_OK == pPinEnum->Next(1, &pin, NULL)) {

STDMETHOD(Next)(ULONG count, IPin** pins, ULONG* fetched) {
    RTC_DCHECK(count > 0);
    RTC_DCHECK(pins);
    // fetched may be NULL.
    if (pos_ > 0) { // 正常不会大于0
      if (fetched)
        *fetched = 0;
      return S_FALSE;
    }
    ++pos_;
    pins[0] = pin_.get();
    pins[0]->AddRef(); // 引用计数的增加
    if (fetched)
      *fetched = 1;
    return count == 1 ? S_OK : S_FALSE;
  }

4、pin->QueryDirection(&pPinDir);

STDMETHODIMP CaptureInputPin::QueryDirection(PIN_DIRECTION* pin_dir) {
  RTC_DCHECK_RUN_ON(&main_checker_);
  *pin_dir = info_.dir;
  return S_OK;
}

5、 if (S_OK != pin->ConnectedTo(&tempPin)) // The pint is not connected
判断当前引脚是否已经和其他引脚连接，正常不连接

STDMETHODIMP CaptureInputPin::ConnectedTo(IPin** pin) {
  RTC_DCHECK_RUN_ON(&main_checker_);
  if (!receive_pin_) 
    return VFW_E_NOT_CONNECTED;  //正常返回这个
  *pin = receive_pin_.get();
  receive_pin_->AddRef();
  return S_OK;
}