CapturerTrackSource::Create()

 static rtc::scoped_refptr<CapturerTrackSource> Create() {
    const size_t kWidth = 640;
    const size_t kHeight = 480;
    const size_t kFps = 30;
    std::unique_ptr<webrtc::test::VcmCapturer> capturer;
    std::unique_ptr<webrtc::VideoCaptureModule::DeviceInfo> info(
        webrtc::VideoCaptureFactory::CreateDeviceInfo());
    if (!info) {
      return nullptr;
    }
    int num_devices = info->NumberOfDevices();
    for (int i = 0; i < num_devices; ++i) {
      capturer = absl::WrapUnique(
          webrtc::test::VcmCapturer::Create(kWidth, kHeight, kFps, i));
      if (capturer) {
        return new rtc::RefCountedObject<CapturerTrackSource>(
            std::move(capturer));
      }
    }
    return nullptr;
  }

—》 int num_devices = info->NumberOfDevices();

uint32_t DeviceInfoDS::NumberOfDevices() {
  MutexLock lock(&_apiLock);
  return GetDeviceInfo(0, 0, 0, 0, 0, 0, 0);
}

—》GetDeviceInfo(0, 0, 0, 0, 0, 0, 0);

int32_t DeviceInfoDS::GetDeviceInfo(uint32_t deviceNumber, 第几个设备
                                    char* deviceNameUTF8, 设备名称
                                    uint32_t deviceNameLength, 设备名称长度
                                    char* deviceUniqueIdUTF8, 唯一标识
                                    uint32_t deviceUniqueIdUTF8Length,
                                    char* productUniqueIdUTF8, 生产的标识
                                    uint32_t productUniqueIdUTF8Length)
{
  // enumerate all video capture devices
  RELEASE_AND_CLEAR(_dsMonikerDevEnum);
    // 指定获取视频设备
  HRESULT hr = _dsDevEnum->CreateClassEnumerator(CLSID_VideoInputDeviceCategory,
                                                 &_dsMonikerDevEnum, 0);
  if (hr != NOERROR) {
    RTC_LOG(LS_INFO) << "Failed to enumerate CLSID_SystemDeviceEnum, error 0x"
                     << rtc::ToHex(hr) << ". No webcam exist?";
    return 0;
  }
  _dsMonikerDevEnum->Reset();
  ULONG cFetched;
  IMoniker* pM;
  int index = 0;
    // 枚举设备，默认只有一个设备，只会循环一次
  while (S_OK == _dsMonikerDevEnum->Next(1, &pM, &cFetched)) {
    IPropertyBag* pBag;
    hr = pM->BindToStorage(0, 0, IID_IPropertyBag, (void**)&pBag);
    if (S_OK == hr) {
      // Find the description or friendly name.
      VARIANT varName;
      VariantInit(&varName);
      hr = pBag->Read(L"Description", &varName, 0);
      if (FAILED(hr)) {
        hr = pBag->Read(L"FriendlyName", &varName, 0);
      }
      if (SUCCEEDED(hr)) {
        // ignore all VFW drivers
        if ((wcsstr(varName.bstrVal, (L"(VFW)")) == NULL) &&
            (_wcsnicmp(varName.bstrVal, (L"Google Camera Adapter"), 21) != 0)) {
          // Found a valid device.
          if (index == static_cast<int>(deviceNumber)) {
            int convResult = 0;
            if (deviceNameLength > 0) {
              convResult = WideCharToMultiByte(CP_UTF8, 0, varName.bstrVal, -1,
                                               (char*)deviceNameUTF8,
                                               deviceNameLength, NULL, NULL);
              if (convResult == 0) {
                RTC_LOG(LS_INFO) << "Failed to convert device name to UTF8, "
                                    "error = "
                                 << GetLastError();
                return -1;
              }
            }
            if (deviceUniqueIdUTF8Length > 0) {
              hr = pBag->Read(L"DevicePath", &varName, 0);
              if (FAILED(hr)) {
                strncpy_s((char*)deviceUniqueIdUTF8, deviceUniqueIdUTF8Length,
                          (char*)deviceNameUTF8, convResult);
                RTC_LOG(LS_INFO) << "Failed to get "
                                    "deviceUniqueIdUTF8 using "
                                    "deviceNameUTF8";
              } else {
                convResult = WideCharToMultiByte(
                    CP_UTF8, 0, varName.bstrVal, -1, (char*)deviceUniqueIdUTF8,
                    deviceUniqueIdUTF8Length, NULL, NULL);
                if (convResult == 0) {
                  RTC_LOG(LS_INFO) << "Failed to convert device "
                                      "name to UTF8, error = "
                                   << GetLastError();
                  return -1;
                }
                if (productUniqueIdUTF8 && productUniqueIdUTF8Length > 0) {
                  GetProductId(deviceUniqueIdUTF8, productUniqueIdUTF8,
                               productUniqueIdUTF8Length);
                }
              }
            }
          }
          ++index;  // increase the number of valid devices
        }
      }
      VariantClear(&varName);
      pBag->Release();
      pM->Release();
    }
  }
  if (deviceNameLength) {
    RTC_DLOG(LS_INFO) << __FUNCTION__ << " " << deviceNameUTF8;
  }
  return index;
}