最近项目中需要通过webrtc进行音视频传输，其中视频源为抓取window系统桌面。

在PeerConnectionClient Demo基础上实现抓屏接口。需要修改Conductor::AddTracks接口中的video_track_，由VcmCapturer 改为DesktopCapturer。

void Conductor::AddTracks() {
  if (!peer_connection_->GetSenders().empty()) {
    return;  // Already added tracks.
  }

  rtc::scoped_refptr<webrtc::AudioTrackInterface> audio_track(
      peer_connection_factory_->CreateAudioTrack(
          kAudioLabel, peer_connection_factory_->CreateAudioSource(
                           cricket::AudioOptions())));
  auto result_or_error = peer_connection_->AddTrack(audio_track, {kStreamId});
  if (!result_or_error.ok()) {
    RTC_LOG(LS_ERROR) << "Failed to add audio track to PeerConnection: "
                      << result_or_error.error().message();
  }
  //视频源修改为抓取window系统桌面
  rtc::scoped_refptr<MyCapturer> video_device = new rtc::RefCountedObject<MyCapturer>();

  if (video_device) {
    video_device->startCapturer();
    rtc::scoped_refptr<webrtc::VideoTrackInterface> video_track_(
        peer_connection_factory_->CreateVideoTrack(kVideoLabel, video_device));
    main_wnd_->StartLocalRenderer(video_track_);

    result_or_error = peer_connection_->AddTrack(video_track_, {kStreamId});
    if (!result_or_error.ok()) {
      RTC_LOG(LS_ERROR) << "Failed to add video track to PeerConnection: "
                        << result_or_error.error().message();
    }
  } else {
    RTC_LOG(LS_ERROR) << "OpenVideoCaptureDevice failed";
  }

CreateVideoTrack接口需要传入VideoTrackSourceInterface* source接口的参数，我们使用webrtc::DesktopCapturer::Callback的接口OnCaptureResult抓取屏幕，再由AdaptedVideoTrackSource的OnFrame接口将转码后的yuv数据传给webrtp内部进行编码和打包传输。

直接上代码：

class MyCapturer : public rtc::AdaptedVideoTrackSource,
                   public rtc::MessageHandler,
                   public webrtc::DesktopCapturer::Callback {
 public:
  MyCapturer();

  void startCapturer();

  void CaptureFrame();

  bool is_screencast() const override;

  absl::optional<bool> needs_denoising() const override;

  webrtc::MediaSourceInterface::SourceState state() const override;

  bool remote() const override;

  void OnCaptureResult(webrtc::DesktopCapturer::Result result,
                               std::unique_ptr<webrtc::DesktopFrame> frame) override;
  void OnMessage(rtc::Message* msg) override;

 private:
  std::unique_ptr<webrtc::DesktopCapturer> capturer_;
  rtc::scoped_refptr<webrtc::I420Buffer> i420_buffer_;
  //mutable volatile int ref_count_;
};

MyCapturer::MyCapturer() {

}

void MyCapturer::startCapturer() {
  auto options = webrtc::DesktopCaptureOptions::CreateDefault();
  options.set_allow_directx_capturer(true);
  capturer_ = webrtc::DesktopCapturer::CreateScreenCapturer(options);
  capturer_->Start(this);
  CaptureFrame();
}

webrtc::MediaSourceInterface::SourceState MyCapturer::state() const {
  return webrtc::MediaSourceInterface::kLive;
}

bool MyCapturer::remote() const {
  return false;
}

bool MyCapturer::is_screencast() const {
  return true;
}

absl::optional<bool> MyCapturer::needs_denoising() const {
  return false;
}

void MyCapturer::OnCaptureResult(webrtc::DesktopCapturer::Result result,
                                 std::unique_ptr<webrtc::DesktopFrame> frame) {
  if (result != webrtc::DesktopCapturer::Result::SUCCESS)
    return;

  int width = frame->size().width();
  int height = frame->size().height();

  if (!i420_buffer_.get() ||
      i420_buffer_->width() * i420_buffer_->height() < width * height) {
    i420_buffer_ = webrtc::I420Buffer::Create(width, height);
  }
  libyuv::ConvertToI420(frame->data(), 0, i420_buffer_->MutableDataY(),
                        i420_buffer_->StrideY(), i420_buffer_->MutableDataU(),
                        i420_buffer_->StrideU(), i420_buffer_->MutableDataV(),
                        i420_buffer_->StrideV(), 0, 0, width, height, width,
                        height, libyuv::kRotate0, libyuv::FOURCC_ARGB);

  OnFrame(webrtc::VideoFrame(i420_buffer_, 0, 0, webrtc::kVideoRotation_0));
}

void MyCapturer::OnMessage(rtc::Message* msg) {
  if (msg->message_id == 0)
    CaptureFrame();
}

void MyCapturer::CaptureFrame() {
  capturer_->CaptureFrame();

  //fps=30
  rtc::Location loc(__FUNCTION__, __FILE__);
  rtc::Thread::Current()->PostDelayed(loc, 33, this, 0);
}

Demo 入口：

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