VideoReceiveStream::Start方法分析

源码：

void VideoReceiveStream::Start() {
  RTC_DCHECK_RUN_ON(&worker_sequence_checker_);

  if (decoder_running_) {
    return;
  }

  const bool protected_by_fec = config_.rtp.protected_by_flexfec ||
                                rtp_video_stream_receiver_.IsUlpfecEnabled();

  if (rtp_video_stream_receiver_.IsRetransmissionsEnabled() &&
      protected_by_fec) {
    frame_buffer_->SetProtectionMode(kProtectionNackFEC);
  }

  transport_adapter_.Enable();
  rtc::VideoSinkInterface<VideoFrame>* renderer = nullptr;
  if (config_.enable_prerenderer_smoothing) {
    incoming_video_stream_.reset(new IncomingVideoStream(
        task_queue_factory_, config_.render_delay_ms, this));
    renderer = incoming_video_stream_.get();
  } else {
    renderer = this;
  }

  for (const Decoder& decoder : config_.decoders) {
    std::unique_ptr<VideoDecoder> video_decoder =
        config_.decoder_factory->LegacyCreateVideoDecoder(decoder.video_format,
                                                          config_.stream_id);
    // If we still have no valid decoder, we have to create a "Null" decoder
    // that ignores all calls. The reason we can get into this state is that the
    // old decoder factory interface doesn't have a way to query supported
    // codecs.
    if (!video_decoder) {
      video_decoder = std::make_unique<NullVideoDecoder>();
    }

    std::string decoded_output_file =
        field_trial::FindFullName("WebRTC-DecoderDataDumpDirectory");
    // Because '/' can't be used inside a field trial parameter, we use ';'
    // instead.
    // This is only relevant to WebRTC-DecoderDataDumpDirectory
    // field trial. ';' is chosen arbitrary. Even t