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 though it's a legal character
// in some file systems, we can sacrifice ability to use it in the path to
// dumped video, since it's developers-only feature for debugging.
absl::c_replace(decoded_output_file, ';', '/');
if (!decoded_output_file.empty()) {
char filename_buffer[256];
rtc::SimpleStringBuilder ssb(filename_buffer);
ssb << decoded_output_file << "/webrtc_receive_stream_"
<< this->config_.rtp.remote_ssrc << "-" << rtc::TimeMicros()
<< ".ivf";
video_decoder = CreateFrameDumpingDecoderWrapper(
std::move(video_decoder), FileWrapper::OpenWriteOnly(ssb.str()));
}
video_decoders_.push_back(std::move(video_decoder));
video_receiver_.RegisterExternalDecoder(video_decoders_.back().get(),
decoder.payload_type);
VideoCodec codec = CreateDecoderVideoCodec(decoder);
const bool raw_payload =
config_.rtp.raw_payload_types.count(decoder.payload_type) > 0;
rtp_video_stream_receiver_.AddReceiveCodec(decoder.payload_type, codec,
decoder.video_format.parameters,
raw_payload);
RTC_CHECK_EQ(VCM_OK, video_receiver_.RegisterReceiveCodec(
decoder.payload_type, &codec, num_cpu_cores_));
}
RTC_DCHECK(renderer != nullptr);
video_stream_decoder_.reset(
new VideoStreamDecoder(&video_receiver_, &stats_proxy_, renderer));
// Make sure we register as a stats observer *after* we've prepared the
// |video_stream_decoder_|.
call_stats_->RegisterStatsObserver(this);
// Start decoding on task queue.
video_receiver_.DecoderThreadStarting();
stats_proxy_.DecoderThreadStarting();
decode_queue_.PostTask([this] {
RTC_DCHECK_RUN_ON(&decode_queue_);
decoder_stopped_ = false;
StartNextDecode();
});
decoder_running_ = true;
rtp_video_stream_receiver_.StartReceive();
}解释：
1运行环境检查：
RTC_DCHECK_RUN_ON(&worker_sequence_checker_);
2检查解码器是否已运行：如果解码器已经在运行，则直接返回，避免重复启动
if (decoder_running_) {
return;
}
3设置保护模式：
检查是否启用了 FEC（前向纠错）保护，并根据需要设置帧缓冲区的保护模式。kProtectionNackFEC
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);
}
4启用传输适配器：
transport_adapter_.Enable();
5设置视频渲染器：
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;
}
6.初始化解码器：
遍历配置中的所有解码器，尝试使用解码器工厂创建解码器对象。如果创建失败，使用一个空解码器（NullVideoDecoder）。
for (const Decoder& decoder : config_.decoders) {
std::unique_ptr<VideoDecoder> video_decoder =
config_.decoder_factory->LegacyCreateVideoDecoder(decoder.video_format,
config_.stream_id);
if (!video_decoder) {
video_decoder = std::make_unique<NullVideoDecoder>();
}
7.处理解码数据转储：
如果启用了解码数据转储功能，则设置解码数据转储路径，并用 CreateFrameDumpingDecoderWrapper 包装解码器，以便将解码帧保存到文件中
std::string decoded_output_file =
field_trial::FindFullName("WebRTC-DecoderDataDumpDirectory");
absl::c_replace(decoded_output_file, ';', '/');
if (!decoded_output_file.empty()) {
char filename_buffer[256];
rtc::SimpleStringBuilder ssb(filename_buffer);
ssb << decoded_output_file << "/webrtc_receive_stream_"
<< this->config_.rtp.remote_ssrc << "-" << rtc::TimeMicros()
<< ".ivf";
video_decoder = CreateFrameDumpingDecoderWrapper(
std::move(video_decoder), FileWrapper::OpenWriteOnly(ssb.str()));
}
8.注册解码器：
将创建的解码器添加到 video_decoders_ 列表中，并注册到 video_receiver_ 中。然后，根据解码器的配置，向 rtp_video_stream_receiver_ 注册接收编解码器，并向 video_receiver_ 注册接收编解码器
video_decoders_.push_back(std::move(video_decoder));
video_receiver_.RegisterExternalDecoder(video_decoders_.back().get(),
decoder.payload_type);
VideoCodec codec = CreateDecoderVideoCodec(decoder);
const bool raw_payload =
config_.rtp.raw_payload_types.count(decoder.payload_type) > 0;
rtp_video_stream_receiver_.AddReceiveCodec(decoder.payload_type, codec,
decoder.video_format.parameters,
raw_payload);
RTC_CHECK_EQ(VCM_OK, video_receiver_.RegisterReceiveCodec(
decoder.payload_type, &codec, num_cpu_cores_));
9设置视频流解码器：
RTC_DCHECK(renderer != nullptr);
video_stream_decoder_.reset(
new VideoStreamDecoder(&video_receiver_, &stats_proxy_, renderer));
10注册统计观察者：
call_stats_->RegisterStatsObserver(this);
11启动解码线程：
启动解码线程，调用 DecoderThreadStarting 方法，并在任务队列上发布任务以启动下一个解码操作。将 decoder_running_ 设置为 true，并启动 RTP 视频流接收
video_receiver_.DecoderThreadStarting();
stats_proxy_.DecoderThreadStarting();
decode_queue_.PostTask([this] {
RTC_DCHECK_RUN_ON(&decode_queue_);
decoder_stopped_ = false;
StartNextDecode();
});
decoder_running_ = true;
rtp_video_stream_receiver_.StartReceive();