前言

呼叫方调用SetLocalDescription ，被呼叫方调用 SetRemoteDescription 。
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第一个都是处理音频。
注意是因为现在音频媒体协商确定后，才会调用AudioCodingModule::SetEncoder。之前只是收集编码器。
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注意，最后是构建AudioEncoderOpusImpl，然后传到左边创建的AudioCodingModuleImpl，再调用AudioCodingModuleImpl::RegisterTransportCallback注册回调对象，这时候注册的回调对象就可以交给其他对象去处理了。
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SdpOfferAnswerHandler::DoSetRemoteDescription

  1. SdpOfferAnswerHandler::DoSetRemoteDescription
  2. {
  3. SdpOfferAnswerHandler::FillInMissingRemoteMids
  4. SdpOfferAnswerHandler::ApplyRemoteDescription
  5. OnSetRemoteDescriptionComplete
  6. SdpOfferAnswerHandler::UpdateNegotiationNeeded
  7. }

SdpOfferAnswerHandler::ApplyRemoteDescription

  1. SdpOfferAnswerHandler::ApplyRemoteDescription
  2. {
  3. SdpOfferAnswerHandler::PushdownTransportDescription
  4. SdpOfferAnswerHandler::UpdateTransceiversAndDataChannels
  5. SdpOfferAnswerHandler::UpdateSessionState
  6. CheckForRemoteIceRestart
  7. UpdateRemoteSendersList
  8. }

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SdpOfferAnswerHandler::UpdateTransceiversAndDataChannels

  1. SdpOfferAnswerHandler::UpdateTransceiversAndDataChannels{
  2. SdpOfferAnswerHandler::AssociateTransceiver
  3. SdpOfferAnswerHandler::UpdateTransceiverChannel   
  4. }
  6. SdpOfferAnswerHandler::UpdateTransceiverChannel
  7. {
  8. ***
  9.  if (transceiver->media_type() == cricket::MEDIA_TYPE_AUDIO) {
  10.         channel = CreateVoiceChannel(content.name);
  11.       } else {
  12.         RTC_DCHECK_EQ(cricket::MEDIA_TYPE_VIDEO, transceiver->media_type());
  13.         channel = CreateVideoChannel(content.name);
  14. }
  15. ***
  16. }

JsepTransportController::SetRemoteDescription

  1. RTCError JsepTransportController::SetRemoteDescription(
  2.     SdpType type,
  3.     const cricket::SessionDescription* description) {
  4.   if (!network_thread_->IsCurrent()) {
  5.     return network_thread_->Invoke<RTCError>(
  6.         RTC_FROM_HERE, [=] { return SetRemoteDescription(type, description); });
  7.   }
  9.   RTC_DCHECK_RUN_ON(network_thread_);
  10.   return ApplyDescription_n(/*local=*/false, type, description);
  11. }

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SdpOfferAnswerHandler::PushdownMediaDescription

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  1.  SdpOfferAnswerHandler::PushdownMediaDescription 
  2.  {
  3.  ***
  4. // Push down the new SDP media section for each audio/video transceiver.
  5.   for (const auto& transceiver : transceivers()->List()) {
  6.     const ContentInfo* content_info =
  7.         FindMediaSectionForTransceiver(transceiver, sdesc);
  8.     cricket::ChannelInterface* channel = transceiver->internal()->channel();
  9.     if (!channel || !content_info || content_info->rejected) {
  10.       continue;
  11.     }
  12.     const MediaContentDescription* content_desc =
  13.         content_info->media_description();
  14.     if (!content_desc) {
  15.       continue;
  16.     }
  17.     std::string error;
  18.     bool success = (source == cricket::CS_LOCAL)
  19.                        ? channel->SetLocalContent(content_desc, type, &error)
  20.                        : channel->SetRemoteContent(content_desc, type, &error);
  21.     if (!success) {
  22.       LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, error);
  23.     }
  24.   }
  25.  }
  26.  ****
  27.  -》
  28.  bool BaseChannel::SetRemoteContent(const MediaContentDescription* content,
  29.                                    SdpType type,
  30.                                    std::string* error_desc) {
  31.   TRACE_EVENT0("webrtc", "BaseChannel::SetRemoteContent");
  32.   return InvokeOnWorker<bool>(RTC_FROM_HERE, [this, content, type, error_desc] {
  33.     RTC_DCHECK_RUN_ON(worker_thread());
  34.     return SetRemoteContent_w(content, type, error_desc);
  35.   });
  36. }
  38. -》
  39. H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\pc\channel.cc
  40.  VoiceChannel::SetRemoteContent_w

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因为 channel->SetRemoteContent(content_desc, type, &error);，此时是音频，所以是VoiceChannel ，因为VoiceChannel 继承BaseChannel，所以会先调用父类的SetRemoteContent，然后再在工作线程中调用VoiceChannel::SetRemoteContent_w，举一反三视频的也一样。