Hey, @dkubrakov I know that packaging libwebrtc and developing flutter windows plug-ins are very challenging. If you have any questions or need my help, please leave a message.

Question 1 - do I need to pre-create a track through the RTCPeerConnectionFactory or just pass information about a new track?

The PeerConnection.AddTrack method in dart needs to use an existing audio/video track as a parameter. You don’t need to create it in flutter_peerconnection.cc#L251. Here you only need to find it from base_->media_tracks_ by trackID. If not found, return error.

The dart side usage looks like this

var localStream = await navigator.mediaDevices.getUserMedia({'audio': true, 'video': true});

 localStream.getTracks().forEach((track) {
        var sender = pc.addTrack(track, localStream);
 });

The cpp side looks like this

void FlutterPeerConnection::AddTrack(RTCPeerConnection *pc, const std::string &track_id, const EncodableList &stream_ids, std::unique_ptr<MethodResult<EncodableValue>> result) {

    scoped_refptr<RTCMediaTrack> track =  base_->MediaTrackForID(track_id);
    if(!track) {
          result->Error("AddTrackFailed", "track [" + track_id + "] not found.");
    }

   std::vector<std::string> streamIds = toStringVector(stream_ids);
   scoped_refptr<RTCRtpSender> sender = pc->AddTrack(track, streamIds[0].c_str());
    ...
}

Next, the result of AddTrack () is the RtpSenderInterface that needs to be returned to the client.
One of the RtpSender parameters is the complex structure RtpParameters (GetParameters()), which consists of the lists RtpCodecParameters, RtpHeaderExtensionParameters, RtpEncodingParameters.

Yes, The parameters here are indeed very complicated and need to be packaged with Encodeable correctly

Question 2 - why is it that when I call AddTrack() when creating an audio track, these lists are empty? (What is recorded in the message.)

We must create an interface wrapper similar to webrtc::RtpParameters in libwebrtc. Let's say it is called RTCRtpParameters, and wrap it with scoped_refptr and return it to the user. Any std::string id; parameter in any webrtc object must be copied as char id_[kMaxStringLength ];, This is very important. The purpose of this is to make the exported libwebrtc.dll applicable to any caller environment, because the user's libstdc++ cannot be consistent with libwebrtc.dll when compiled.

please refer to this example.
https://github.com/dkubrakov/libwebrtc/blob/955b4c1e082f8bff7d82706c6cc82b91cf038527/src/rtc_audio_track_impl.cc#L9

Question 3 - how to correctly return the structure and lists/map from virtual functions to the client application?

When we export using the new RTCRtpParameters interface, it will look like this

#define Vector bsp::inlined_vector

class RTCRtpParameters : public RefCountInterface {
  virtual const char*  transaction_id() = 0;
  
  vitrual const char* mid()   = 0;
  
  Vector<RTCRtpCodecParameters> codecs;

  Vector<RTCRtpExtension> header_extensions;

  Vector<RTCRtpEncodingParameters> encodings;

  vitrual  scoped_refptr<RtcpParameters> rtcp() = 0;
}

Uh oh!

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