peerconnection是webrtc面向外面的音视频交互的统一接口，可以理解为一个功能特别强大的socket接口，里面保存了实时交互的所有信息,同时音视频的转发与接收也是通过peerconnection来完成

1 peerconnection 概述

peerconnection继承自PeerConnectionInterface 接口，所以作为对外暴露的统一接口为PeerConnectionInterface，通过工厂方法来通过统一创建PeerConnectionInterface 来创建peerconnection，在peerConnection.h中，// It uses WebRtcSession to implement the PeerConnection functionality. 说明了，webRtcSession类的重要

2 peerconnection类简要分析

  rtc::scoped_refptr<PeerConnectionFactory> factory_; //创建工厂
  PeerConnectionObserver* observer_;    //回调观察者，一般在外部类创建成功时，传入this指针，在示例中是Conductor,方便时间回调
  UMAObserver* uma_observer_; //暂时不清楚
  SignalingState signaling_state_; // signal slot相关
  // TODO(bemasc): Remove ice_state_.
  IceState ice_state_;    //ice状态
  IceConnectionState ice_connection_state_;//ice链接状态（相当于ice_state_的子状态的一个状态）
  IceGatheringState ice_gathering_state_;//ice 收集状态（相当于ice_state_的子状态的一个状态）

  std::unique_ptr<cricket::PortAllocator> port_allocator_;//sdp相关？
  std::unique_ptr<MediaControllerInterface> media_controller_;//媒体控制相关

  // One PeerConnection has only one RTCP CNAME.
  // https://tools.ietf.org/html/draft-ietf-rtcweb-rtp-usage-26#section-4.9
  std::string rtcp_cname_;

  // Streams added via AddStream.
  rtc::scoped_refptr<StreamCollection> local_streams_;//本地stream管理
  // Streams created as a result of SetRemoteDescription.
  rtc::scoped_refptr<StreamCollection> remote_streams_;//远端stream管理

  std::vector<std::unique_ptr<MediaStreamObserver>> stream_observers_;

  // These lists store track info seen in local/remote descriptions.
  TrackInfos remote_audio_tracks_;
  TrackInfos remote_video_tracks_;
  TrackInfos local_audio_tracks_;
  TrackInfos local_video_tracks_;

  SctpSidAllocator sid_allocator_;//暂不清楚
  // label -> DataChannel
  std::map<std::string, rtc::scoped_refptr<DataChannel>> rtp_data_channels_;//数据通道
  std::vector<rtc::scoped_refptr<DataChannel>> sctp_data_channels_;
  std::vector<rtc::scoped_refptr<DataChannel>> sctp_data_channels_to_free_;

  bool remote_peer_supports_msid_ = false;

  std::vector<rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>>>
      senders_;
  std::vector<
      rtc::scoped_refptr<RtpReceiverProxyWithInternal<RtpReceiverInternal>>>
      receivers_;

  std::unique_ptr<WebRtcSession> session_; //session管理类
  std::unique_ptr<StatsCollector> stats_;