结合 AppRTC 源码分析 WebRTC 建立连接的过程

//创建peerConnectionFactory，以及对应的 video codec factory
RTCDefaultVideoDecoderFactory *decoderFactory = [[RTCDefaultVideoDecoderFactory alloc] init];
RTCDefaultVideoEncoderFactory *encoderFactory = [[RTCDefaultVideoEncoderFactory alloc] init];
encoderFactory.preferredCodec = [settings currentVideoCodecSettingFromStore];
_factory = [[RTCPeerConnectionFactory alloc] initWithEncoderFactory:encoderFactory decoderFactory:decoderFactory];

// Request TURN.
__weak ARDAppClient *weakSelf = self;
[_turnClient requestServersWithCompletionHandler:^(NSArray *turnServers,
                                                 NSError *error) {
    //此处省略错误检查代码 
    ARDAppClient *strongSelf = weakSelf;
    [strongSelf.iceServers addObjectsFromArray:turnServers];
    strongSelf.isTurnComplete = YES;
    [strongSelf startSignalingIfReady];
}];

// Join room on room server.
[_roomServerClient joinRoomWithRoomId:roomId
                         isLoopback:isLoopback
  completionHandler:^(ARDJoinResponse *response, NSError *error) {
    ARDAppClient *strongSelf = weakSelf;
	
	// 此处省略错误检查代码 
    
    RTCLog(@"Joined room:%@ on room server.", roomId);
    strongSelf.roomId = response.roomId; //获得roomId
    strongSelf.clientId = response.clientId;
    strongSelf.isInitiator = response.isInitiator; //服务器决定谁是发起者（先加入房间的是发起者，后加入的是非发起者）
    for (ARDSignalingMessage *message in response.messages) {
        if (message.type == kARDSignalingMessageTypeOffer ||
            message.type == kARDSignalingMessageTypeAnswer) {
            strongSelf.hasReceivedSdp = YES;
            [strongSelf.messageQueue insertObject:message atIndex:0];
        } else {
            [strongSelf.messageQueue addObject:message];
        }
    }
    //获得 WebSocket 地址
    strongSelf.webSocketURL = response.webSocketURL;
    strongSelf.webSocketRestURL = response.webSocketRestURL;
    [strongSelf registerWithColliderIfReady]; // 连接WebSocket 服务器
    [strongSelf startSignalingIfReady]; 
}];

//RTCMediaConstraints 主要是描述音视频媒体的参数，比如分辨率大小，音频声道数等等
RTCMediaConstraints *constraints = [self defaultPeerConnectionConstraints]; 
RTCConfiguration *config = [[RTCConfiguration alloc] init];
RTCCertificate *pcert = [RTCCertificate generateCertificateWithParams:@{
    @"expires" : @100000,
    @"name" : @"RSASSA-PKCS1-v1_5"
}];
config.iceServers = _iceServers; //上面从服务器获得的 ICE server 地址
config.sdpSemantics = RTCSdpSemanticsUnifiedPlan; //这里使用 unified plan
config.certificate = pcert;
    
//创建 PeerConnection！！！
❶_peerConnection = [_factory peerConnectionWithConfiguration:config
                                              constraints:constraints
                                                 delegate:self];
    
❷[self createMediaSenders]; 
  if (_isInitiator) { //只有发起方创建 offer，接收方只需等待对方的offer，然后创建answer即可
    // Send offer.
    __weak ARDAppClient *weakSelf = self;
    
  ❸[_peerConnection offerForConstraints:[self defaultOfferConstraints]
                       completionHandler:^(RTCSessionDescription *sdp,
                                           NSError *error) {
      ARDAppClient *strongSelf = weakSelf;
      [strongSelf peerConnection:strongSelf.peerConnection
          didCreateSessionDescription:sdp
                                error:error];
    }];
  } else {
    // Check if we've received an offer.
  ❹[self drainMessageQueueIfReady];
  }

- (void)createMediaSenders {
	//创建 local audio track，并添加到 peerconnection 中
  RTCMediaConstraints *constraints = [self defaultMediaAudioConstraints];
  RTCAudioSource *source = [_factory audioSourceWithConstraints:constraints];
  RTCAudioTrack *track = [_factory audioTrackWithSource:source
                                                trackId:kARDAudioTrackId];
  [_peerConnection addTrack:track streamIds:@[ kARDMediaStreamId ]];
	
	//创建 local video track 并添加到 peerconnection 中（最终会添加到 peerconnection.transceiver.sender 里）
  _localVideoTrack = [self createLocalVideoTrack];
  if (_localVideoTrack) {
    [_peerConnection addTrack:_localVideoTrack streamIds:@[ kARDMediaStreamId ]];
    [_delegate appClient:self didReceiveLocalVideoTrack:_localVideoTrack];
    
    // We can set up rendering for the remote track right away since the transceiver already has an
    // RTCRtpReceiver with a track. The track will automatically get unmuted and produce frames
    // once RTP is received.
    RTCVideoTrack *track = (RTCVideoTrack *)([self videoTransceiver].receiver.track);
    [_delegate appClient:self didReceiveRemoteVideoTrack:track];
  }
}

//只保留了核心逻辑，其他省略了
- (RTCVideoTrack *)createLocalVideoTrack {
	//factory 创建一个新的 source
  RTCVideoSource *source = [_factory videoSource];
  //用 source 创建一个 RTCCameraVideoCapturer，RTCCameraVideoCapturer 负责 iOS 上的采集，
  //采集到的视频帧发送给 delegate。RTCVideoSource 实现了 RTCVideoCapturerDelegate，
  //所以可以接收到 RTCCameraVideoCapturer 采集到的视频帧
  RTCCameraVideoCapturer *capturer = [[RTCCameraVideoCapturer alloc] initWithDelegate:source];
    
	//这里的delegate 就是 ARDVideoCallViewController，它收到消息后，开始通过 capturer 进行视频采集
  [_delegate appClient:self didCreateLocalCapturer:capturer];
    
  return [_factory videoTrackWithSource:source trackId:kARDVideoTrackId];
}

- (void)peerConnection:(RTCPeerConnection *)peerConnection
    didCreateSessionDescription:(RTCSessionDescription *)sdp
                          error:(NSError *)error {
  dispatch_async(dispatch_get_main_queue(), ^{
		//...省略错误检查
    __weak ARDAppClient *weakSelf = self;
		//设置 setLocalDescription
    [self.peerConnection setLocalDescription:sdp
                           completionHandler:^(NSError *error) {
                             ARDAppClient *strongSelf = weakSelf;
                             [strongSelf peerConnection:strongSelf.peerConnection
                                 didSetSessionDescriptionWithError:error];
                           }];
    ARDSessionDescriptionMessage *message =
        [[ARDSessionDescriptionMessage alloc] initWithDescription:sdp];
		//发送 SDP 到 remote peer
    [self sendSignalingMessage:message];
		//设置 video sender 的最大码率
    [self setMaxBitrateForPeerConnectionVideoSender];
  });
}

case kARDSignalingMessageTypeAnswer: {
      ARDSessionDescriptionMessage *sdpMessage =
          (ARDSessionDescriptionMessage *)message;
      RTCSessionDescription *description = sdpMessage.sessionDescription;
      __weak ARDAppClient *weakSelf = self;
			
			//把对方的 SDP 设置为 remote SDP
      [_peerConnection setRemoteDescription:description
                          completionHandler:^(NSError *error) {
                            ARDAppClient *strongSelf = weakSelf;
														//回调 delegate
                            [strongSelf peerConnection:strongSelf.peerConnection
                                didSetSessionDescriptionWithError:error];
                          }];
      break;
    }

- (void)peerConnection:(RTCPeerConnection *)peerConnection
    didSetSessionDescriptionWithError:(NSError *)error {
  dispatch_async(dispatch_get_main_queue(), ^{
		//省略错误处理
    // If we're answering and we've just set the remote offer we need to create
    // an answer and set the local description.
    if (!self.isInitiator && !self.peerConnection.localDescription) {
      RTCMediaConstraints *constraints = [self defaultAnswerConstraints];
      __weak ARDAppClient *weakSelf = self;
			//创建 Answer
      [self.peerConnection answerForConstraints:constraints
                              completionHandler:^(RTCSessionDescription *sdp, NSError *error) {
                                ARDAppClient *strongSelf = weakSelf;
																//回调 delegate 的 didCreateSessionDescription 方法（和发起者创建完 offer 回调的方法一致，行为也一致，
																//首先 setLocalDescription，然后发送到 remote peer，然后设置最大发送码率）
                                [strongSelf peerConnection:strongSelf.peerConnection
                                    didCreateSessionDescription:sdp
                                                          error:error];
                              }];
    }
  });
}

- (void)peerConnection:(RTCPeerConnection *)peerConnection
    didCreateSessionDescription:(RTCSessionDescription *)sdp
                          error:(NSError *)error {
  dispatch_async(dispatch_get_main_queue(), ^{
		//...省略错误检查
    __weak ARDAppClient *weakSelf = self;
		//设置 setLocalDescription
    [self.peerConnection setLocalDescription:sdp
                           completionHandler:^(NSError *error) {
                             ARDAppClient *strongSelf = weakSelf;
                             [strongSelf peerConnection:strongSelf.peerConnection
                                 didSetSessionDescriptionWithError:error];
                           }];
    ARDSessionDescriptionMessage *message =
        [[ARDSessionDescriptionMessage alloc] initWithDescription:sdp];
		//发送 SDP 到 remote peer
    [self sendSignalingMessage:message];
		//设置 video sender 的最大码率
    [self setMaxBitrateForPeerConnectionVideoSender];
  });
}

- (void)peerConnection:(RTCPeerConnection *)peerConnection
    didGenerateIceCandidate:(RTCIceCandidate *)candidate {
  dispatch_async(dispatch_get_main_queue(), ^{
    ARDICECandidateMessage *message =
        [[ARDICECandidateMessage alloc] initWithCandidate:candidate];
    [self sendSignalingMessage:message];
  });
}

- (void)processSignalingMessage:(ARDSignalingMessage *)message {
  NSParameterAssert(_peerConnection ||
      message.type == kARDSignalingMessageTypeBye);
  switch (message.type) {
    //...省略
    case kARDSignalingMessageTypeCandidate: {
      ARDICECandidateMessage *candidateMessage =
          (ARDICECandidateMessage *)message;
      [_peerConnection addIceCandidate:candidateMessage.candidate];
      break;
    }
    //...省略
  }
}