问[InvalidStateError："setRemoteDescription需要先调用addIceCandidate“代码: 11

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var localVideo;
var remoteVideo;
var peerConnection;
var uuid;
var localStream;
var peerConnectionConfig = {
'iceServers': [
    {'urls': 'stun:stun.services.mozilla.com'},
    {'urls': 'stun:stun.l.google.com:19302'},
]
};

function pageReady() {
    uuid = uuid();
    console.log('Inside Page Ready');
    localVideo = document.getElementById('localVideo');
    remoteVideo = document.getElementById('remoteVideo');

   serverConnection = new WebSocket('wss://' + window.location.hostname + 
   ':8443');
   serverConnection.onmessage = gotMessageFromServer;

   var constraints = {
       video: true,
       audio: true,
   };

   if(navigator.mediaDevices.getUserMedia) {

   navigator.mediaDevices.getUserMedia(constraints)
   .then(getUserMediaSuccess).catch(errorHandler);
   }else
   {
       alert('Your browser does not support getUserMedia API');
   }
   }

   function getUserMediaSuccess(stream) {
        localStream = stream;
        localVideo.src = window.URL.createObjectURL(stream); 
   }

   function start(isCaller) {
       console.log('Inside isCaller');
       peerConnection = new RTCPeerConnection(peerConnectionConfig);
       peerConnection.onicecandidate = gotIceCandidate;
       peerConnection.onaddstream = gotRemoteStream;
       peerConnection.addStream(localStream);

       if(isCaller) {
            console.log('Inside Caller to create offer');
            peerConnection.createOffer().
            then(createdDescription).catch(errorHandler);
       }
      }

   function gotMessageFromServer(message) {
   console.log('Message from Server');
   if(!peerConnection) 
   {
        console.log('Inside !Peer Conn');
        start(false);
   }

   var signal = JSON.parse(message.data);

   // Ignore messages from ourself
   if(signal.uuid == uuid) return;

   if(signal.sdp) {
        console.log('Inside SDP');
        peerConnection.setRemoteDescription(new 
        RTCSessionDescription(signal.sdp)).then(function() {
        // Only create answers in response to offers
        if(signal.sdp.type == 'offer') {
            console.log('Before Create Answer');
            peerConnection.createAnswer().then(createdDescription)
            .catch(errorHandler);
        }
     }).catch(errorHandler);
     } else if(signal.ice) {
           console.log('Inside Signal Ice');
           peerConnection.addIceCandidate(new 
           RTCIceCandidate(signal.ice)).catch(errorHandler);
     }

    }

    function gotIceCandidate(event) {
         console.log('Inside Got Ice Candi');
         if(event.candidate != null) {
         serverConnection.send(JSON.stringify({'ice': event.candidate, 
         'uuid': uuid}));
    }
  }

  function createdDescription(description) {
  console.log('got description');

    peerConnection.setLocalDescription(description).then(function() {
    console.log('Inside Setting ');
    serverConnection.send(JSON.stringify({'sdp': 
    peerConnection.localDescription, 'uuid': uuid}));
   }).catch(errorHandler);
  }

  function gotRemoteStream(event) {
  console.log('got remote stream');
  remoteVideo.src = window.URL.createObjectURL(event.stream);
  }

  function errorHandler(error) {
     console.log(error);
  }

  // Taken from http://stackoverflow.com/a/105074/515584
  // Strictly speaking, it's not a real UUID, but it gets the job done here
  function uuid() {
      function s4() {
      return Math.floor((1 + Math.random()) * 
      0x10000).toString(16).substring(1);
      }

  return s4() + s4() + '-' + s4() + '-' + s4() + '-' + s4() + '-' + s4() + 
  s4() + s4();
  }