function initialize() {

    console.log("Initializing; room=99688636.");

    card = document.getElementById("card");

    localVideo = document.getElementById("localVideo");

    miniVideo = document.getElementById("miniVideo");

    remoteVideo = document.getElementById("remoteVideo");

    resetStatus();

    openChannel('AHRlWrqvgCpvbd9B-Gl5vZ2F1BlpwFv0xBUwRgLF/* ...*/');/*room token 由Google App Engine app 提供*/

    doGetUserMedia();//确认浏览器是否支持getUserMedia API 如果支持则调用onUserMediaSuccess

}

/* 建立通道过程

1.客户端A生成一个唯一的ID

2.客户端A把ID传给App Engine app，请求获得Channel token

3.App Engine app 把ID传给 Channel API,请求获得一个channel和token

4.App把token传给客户端A

5.客户端A打开socket,监听channel

*/

function openChannel(channelToken) {

  console.log("Opening channel.");

  var channel = new goog.appengine.Channel(channelToken);

  var handler = {

    'onopen': onChannelOpened,

    'onmessage': onChannelMessage,

    'onerror': onChannelError,

    'onclose': onChannelClosed

  };

  socket = channel.open(handler);

}

/*Sending a message works like this:

1.Client B makes a POST request to the App Engine app with an update.

2.The App Engine app passes a request to the channel.

3.The channel carries a message to Client A.

4.Client A's onmessage callback is called.

*/

//如果浏览器支持getUserMedia,则函数被调用

function onUserMediaSuccess(stream) {

  console.log("User has granted access to local media.");

  // Call the polyfill wrapper to attach the media stream to this element.

  attachMediaStream(localVideo, stream);//localVideo.src = ... localViedo代表一个标签

  localVideo.style.opacity = 1;

  localStream = stream;

  // Caller creates PeerConnection.

  if (initiator) maybeStart();//initiator 已经被设置为1，直到caller的session终止 所以这里会调用maybeStart

}

//connection只会被建立一次

//建立前提1.第一次建立 2.localStream已经准备好了，即本地视频 3.信令通道准备好了

function maybeStart() {

  if (!started && localStream && channelReady) {

    // ...调用func,使用STUN创建RTCPeerConnection(pc),设置各种事件监听函数

    createPeerConnection();

    // ...

    pc.addStream(localStream);

    started = true;

    // Caller initiates offer to peer.

    if (initiator)

      doCall();

  }

}

//被maybeStart调用

//主要目的是使用STUN服务器和回调函数onIceCandidata来建立connection

//为每一个RTCPeerConnection事件建立handlers

function createPeerConnection() {

  var pc_config = {"iceServers": [{"url": "stun:stun.l.google.com:19302"}]};

  try {

    // Create an RTCPeerConnection via the polyfill (adapter.js).

    pc = new RTCPeerConnection(pc_config);//在adapter.js中被包装过了

    pc.onicecandidate = onIceCandidate;

    console.log("Created RTCPeerConnnection with config:\n" + "  \"" +

      JSON.stringify(pc_config) + "\".");

  } catch (e) {

    console.log("Failed to create PeerConnection, exception: " + e.message);

    alert("Cannot create RTCPeerConnection object; WebRTC is not supported by this browser.");

      return;

  }

  pc.onconnecting = onSessionConnecting;//log status messages作用

  pc.onopen = onSessionOpened;            //log status messages作用

  pc.onaddstream = onRemoteStreamAdded;    //log status messages作用

  pc.onremovestream = onRemoteStreamRemoved;//为remoteVideo标签设置内容

}

//handler

function onRemoteStreamAdded(event) {

  // ...

  miniVideo.src = localVideo.src;

  attachMediaStream(remoteVideo, event.stream);

  remoteStream = event.stream;

  waitForRemoteVideo();

}

//在maybeStart()调用createPeerConnection()之后, a call is intitiated by creating and offer and sending it to the callee

function doCall() {

  console.log("Sending offer to peer.");

  pc.createOffer(setLocalAndSendMessage, null, mediaConstraints);

}

//创建offer的过程和非信令的例子（caller callee都在一个浏览器内）类似。

//不同点:message被发送到远端（remote peer），giving a serialized SessionDescription

//不同点的功能有setLocalAndMessage()完成

//客户端配置信息叫做Session Description

function setLocalAndSendMessage(sessionDescription) {

  // Set Opus as the preferred codec in SDP if Opus is present.

  sessionDescription.sdp = preferOpus(sessionDescription.sdp);

  pc.setLocalDescription(sessionDescription);

  sendMessage(sessionDescription);

}

/*signaling with the Channel API*/

/*当createPeerConnection()成功创建RTCPeerConnetion后 回调函数onIceCandidate被调用:

发送收集来的candidates的信息

*/

 function onIceCandidate(event) {

    if (event.candidate) {

    //使用XHR请求，客户端向服务器发送出站信息

      sendMessage({type: 'candidate',

        label: event.candidate.sdpMLineIndex,

        id: event.candidate.sdpMid,

        candidate: event.candidate.candidate});

    } else {

      console.log("End of candidates.");

    }

  }

//使用XHR请求，从客户端向服务器发送出站消息（Outbound messaging）

function sendMessage(message) {

  var msgString = JSON.stringify(message);

  console.log('C->S: ' + msgString);

  path = '/message?r=99688636' + '&u=92246248';

  var xhr = new XMLHttpRequest();

  xhr.open('POST', path, true);

  xhr.send(msgString);

}

/*

客户端->服务器发送信令消息：使用XHR

服务器->客户端发送信令消息：使用Google App Engine Channel API

*/

//处理由App Engine server发送来的消息

function processSignalingMessage(message) {

  var msg = JSON.parse(message);

  if (msg.type === 'offer') {

    // Callee creates PeerConnection

    if (!initiator && !started)//initiator代表session是否创建 RTCPeerConnection是否被创建

      maybeStart();

    pc.setRemoteDescription(new RTCSessionDescription(msg));

    doAnswer();

  } else if (msg.type === 'answer' && started) {

    pc.setRemoteDescription(new RTCSessionDescription(msg));

  } else if (msg.type === 'candidate' && started) {

    var candidate = new RTCIceCandidate({sdpMLineIndex:msg.label,

                                         candidate:msg.candidate});

    pc.addIceCandidate(candidate);//??

  } else if (msg.type === 'bye' && started) {

    onRemoteHangup();

  }

}

function doAnswer() {

  console.log("Sending answer to peer.");

  pc.createAnswer(setLocalAndSendMessage, null, mediaConstraints);

}

//在哪里设置msg.type？