第一章 WebRTC入门

1.1 绪论

WebRTC是Web Real-Time Communications (RTC) 。WebRTC是一个基于标准化技术的行业性项目，旨在将实时通信功能引入到所有浏览器中，并通过标准的[HTML5]标签和JavaScript APl使这些功能可为Web开发者所用，目前由万维网联盟（ World Wide Web Consortium (W3C)）和互联网工程任务组（Internet Engineering Task Force (IETF)）联合负责WebRTC的标准化工作。

1.2 资源

• adapter.js
• webrtc官网https://webrtc.org/
• W3C Spec (getUserMedia): http://w3c.github.io/mediacapture-main/getusermedia.html
• W3C Spec (WebRTC 1.0: Real-time Communication Between Browsers): http://dev.w3.org/2011/webrtc/editor/webrtc.html
  https://www.w3.org/TR/webrtc/
• ❤❤❤ w3c标准： WebRTC1.0 浏览器间的实时通信(网易云信翻译版，在yasepix@163.com中有)
• W3C Spec (mediacapture-streams): https://www.w3.org/TR/mediacapture-streams/
• W3C Spec (WebRTC Stats): http://w3c.github.io/webrtc-stats/
• W3C Spec (Screen Capture): https://w3c.github.io/mediacapture-screen-share/

1.3 Simple Peer-to-peer Example

const signaling = new SignalingChannel(); // handles JSON.stringify/parse
const constraints = {audio: true, video: true};
const configuration = {iceServers: [{urls: 'stuns:stun.example.org'}]};
const pc = new RTCPeerConnection(configuration);

// send any ice candidates to the other peer
pc.onicecandidate = ({candidate}) => signaling.send({candidate});

// let the "negotiationneeded" event trigger offer generation
pc.onnegotiationneeded = async () => {
  try {
    await pc.setLocalDescription(await pc.createOffer());
    // send the offer to the other peer
    signaling.send({desc: pc.localDescription});
  } catch (err) {
    console.error(err);
  }
};

// once media for a remote track arrives, show it in the remote video element
pc.ontrack = (event) => {
  // don't set srcObject again if it is already set.
  if (remoteView.srcObject) return;
  remoteView.srcObject = event.streams[0];
};

// call start() to initiate
async function start() {
  try {
    // get a local stream, show it in a self-view and add it to be sent
    const stream = await navigator.mediaDevices.getUserMedia(constraints);
    stream.getTracks().forEach((track) => pc.addTrack(track, stream));
    selfView.srcObject = stream;
  } catch (err) {
    console.error(err);
  }
}

signaling.onmessage = async ({desc, candidate}) => {
  try {
    if (desc) {
      // if we get an offer, we need to reply with an answer
      if (desc.type == 'offer') {
        await pc.setRemoteDescription(desc);
        const stream = await navigator.mediaDevices.getUserMedia(constraints);
        stream.getTracks().forEach((track) => pc.addTrack(track, stream));
        await pc.setLocalDescription(await pc.createAnswer());
        signaling.send({desc: pc.localDescription});
      } else if (desc.type == 'answer') {
        await pc.setRemoteDescription(desc);
      } else {
        console.log('Unsupported SDP type. Your code may differ here.');
      }
    } else if (candidate) {
      await pc.addIceCandidate(candidate);
    }
  } catch (err) {
    console.error(err);
  }
};

从这个程序中抽取几个基本的对象出来，以后各节详细解析。这几个对象是：

• navigator.mediaDevices.getUserMedia
• RTCPeerConnection
• SignalingChannel