用代码实现线上多人视频

实现线上多人视频的关键是要使用视频流媒体服务器。下面是一个基于 Node.js 和 WebRTC 的简单示例：

1. 安装 Node.js 和相关的模块：

npm install express socket.io webrtc

2. 创建一个 Express 应用程序并启动服务器：

const express = require('express');
const app = express();
const server = require('http').Server(app);
const io = require('socket.io')(server);
const port = 3000;

app.use(express.static('public'));

server.listen(port, () => {
  console.log(`Server listening on port ${port}`);
});

3. 在客户端使用 WebRTC 进行视频通信，使用 Socket.IO 进行信令传输：

const socket = io();

const constraints = { audio: true, video: true };
navigator.mediaDevices.getUserMedia(constraints)
  .then(stream => {
    const video = document.querySelector('video');
    video.srcObject = stream;

    const peerConnection = new RTCPeerConnection();
    stream.getTracks().forEach(track => peerConnection.addTrack(track, stream));

    peerConnection.addEventListener('icecandidate', event => {
      if (event.candidate) {
        socket.emit('icecandidate', event.candidate);
      }
    });

    socket.on('icecandidate', candidate => {
      peerConnection.addIceCandidate(candidate);
    });

    socket.on('offer', async offer => {
      await peerConnection.setRemoteDescription(offer);
      const answer = await peerConnection.createAnswer();
      await peerConnection.setLocalDescription(answer);
      socket.emit('answer', answer);
    });

    socket.on('answer', async answer => {
      await peerConnection.setRemoteDescription(answer);
    });

    peerConnection.addEventListener('track', event => {
      const remoteVideo = document.querySelector('remoteVideo');
      remoteVideo.srcObject = event.streams[0];
    });

    socket.emit('join');
  })
  .catch(error => {
    console.error(error);
  });

4. 在服务器端处理信令：

const peers = new Map();

io.on('connection', socket => {
  socket.on('join', () => {
    peers.set(socket.id, socket);
    if (peers.size === 1) {
      socket.emit('offer', offer);
    }
  });

  socket.on('icecandidate', candidate => {
    socket.broadcast.emit('icecandidate', candidate);
  });

  socket.on('answer', answer => {
    socket.broadcast.emit('answer', answer);
  });

  socket.on('disconnect', () => {
    peers.delete(socket.id);
  });
});

这是一个简单的示例，实际上在实现多人视频时还需要考虑更多的问题，如信令服务器的负载平衡、网络带宽管理等。建议在实际应用中使用成熟的视频流媒体服务器，如 Jitsi、Kurento 等。