使用信令服务器来限制发送方数量

当使用 Android TV 进行投屏时，系统默认会限制发送方的数量，以避免网络拥堵和降低投屏质量。如果需要增加发送方数量，需要使用信令服务器来进行限制。

下面是一个使用 WebSocket 进行信令的代码示例：

// 客户端代码
const signaling = new WebSocket('ws://example.com/signaling');

// 发送 offer
signaling.send(JSON.stringify({
  type: 'offer',
  sender: 'abc',
  receiver: 'def',
  offer: '...'
}));

// 接收 answer
signaling.onmessage = ({ data }) => {
  const { type, sender, receiver, answer } = JSON.parse(data);
  if (type === 'answer' && sender === 'def' && receiver === 'abc') {
    // 处理 answer
  }
};

// 服务器代码
const WebSocket = require('ws');
const wss = new WebSocket.Server({ port: 8080 });

const clients = new Map();

wss.on('connection', (socket) => {
  socket.on('message', (msg) => {
    const { type, sender, receiver } = JSON.parse(msg);
    if (type === 'offer') {
      if (!clients.has(receiver)) {
        socket.send(JSON.stringify({ type: 'error', message: 'Receiver not found' }));
      } else {
        const receiverSocket = clients.get(receiver);
        if (receiverSocket.ready) {
          socket.send(JSON.stringify({ type: 'error', message: 'Receiver busy' }));
        } else {
          receiverSocket.send(msg);
          receiverSocket.ready = true;
        }
      }
    } else if (type === 'answer') {
      if (!clients.has(sender)) {
        socket.send(JSON.stringify({ type: 'error', message: 'Sender not found' }));
      } else {
        const senderSocket = clients.get(sender);
        senderSocket.send(msg);
        senderSocket.ready = false;
      }
    }
  });

  socket.on('close', () => {
    for (const [id, client] of clients.entries()) {
      if (client === socket