复制
1function EventEmitter() {
2  this.events = new Map();
3}
4
5// 需要实现的一些方法：
6// addListener、removeListener、once、removeAllListeners、emit
7
8// 模拟实现addlistener方法
9const wrapCallback = (fn, once = false) => ({ callback: fn, once });
10EventEmitter.prototype.addListener = function(type, fn, once = false) {
11  const hanlder = this.events.get(type);
12  if (!hanlder) {
13    // 没有type绑定事件
14    this.events.set(type, wrapCallback(fn, once));
15  } else if (hanlder && typeof hanlder.callback === 'function') {
16    // 目前type事件只有一个回调
17    this.events.set(type, [hanlder, wrapCallback(fn, once)]);
18  } else {
19    // 目前type事件数>=2
20    hanlder.push(wrapCallback(fn, once));
21  }
22}
23// 模拟实现removeListener
24EventEmitter.prototype.removeListener = function(type, listener) {
25  const hanlder = this.events.get(type);
26  if (!hanlder) return;
27  if (!Array.isArray(this.events)) {
28    if (hanlder.callback === listener.callback) this.events.delete(type);
29    else return;
30  }
31  for (let i = 0; i < hanlder.length; i++) {
32    const item = hanlder[i];
33    if (item.callback === listener.callback) {
34      hanlder.splice(i, 1);
35      i--;
36      if (hanlder.length === 1) {
37        this.events.set(type, hanlder[0]);
38      }
39    }
40  }
41}
42// 模拟实现once方法
43EventEmitter.prototype.once = function(type, listener) {
44  this.addListener(type, listener, true);
45}
46// 模拟实现emit方法
47EventEmitter.prototype.emit = function(type, ...args) {
48  const hanlder = this.events.get(type);
49  if (!hanlder) return;
50  if (Array.isArray(hanlder)) {
51    hanlder.forEach(item => {
52      item.callback.apply(this, args);
53      if (item.once) {
54        this.removeListener(type, item);
55      }
56    })
57  } else {
58    hanlder.callback.apply(this, args);
59    if (hanlder.once) {
60      this.events.delete(type);
61    }
62  }
63  return true;
64}
65EventEmitter.prototype.removeAllListeners = function(type) {
66  const hanlder = this.events.get(type);
67  if (!hanlder) return;
68  this.events.delete(type);
69}
70