原理

原理就是发布-订阅。
本次实现了几个功能

  • on 注册事件
  • emit 触发事件
  • off 注销事件
  • once 执行一次后注销事件
  • newListener 每次注册新事件时触发

代码如下：

  1. function EventEmitter() {
  2.     this._events = {};
  3. }
  5. EventEmitter.prototype.on = function (eventName, callback) {
  6.     if (!this._events) {
  7.         this._events = Object.create(null);
  8.     }
  9.     if (eventName !== 'newListener') {
  10.         this._events['newListener'] ? this._events['newListener'].forEach(fn => fn.call(this, eventName)) : void 0;
  11.     }
  12.     if (this._events[eventName]) {
  13.         this._events[eventName].push(callback);
  14.     } else {
  15.         this._events[eventName] = [callback];
  16.     }
  17. }
  19. EventEmitter.prototype.emit = function (eventName, ...args) {
  20.     if (!this._events) {
  21.         this._events = Object.create(null);
  22.     }
  23.     if (this._events[eventName]) {
  24.         this._events[eventName].forEach(fn => fn.call(this, ...args))
  25.     }
  26. }
  28. EventEmitter.prototype.off = function (eventName, callback) {
  29.     if (!this._events) {
  30.         this._events = Object.create(null);
  31.     }
  32.     if (this._events[eventName]) {
  33.         this._events[eventName] = this._events[eventName].filter(fn => {
  34.             return fn !== callback && fn.l !== callback
  35.         })
  36.     }
  37. }
  39. EventEmitter.prototype.once = function (eventName, callback) {
  40.     if (!this._events) {
  41.         this._events = Object.create(null);
  42.     }
  43.     function one(...args) {
  44.         callback(...args);
  45.         this.off(eventName, one)
  46.     }
  47.     one.l = callback;
  48.     this.on(eventName, one);
  49. }
  51. module.exports = EventEmitter;