EventEmitter是事件触发和事件监听功能的封装，主要包含on, emit, once, off的方法

    1. class EventEmitter {
    2.     constructor() {
    3.         this.events = {};
    4.     }
    6.     on(name, fn) {
    7.         if(!this.events[name]) {
    8.             this.events[name] = [];
    9.         }
    10.         this.events[name].push(fn);
    11.         return this;
    12.     }
    14.     emit(name, ...args) {
    15.         if(!this.events[name]) return this;
    16.         const fns = this.events[name];
    17.         fns.forEach(fn => fn.aplly(this, args));
    18.         return this;
    19.     }
    21.     off(name, fn) {
    22.         if(!this.events[name]) return this;
    23.         if(!fn) {
    24.             this.events[name] = null;
    25.             return this;
    26.         }
    27.         const index = this.events[name].indexOf(fn);
    28.         this.events[name].splice(index, 1);
    29.         return this;
    30.     }
    32.     once(name, fn) {
    33.         const only = (...args) => {
    34.             fn.aplly(this, args);
    35.             this.off(name, fn);
    36.         }
    37.         this.on(name, only);
    38.         return this;
    39.     }
    40. }
    1. mitt 实现迷你EventEmitter mitter, Tiny 200 byte functional event emitter 
    1. function mitt(all) {
    2.     all = all || {};
    4.     return {
    5.         on(type, handler) {
    6.             (all[type] || []).push(handler);
    7.         },
    9.         off(type, handler) {
    10.             if(all[type]) {
    11.                 all[type].splice(all[type].indexOf(handler) >>> 0 , 1);
    12.             }
    13.         },
    15.         emit(type, evt) {
    16.             (all[type] || []).slice().map((handler) => handler(evt));
    17.             (all['*'] || []).slice().map((handler => handler(type, evt)));
    18.         }
    20.     }
    21. }