详解如何模拟实现node中的Events模块(通俗易懂版)

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('嗨', (str) => {
 console.log(str);
});
myEmitter.emit('嗨','你好');

class EventEmitter{
 constructor(){
  #事件监听函数保存的地方
  this.events={};
 }
 on(eventName,listener){
  if (this.events[eventName]) {
   this.events[eventName].push(listener);
  } else {
   #如果没有保存过，将回调函数保存为数组
   this.events[eventName] = [listener];
  }
 }
 emit(eventName){
  #emit触发事件，把回调函数拉出来执行
  this.events[eventName] && this.events[eventName].forEach(listener => listener())
 }
}

let event = new EventEmitter();
event.on('嗨',function(){
 console.log('你好');
});
event.emit('嗨');
#输出：你好

emit(eventName,...rest){
 #emit触发事件，把回调函数拉出来执行
 this.events[eventName] && this.events[eventName].forEach(listener => listener.apply(this,rest))
}

let event = new EventEmitter();
event.on('嗨',function(str){
 console.log(str);
});
event.emit('嗨','你好');
#输出：你好

EventEmitter.prototype.addListener=EventEmitter.prototype.on

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let callback = (str) => {
 console.log(str);
}
myEmitter.on('嗨', callback);
myEmitter.emit('嗨','你好');#输出：你好
myEmitter.removeListener('嗨',callback);
myEmitter.emit('嗨','你好');#无输出

removeListener (eventName,listener) {
 #保证回调函数数组存在，同时去除指定的listener
 this.events[eventName] && this.events[eventName] = this.events[eventName].filter(l => l != listener);
}

EventEmitter.prototype.removeListener=EventEmitter.prototype.off

removeAllListeners (eventName) {
 #移除全部监听器
 delete this.events[eventName]
}

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.once('嗨', (str) => {
 console.log(str);
});
myEmitter.emit('嗨','你好');
myEmitter.emit('嗨','你好');
myEmitter.emit('嗨','你好'); #只能输出一次你好

once (eventName, listener) {
 #重新改变监听回调函数，使其执行之后可以被销毁
 let reListener = (...rest) => {
  listener.apply(this,rest);
  #执行完之后解除事件绑定
  this.removeListener(type,wrapper);
 }
 this.on(eventName,reListener);
}

listeners (eventName) {
 return this.events[eventName]
}

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
let callback = (str) => {
 console.log(str);
}
for (let i = 0; i <= 11; i++) {
 myEmitter.on('嗨', callback);
}
myEmitter.emit('嗨', '你好');

constructor(){
 #事件监听函数保存的地方
 this.events={};
 #最大监听器数量
 this._maxListeners = 10;
}

on(eventName,listener){
 if (this.events[eventName]) {
  this.events[eventName].push(listener);
  #如果超过最大限度，以及不为0，则作出内存泄漏提示
  if (this._maxListeners != 0 && this.events[type].length >= this._maxListeners) {
   console.error('超过最大的监听数量可能会导致内存泄漏');
  }
 } else {
  #如果没有保存过，将回调函数保存为数组
  this.events[eventName] = [listener];
 }
}

setMaxListeners(MaxListeners) {
 this._maxListeners = MaxListeners
}