手写一个发布-订阅模式 #110

class EventEmitter {
  constructor() {
    // 初始值为空对象
    this.listeners = Object.create(null)
  }
  
  // 注册事件
  on = (event, listener) => {
    // this.bindEvent(event, listener, false)
    if (!event || !listener) {
      return
    }
    if (this.listeners[event]) {
        //如果有就放一个新的
        this.listeners[event].push(listener)
    } else {
        //如果没有就创建一个数组
        this.listeners[event] = [listener]
    }
  }
  
  // 只订阅一次
  once = (event, listener) => {
    // this.bindEvent(event, listener, true)
    function one() {
        // 在one函数运行原来的函数，只有将one清空
        listener.apply(this, arguments)
        // 先绑定 执行后再删除
        this.off(event)
    }
      this.on(event, one)
      //此时emit触发会执行此函数，会给这个函数传递rest参数
  }
  
  // 发布事件
  emit = (event, ...args) => {
    if (!this.hasBind(event)) {
      console.warn(`this event: ${event} don't has bind listener.`)
      return
    }

    this.listeners[event].forEach(listener => listener.call(this, ...args))
  }
  
  // 取消订阅
  off = (event, listener) => {
    if (!this.hasBind(event)) {
      console.warn(`this event: ${event} don't exist.`)
      return
    }

    // remove all listener
    if (!listener) {
      delete this.listeners[event]
      return
    }

    // remove specific listener
    this.listeners[event] = this.listeners[event].filter(item => item !== listener)
  }

  hasBind = event => {
    return this.listeners[event]
      && this.listeners[event].length
  }
}

// 测试
const baseEvent = new EventEmitter()
function cb(value){
    console.log("hello "+value)
}
baseEvent.once("click",cb)
// baseEvent.off("click")
baseEvent.emit("click",'2020')
// hello 2020
console.log(baseEvent.listeners)