class EventBus {
  constructor(){
    this.eventList = new Map()
  }

  $emit(ename, params, once = false){
    const cbList = this.eventList.get(ename)
    cbList && cbList instanceof Array && cbList.forEach(
        cb => typeof cb === 'function' && cb(params)
      );
    once && this.$off(ename)
  }

  $on(ename, cb){
    if(!ename || !cb) return;
    let cbList = this.eventList.get(ename)
    !cbList && (cbList = [])
    cbList.push(cb)
    this.eventList.set(ename,cbList)
    return cb
  }

  $off(ename,cb){
    // 默认清空所有事件
    !ename && (this.eventList = new Map())
    // 不传cb, 则清除ename下所有事件
    !cb && (this.eventList.get(ename) = [])
    const cbList = this.eventList.get(ename)?.filter(callback=>callback!=cb)
    this.eventList.set(ename,cbList)
  }
}

// 防抖：函数在某一时间内多次调用只有一次执行
// 第一次执行会设置一个定时器并缓存，
// 之后每次执行就会清除之前的定时器并重新设置
// 定时器计时结束后触发函数执行
function debounce(cb, time){
  // 定时器
  let timer = null;
  return (...args)=>{
    if(timer) {
      clearTimeout(timer)
    }
    timer = setTimeout(()=>{
      cb(...args)
    }, time)
  }
}

// 节流函数，在一定时间内只允许一次函数执行
function throttle(cb,time){
  let flag = false
  return (...args)=>{
    if(flag) return
    flag = true
    setTimeout(()=>{
      cb(...args)
      flag = false
    },time)
  }
}

window.onmouseover = debounce(e=>{
  console.log(e);
},500)