// 手寫EventBus
class EventBus{
  constructor(){
    this._event = {}
  }
  // 注冊(cè)
  $on(eventName,fn){
    this._event[eventName] = this._event[eventName] || []
    this._event[eventName].push(fn)
  }
  // 觸發(fā)
  $emit(eventName,args){
    if(!this._event[eventName]) {
      console.log('未注冊(cè)該事件');
      return // 未注冊(cè)該事件
    }
    this._event[eventName].forEach(fn => {
      fn(args)
    });
  }
  // 移除
  $remove(eventName,fn){
    if(!this._event[eventName]){
      console.log('未綁定該事件');
      return // 未綁定該事件
    }
    if(!fn){
      delete this._event[eventName]
      return
    }
    const eventHandler = this._event[eventName]
    const handlerIndex = eventHandler.findIndex(it => it === handler)
    this._event[eventName].splice(handlerIndex,1)
    if(this._event[eventName].length === 0){
      delete this._event[eventName]
    }
  }
  $once(eventName,args){
    this.$emit(eventName,args)
    this.$remove(eventName)
  }
}
const event_bus = new EventBus
const fn1 = (param) => {
  console.log('fn1',param)
}
event_bus.$on('testfunc',fn1)
event_bus.$once('testfunc',123)
event_bus.$emit('testfunc',123)