event

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事件 demo 
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class EventEmitter {
  constructor() {
    this._events = new Map()
  }
  addListener(type, fn) {
    const handler = this._events.get(type)
    if (!handler) {
      this._events.set(type, fn)
    } else if (handler && typeof handler === 'function') {
      this._events.set(type, [handler, fn])
    } else {
      handler.push(fn)
    }
  }
  emit(type, ...args) {
    let handler = this._events.get(type)
    if (Array.isArray(handler)) {
      for (let i = 0; i < handler.length; i++) {
        if (!!args.length) {
          handler[i].apply(this, args)
        } else {
          handler[i].call(this)
        }
      }
    } else {
      if (!!args.length) {
        handler.apply(this, args)
      } else {
        handler.call(this)
      }
    }
    return true
  }
}

let emitter = new EventEmitter()
emitter.addListener('say', (what) => {
  console.log(`say ${what}`)
})
emitter.addListener('say', (what) => {
  console.log(`say ${what} too`)
})
emitter.emit('say', 'hello')