原文參考： https://www.cxymsg.com/guide/jsWritten.html#%E5%AE%9E%E7%8E%B0event-event-bus

class EventEmitter {
  constructor() {
    this._event = this._event || new Map() // 儲(chǔ)存事件
    this._maxListeners = this._maxListeners || 10 // 設(shè)置監(jiān)聽(tīng)上限
  }
}

EventEmitter.prototype.emit = function (type, ...args) {
  let handler = null
  handler = this._event.get(type)

  if (Array.isArray(handler)) {
    // 有多個(gè)監(jiān)聽(tīng)者,需要依次觸發(fā)
    for (let i = 0; i < handler.length; i++) {
      if (args.length > 0) {
        handler[i].apply(this, args)
      } else {
        handler[i].call(this)
      }
    }
  } else if (handler && typeof handler === 'function') {
    if (args.length > 0) {
      handler.apply(this, args)
    } else {
      handler.call(this)
    }
  }

  return true
}

EventEmitter.prototype.addListener =  function (type, fn) {
  let handler = this._event.get(type)

  if (!handler) {
    this._event.set(type, fn)
  } else if (handler && typeof handler === 'function') {
    // 如果handler是函數(shù)，說(shuō)明目前已經(jīng)存在一個(gè)監(jiān)聽(tīng)者
    this._event.set(type, [handler, fn])
  } else {
    // 已經(jīng)有多個(gè)監(jiān)聽(tīng)者，直接push
    handler.push(fn)
  }
}

EventEmitter.prototype.removeListener = function (type, fn) {
  let handler = this._event.get(type)

  if (handler && typeof handler === 'function') {
    // 只有一個(gè)監(jiān)聽(tīng)者，直接刪除
    this._event.delete(type, fn)
  } else if (Array.isArray(handler)) {
    // 是數(shù)組，說(shuō)明被監(jiān)聽(tīng)多次，要找到對(duì)應(yīng)的函數(shù)
    let position = -1

    for(let i = 0; i < handler.length; i++) {
      if (handler[i] === fn) {
        position = i
      }
    }

    // 如果匹配，從數(shù)組中移除
    if (position !== -1) {
      handler.splice(position, 1)

      // 移除后，如果監(jiān)聽(tīng)只剩一個(gè)，那么取消數(shù)組，以函數(shù)形式保存
      if (handler.length === 1) {
        this._event.set(type, handler[0])
      }
    } else {
      return this
    }
  }
}