EventEmitter.ts

/**
 * @see {@link https://nodejs.org/api/events.html|node EventEmitter}
 */

type listener = {
  listener?: Function
} & Function

class EventEmitter {
  // 默认最大订阅数10
  static defaultMaxListeners = 10
  listenerStore: Record<string, listener[] | undefined>
  maxListenerCount: number
  constructor({ maxListenerCount }: { maxListenerCount?: number }) {
    // 使用对象而不是数组是因为一个 emitter 可以订阅不同的事件
    this.listenerStore = {}
    this.maxListenerCount = maxListenerCount || EventEmitter.defaultMaxListeners
  }
  // 添加订阅
  on(eventName: string, listener: Function, prepend: boolean = false) {
    if (!this.listenerStore[eventName]) {
      this.listenerStore[eventName] = [listener]
    } else if (prepend) {
      this.listenerStore[eventName]!.unshift(listener)
    } else {
      this.listenerStore[eventName]!.push(listener)
    }

    // maxListenersCount 等于 0 时等同于 Infinity
    const currentListenersCount = this.listenerStore[eventName]!.length
    // 添加的事件超出最大事件数会报警告
    if (currentListenersCount > (this.maxListenerCount || Infinity)) {
      console.warn(
        `You had add ${currentListenersCount}listeners, more than the max listeners count: ${this.maxListenerCount}`
      )
    }

    return this
  }

  // 删除订阅 如果同一个函数被多次添加为 listener，需要多次调用 off
  off(eventName: string, listener: Function) {
    if (!this.listenerStore[eventName]) {
      return
    }

    const index = this.listenerStore[eventName]!.findIndex(
      (existListener) => existListener === listener || existListener.listener === listener
    )
    if (index !== -1) {
      this.listenerStore[eventName]!.splice(index, 1)
    }

    return this
  }

  // 只订阅一次 使用闭包在执行后 off 掉自身即可
  once(eventName, listener, prepend = false) {
    const onceListener = (...args) => {
      listener(...args)
      this.off(eventName, onceListener)
    }
    onceListener.listener = listener
    this.on(eventName, onceListener, prepend)

    return this
  }

  // 按照注册的顺序同步执行所有事件名为 event 的 listener
  // 如果还有事件名为 event 的 listener 返回 true，否则返回 false
  emit(eventName, ...args) {
    if (this.listenerStore[eventName] && this.listenerStore[eventName]!.length > 0) {
      const listenersCopy = [...this.listenerStore[eventName]!]
      listenersCopy.forEach((listener) => listener(...args))
      return true
    }

    return false
  }

  listeners(eventName) {
    return this.listenerStore[eventName]
      ? this.listenerStore[eventName]!.map((listener) =>
        listener.listener ? listener.listener : listener,
      )
      : []
  }

  rawListeners(eventName) {
    return this.listenerStore[eventName] ? this.listenerStore[eventName] : []
  }

  prependListener(eventName, listener) {
    return this.on(eventName, listener, true)
  }

  prependOnceListener(eventName, listener) {
    return this.once(eventName, listener, true)
  }

  addListener(eventName, listener) {
    return this.on(eventName, listener)
  }

  removeListener(eventName, listener) {
    return this.off(eventName, listener)
  }

  getMaxListeners() {
    return this.maxListenerCount
  }

  setMaxListeners(n) {
    // 必须为非负数
    if (typeof n !== 'number' || n < 0) {
      throw new Error(
        `The value of "n" is out of range. It must be a non-negative number. Received '${n}'`,
      )
    }

    this.maxListenerCount = n

    return this
  }

  removeAllListeners(eventName) {
    if (!eventName) {
      Object.keys(this.listenerStore).forEach((_eventName) => {
        this.listenerStore[_eventName] = undefined
      })
    } else {
      this.listenerStore[eventName] = undefined
    }

    return this
  }
}