手写事件监听（发布订阅） ​

export class Observer {
  private eventsMap = new Map()

  /**
   * 绑定
   * @param eventName
   * @param callback
   */
  on(eventName: string, callback: (...args) => void) {
    if (this.eventsMap.has(eventName))
      this.eventsMap.get(eventName).push(callback)
    else
      this.eventsMap.set(eventName, [callback])
  }

  /**
   * 发射
   * @param eventName
   * @param args
   */
  emit(eventName: string, ...args) {
    this.eventsMap.get(eventName).forEach((fn: (...args) => void) => {
      fn(...args)
    })
  }

  /**
   * 移除
   * @param eventName
   */
  off(eventName: string) {
    this.eventsMap.delete(eventName)
  }
}