发布-订阅模式

class EventEmitter {
  constructor(options = {}) {
    this.events = {};
    this.maxListeners = options.maxListeners || Infinity;
  }
  /**
   * 发布事件
   * @param {String} event 事件类型
   * @param  {...any} args 参数列表，把emit传递的参数赋给回调函数
   */
  emit = (event, ...args) => {
    const cbs = this.events[event];

    if (!cbs) {
      console.warn(`${event} is not registered`);
    }

    cbs.forEach((cb) => cb.apply(this, args));
    return this;
  };
  /**
   * 注册事件监听者
   * @param {String} event 事件类型
   * @param {Function} cb 回调函数
   */
  on = (event, cb) => {
    if (!this.events[event]) {
      this.events[event] = [];
    }
    if (
      this.maxListeners !== Infinity &&
      this.events[event].length >= this.maxListeners
    ) {
      console.log(`${event} has reached max listeners`);
      return this;
    }
    this.events[event].push(cb);

    return this;
  };

  /**
   * 注册事件监听者，仅执行依次回调
   * @param {String} event 事件类型
   * @param {Function} cb 回调函数
   */
  once = (event, cb) => {
    // 用完进行销毁
    const func = (...args) => {
      this.off(event, func);
      cb.apply(this, args);
    };

    this.on(event, func);
    return this;
  };
  /**
   * 移除某个事件的一个监听者
   * @param {String} event 事件类型
   * @param {Function} cb 回调函数
   */
  off = (event, cb) => {
    if (!cb) {
      this.events[event] = null;
    } else {
      this.events[event] = this.events[event].filter((item) => cb !== item);
    }

    return this;
  };
}