class EventEmitter {
    constructor() {
      this.events = {}
    }

    // 订阅事件
    on(evt, callback, ctx) {
      if (!this.events[evt]) {
        this.events[evt] = []
      }
      this.events[evt].push(callback)

      return this
    }

    // 发布事件
    emit(evt, ...payload) {
      const callbacks = this.events[evt]

      if (callbacks) {
        callbacks.forEach(cb => cb.call(this, payload));
      }

      return this
    }

    // 删除订阅
    off(evt, callback) {
      if (typeof evt === "undefined") {
        delete this.events
      } else if (typeof evt === "string") {
        if (typeof callback === 'function') {
          this.events[evt] = this.events[evt].filter((cb) => cb !== callback)
        } else {
          delete this.events[evt]
        }
      }
    }

    // 只触发一次的订阅
    once(evt, callback, ctx) {
      const proxyCallback = (...payload) => {
        callback.apply(ctx, payload)
        this.off(evt, proxyCallback)
      }

      this.on(evt, proxyCallback, ctx)
    }
}