import GLOBAL from './global';
class Observer {
  constructor() {
    this._event = {}
  }
  $on(eventName, handler) {
    if (this._event[eventName]) {
      this._event[eventName].push(handler)
    } else {
      this._event[eventName] = [handler]
    }
  }

  $emit(eventName) {
    let events = this._event[eventName]
    let otherArgs = Array.prototype.slice.call(arguments, 1)
    let that = this
    if (events) {
      events.forEach(event => {
        event.apply(that, otherArgs)
      })
    }
  }
  $off(eventName, handler) {
    let events = this._event[eventName]
    if (events) {
      this._event[eventName] = events.filter(event => {
        return event !== handler
      })
    }
  }
  $once(eventName, handler) {
    let that = this
    function func() {
      let args = Array.prototype.slice.call(arguments, 0)
      handler.apply(that, args)
      this.off(eventName, func)
    }
    this.on(eventName, func)
  }
}
if (!GLOBAL.observer) {
  GLOBAL.observer = new Observer()
}
export default GLOBAL.observer