code: function (fn, after) {
      if (typeof fn === FUNCTION) {
        this._subs[after ? AFTER : ON].push(fn)
      }
  
      return this
    }
docs: 添加change事件
  @method change
  @param {Function} fn 回调函数
  @param {Boolean} after 时候绑定after事件
  @chainable