code: function Headroom(selector, opts) {
      var _this = this;
  
      _this.$headroom = $(selector).eq(0);
      if (!_this.$headroom.length) {
        return;
      }
  
      // 已通过自定义属性实例化过，不再重复实例化
      var oldInst = _this.$headroom.data('mdui.headroom');
      if (oldInst) {
        return oldInst;
      }
  
      _this.options = $.extend({}, DEFAULT, (opts || {}));
  
      // 数值转为 {down: bum, up: num}
      var tolerance = _this.options.tolerance;
      if (tolerance !== Object(tolerance)) {
        _this.options.tolerance = {
          down: tolerance,
          up: tolerance,
        };
      }
  
      _this._init();
    }
docs: Headroom
  @param selector
  @param opts
  @constructor