code: function(){
              var top = reElem.offset().top + reElem.outerHeight() + 5 - $win.scrollTop()
              ,dlHeight = dl.outerHeight();
              
              index = select[0].selectedIndex; //获取最新的 selectedIndex
              reElem.addClass(CLASS+'ed');
              dds.removeClass(HIDE);
              nearElem = null;
  
              //初始选中样式
              dds.eq(index).addClass(THIS).siblings().removeClass(THIS);
  
              //上下定位识别
              if(top + dlHeight > $win.height() && top >= dlHeight){
                reElem.addClass(CLASS + 'up');
              }
              
              followScroll();
            }
docs: 展开下拉