code: function(item, height){
        var start = scrollElem.scrollTop(), end = start + height;
        var elemTop = notDocment ? function(){
          return item.offset().top - scrollElem.offset().top + start;
        }() : item.offset().top;
  
        /* 始终只加载在当前屏范围内的图片 */
        if(elemTop >= start && elemTop <= end){
          if(!item.attr('src')){
            var src = item.attr('lay-src');
            layui.img(src, function(){
              var next = that.lazyimg.elem.eq(index);
              item.attr('src', src).removeAttr('lay-src');
              
              /* 当前图片加载就绪后，检测下一个图片是否在当前屏 */
              next[0] && render(next);
              index++;
            });
          }
        }
      }
docs: 显示图片