src/lay/modules/flow.js
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++;
          });
        }
      }
    }
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++;
          });
        }
      }
    }
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