(1)利用原生的js进行判断：

window.οnscrοll=function(){

        var seeHeight = document.documentElement.clientHeight;

        var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;

        for(var i=l;i<imgNum;i++){

            if(imgObj[i].offsetTop < seeHeight + scrollTop){

                if(imgObj[i].src == ""){

                    imgObj[i].src = imgObj[i].getAttribute("lazysrc");

                }

            }

            if(imgObj[i].offsetTop > seeHeight + scrollTop){

                l=i;

                break;

            }

        }

}

（2）利用jquery进行判断:

$(window).bind("scroll", function(event){

     

      $("div").each(function(){

         

           var fold = $(window).height() + $(window).scrollTop();

          

           if( fold <= $(this).offset().top){

              

                 $(this).trigger("appear");

       }

      });

 });