图片预加载

function aftLoadImg(obj,url){ //图片地址切换
     var oImg = new Image();
          oImg.src = url;
     if(oImg.complete){//图片加载完毕并且显示
            obj.src = oImg.src;
      }else{
             oImg.onload = function() {
             obj.src = oImg.src;
      };
}