function getTop(e)
{
var offset=e.offsetTop;
if(e.offsetParent!=null) //只要还有父元素,也就是当前元素不是根节点就继续往上累计元素的高度
offset+=getTop(e.offsetParent);
return offset;
}
var myBlockTop = getTop(document.getElementById("homepageBanner"));
var oneDiv=document.getElementById("homepageBanner");
if(!!window.attachEvent)//ie浏览器下。
{
window.attachEvent('onscroll',function(){
if(document.documentElement.scrollTop/* + (document.body.clientHeight || window.innerHTML) */>= myBlockTop)
{oneDiv.style.position="fixed";oneDiv.style.top="0";oneDiv.style.left="50%";oneDiv.style.marginLeft="-500px";}
else{oneDiv.removeAttribute("style");}
});
}
if(!!window.addEventListener)//非ie浏览器下
{
window.addEventListener("scroll",function(){//document.body.scrollTop可保证chrome的正常。
if(document.documentElement.scrollTop/* + (document.body.clientHeight || window.innerHTML) */>= myBlockTop||document.body.scrollTop>=myBlockTop)
{oneDiv.style.position="fixed";oneDiv.style.top="0";oneDiv.style.left="50%";oneDiv.style.marginLeft="-500px";}
else{oneDiv.style.position="";oneDiv.style.top="auto";oneDiv.style.left="auto";oneDiv.style.marginLeft="0";oneDiv.removeAttribute("style");}
});
}