Jquery - 超出屏幕时把固定层变为定位层

相信很多人都上过taobao吧，在taobao的产品列表页有一个浮动的用来排序的浮动层，当你拖动滚动条而导致那个排序栏看不到的时候它会自动变为浮动层，一直固定在那里。非常好的一个用户体验实例。如果不明白的话就看下面的图片吧
淘宝产品列表 http://search1.taobao.com/browse/50010388/…

腾讯财经产业经济 http://finance.qq.com/industry/index.htm

如何实现？
scrollTop是什么？当元素内部的内容高度超过元素本身的高度时就会出现滚动条，随着拖动滚动条而被隐去的上面部分的高度就是scrollTop值。
这种定位层就是当页面触发onscroll事件时，取得scrollTop值并写入定位层CSS属性，使之保持位置。
下面是实例代码，请复制到本地运行

< !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" >
< html xmlns ="http://www.w3.org/1999/xhtml" >
< head >
< meta http-equiv ="Content-Type" content ="text/html; charset=utf-8" />
< title >定位层 </title>
< script type ="text/javascript" src ="http://www.jqueryajax.com/wp-content/uploads/2009/03/jquery-132min2.js" > </script>
< style type ="text/css" >
body{padding:10px;margin:0;}
#box{width:960px;margin:0 auto;}
#box1{width:250px;height:25px;position:relative;}
#box1_1{width:250px;height:25px;line-height:25px;background:#f00;color:#fff;}
p{font-size:24px;margin:10px 0;}
</style>
< script type ="text/javascript" >
function tools(){
var top=$(document).scrollTop();
if(($.browser.msie==true)&&($.browser.version==6.0)){
if(top>168)$("#box1_1").css({position:"absolute",top:top-168});
}else{
if(top>168)$("#box1_1").css({position:"fixed",top:"-"&top+"px"});
}
if(top<=168)$("#box1_1").css({position:"static",top:0});
}
$(function(){
window.onscroll=tools;
window.onresize=tools;
});
</script>
</head>
< body >
< div id ="box" >
< p >往下拖动滚动条↓ < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< div id ="box1" > < div id ="box1_1" >这里是定位层 </div> </div>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
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< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
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< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
< p > < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jqueryajax </a> < a href ="http://www.jqueryajax.com" >jquery教程网 </a> < a href ="http://www.itfeed.cn" > </a> </p>
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</div>
</body>
</html>