EventUtil.addHandler(window,‘scroll‘,function(){
var dafid = document.getElementById(‘dafid‘),
eletop = dafid.getBoundingClientRect().top;
if(eletop-549 <= 0){
dafid.className = ‘daf‘;
}else{
dafid.className = ‘‘;
}
});
原文：http://www.cnblogs.com/xupeiyu/p/3850290.html