如何判斷iframe中的頁面加載完畢

<script type="text/javascript">

var a = document.getElementById('aaa');
var iframeLoad = function(){
alert('IFRAME加載完成了!!');
}
if(!a.addEventListener){
a.attachEvent('onload', iframeLoad)
}
a.addEventListener('load', iframeLoad, true);
</script>