iframe监听页面是否加载完成

<iframe id="aaa" src=" http://www.baidu.com"></iframe>

<script type="text/javascript">

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