如何提取iframe里的内容

怎么提取iframe里的内容
<iframe   id= "test "   src= "a.htm "> </iframe>
怎样获取iframe里的a.htm内容?

[解决办法]
document.getElementById( 'test ').document.body.innerHTML
[解决办法]
<iframe src= " " id= "innerFrame " width= "0 " height= "0 " onload= "if(this.src)main_add.innerHTML=innerFrame.document.body.innerHTML; "> </iframe>
<script>
<div id= "main_add "> 加载中。。。 </div>
document.getElementById( 'innerFrame ').src = url;
</script>
[解决办法]
document.getElementById( 'iframeName ').document.body.innerHTML