获取iframe的内容(checkbox)

<script type="text/javascript">
var id;
function selectcheckbox(){
var t=0;
var boxnum = document.frames["dzdaiframe"].document.getElementsByName('listForm.ids');
id = boxnum[0].value;
//此处是getElementsByName，得到的是数组，取值为boxnum[0].value;
//如果是id，则document.getElementById('listForm.ids'); 取值为boxnum.value;
for (var i=0;i<boxnum.length;i++){
if(boxnum[i].checked){
t++;
}
}
if(t==0) {
alert("请至少选择一条记录");
return false;
}
if(t>1){
alert("对不起，一次只能操作1条记录.");
return false;
}
}
</script>

<div id="dzdaall" >
<iframe id="dzdaiframe" name="dzdaiframe" width="100%" onload="Javascript:SetWinHeightall(this)" frameborder="0"></iframe>
<center>
<input id="adddzda" type="button" value="增 加">
<input id="uppdzda" type="button" value="修 改" onclick="selectcheckbox()">
</center>
</div>

$(function(){
$("#mhdp").click(function(){
$("#dzdahdgi").hide();
$("#dzdaall").show();
$("#dzdaiframe").attr("src","${Context}/background/mhdp!search?Entity.root.class=com.egf.qingbao.jzyp.entities.YpMhdp&Entity.root.alias=o&o.sfzjhm.value=${zd.zdgzrysfzh}&Entity.forward=list&Entity.listStyle=mhdp");
$("#adddzda").click(function(){
window.location.href="${Context}/background/mhdp!addmhdp?sfzh=${zd.zdgzrysfzh}";
});
$("#uppdzda").click(function(){
window.location.href="${Context}/background/mhdp!uppmhdp?id="+id;
});
});
});