Do something before user close the page

usually we have to do something before user exit, for example, close the connection for databse, or destroy the session.

one way is to use DesktopCleanup listener,but i've failed.

---

define listener in zk.xml:
<listener>
<description>Called when the ZK Desktop is detroyed</description>
<listener-class>mypackage.MyDesktopCleanup</listener-class>
</listener>
define method in MyDesktopCleanup:
public void cleanup(Desktop desktop) throws Exception {}

---

sencond way is to listen it in client via javascript,and send it back to the server:

---

<?page title="new page title" contentType="text/html;charset=UTF-8"?>
<script>
function window.onbeforeunload()
{window.event.returnValue="are you sure to quit ?";
comm.onEvent(this,"onExit");
}
</script>
<window id="mainwindow" apply="mypackage.MainController" />

---

but i don't know where to receive this event.
dose "this" mean the page ?
why couldn't i use "#{mainwindow}" instead of "this", then i could receive this event in MainController.