For some reason, I have to catch the event when a user is leaving a visualforce page (closing the page, clicking on another link, ...) and execute a method of my controller.

The idea was to use something like window.onbeforeunload = function(){ Unload();return false} which should call the function "unload()". It's working pretty well when I replace the "Unload()" with a javascript alert, meaning the function is executed but the aim is to call the controller... So I have a simple actionfunction  <apex:actionFunction action="{!OnUnload}" name="Unload"/> which should call the method "OnUnload" in my controller. Unfortunately, it is not working... It's not the first time I'm using actionfunction called from Javascript but it looks like that the line "actionfunction" is already "unloaded" even on "onbeforeunload" :/

Has someone already been confronted to this problem ??