[jQuery] accessing iframe window object

[elesueur]

I have a set of iframes which are accessed the following way
(example):

        $('#' + windowID + 'Frame').load(
                function() {
                        $('#' + windowID + 'Loader').css('display', 'none');
                }
        );

This sets a loader image to hide once the document in the iframe has
loaded.

What I would like to do, is display the loader image the instant the
source of the iframe changes.

So I was trying something like:

        $('#' + windowID + 'Frame').unload(
                function() {
                        $('#' + windowID + 'Loader').css('display', 'block');
                }
        );

However, this isn't working because I guess the iframe itself isn't
actually being unloaded, it's the document within the iframe that is
being unloaded. If I do something like:

        $(window).unload( function() {
                alert('unloaded');
        });

in the source page of the iframe, the message is correctly displayed,
however I would like to do this from the parent page.

How can I access the iframe document's window object from the parent?

Can anyone see a better way of doing this?