catching window focus problem

["Matt M." <matt boda virtualave net>]

I need to know when a window has received focus. I tried it originally by
connecting a signal handler (checking for focus_in_event) to the window, but
that did nothing. So then I tried placing the window in an event box and
connecting the handler to the event box, but that crashes the app. So,
basically, how can I tell when a window receives focus?