How do I know when app is iconified?

["HOCHSTETLER, BRUCE W. (JSC-DV3/T) (TSI)" <bruce w hochstetler1 jsc nasa gov>]

I'm trying to determine when my application has been iconified. I've tried
various signal handlers with no success: unmap-event,
visibility-notify-event, and state-changed. Does anyone know what event is
sent if any when an application is iconified? Or is there some other method
that is used?

Thanks,
Bruce