Is widget in Notebook visible

[Jeffrey Barish <jeff_barish earthlink net>]

I have a custom widget that resides on a page of a notebook.  I would like 
for the widget to be able to detect by itself when it becomes visible or 
invisible (as the user selects pages of the notebook).  The expose-event 
tells me when the widget becomes visible, but I can't find a corresponding 
event that tells me the converse.  Likewise, the visibility-notify-event 
tells me only when the widget becomes visible, not the converse.  Any 
suggestions?
-- 
Jeffrey Barish