[NSOutlineView] How to know an item is expanded if its parent item is not?

FROM : Stéphane Sudre
DATE : Sun Mar 02 00:02:10 2008

There seems to be a missing method in NSOutlineView.

You can know an item is expanded only if its parent is expanded (so 
that the item itself is visible).

This is problematic if you want to cache the current list of expanded 
items. Instead of just iterating through the item hierarchy when 
needed, you would need to use the notifications sent when an item is 
about to be disclosed or closed.

This is a bit strange if you consider that NSOutlineView probably 
caches this information in _expandSet or _expandSetToExpandItemsInto 
so that it can remember to expand the children of an item if needed.

An I missing something and is there a way to know which items are 
expanded (either visible or not)?