Disabling undo between events

FROM : Jonathan Grynspan
DATE : Fri Jul 21 20:30:05 2006

I'm currently working on a small graphics application used for 
editing the images in a game. I've implemented selection, line, rect, 
etc. tools, but I've noticed that while the mouse button is pressed, 
I can still activate Undo and Redo with Cmd+Z and Cmd+Shift+Z. I took 
a look at -[NSUndoManager disableUndoRegistration/
enableUndoRegistration], but they only affect the creation of new 
Undo events, rather than stopping me from firing old ones. -
validateMenuItem: is never sent to my view for the Undo or Redo menu 
items, so I can't use that to disable them. Anybody have any ideas?

Thanks,
Jonathan Grynspan