I've implemented undo/redo the standard way (NSUndoManager) but can't figure out how I disable undo/redos when my app is in a specific state. 

Users draw things in my app and when what they've drawn is uploading I disable the UI and of course don't want the user to be able to undo/redo. 

I use a NSView's Undo Manager so I guess one way could be to just make that view resign first responder. Is there another way?