There is scant (or hard to find) information on how to support undo within a modal sheet in a Cocoa application. I’ve managed to get it working, so thought I’d document it here.

I came across the NSPersistentDocument Core Data Tutorial (http://developer.apple.com/documentation/Cocoa/Conceptual/NSPersistentDocumentTutorial/08_CreationSheet/chapter_9_section_6.html), which has a section on supporting Undo in a sheet.

Unfortunately, the code in the document doesn’t do everything related to undo/redo - the menu titles aren’t updated for some reason. To get undo/redo working properly in a sheet, follow these steps:

1. Implement the changes and workarounds mentioned in the tutorial (above).

2. Modify the targetForAction:to:from: method in your NSApplication subclass to modify the menu item titles, so it looks like:

• (id)targetForAction:(SEL)anAction to:(id)aTarget from:(id)sender { if ((anAction != @selector(undo:)) && (anAction != @selector(redo:))) {

    return [super targetForAction:anAction to:aTarget from:sender];   }
  

  id keyWindowDelegate = self keyWindow] delegate]; if ([keyWindowDelegate isKindOfClass:[[[YourSheetController class]]) { // Work-around the fact that the undo/redo action names don’t show up in the edit recipe – CHANGES START if (anAction == @selector(undo:)) [sender setTitleWithMnemonic:[keyWindowDelegate managedObjectContext] undoManager] undoMenuItemTitle; else if (anAction == @selector(redo:)) [sender setTitleWithMnemonic:[keyWindowDelegate managedObjectContext] undoManager] redoMenuItemTitle; // – CHANGES END

    return keyWindowDelegate;   }   return [super targetForAction:anAction to:aTarget from:sender]; }
  

– JeremyHiggs