The undo manager for the document.
- iOS 5.0+
This property holds the document's undo manager (an
Undo object). Accessing this property lazily creates a default undo manager if a custom undo manager has not been set.
The undo manager for the document is registered as an observer of various
Undo notifications so that it can call
update as the user makes undoable changes to the document. When a subclass sets this property and implements registers changes with it, it does not need to call
update directly or (more rarely) override