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The most important code supporting undo should be in your model layer. Each model object in your application should be able to register undo invocations for all primitive methods that change the object.
It is often useful to structure the APIs of your model object to consist of primitive methods and extended methods. Examples of this sort of separation can be found throughout the Foundation framework (including NSString, NSArray, NSDictionary) as well as in the Sketch example project. If you have such a separation in your model objects, remember that only the primitives should register for undo since, by definition, the extended methods are implemented in terms of the primitives.
Some situations might require you to temporarily suspend undo registration for certain actions. For example, a Sketch application lets the user resize a graphic by grabbing a resize knob and dragging it. During this dragging, hundreds or thousands of changes may be made to the bounds rectangle of the selected graphic. Changing the bounds of a graphic is a primitive operation and would normally result in an undo registration. While the user is actively resizing, though, it would be better if those thousands of undo registrations did not happen. In these cases, your model object might provide API to temporarily suspend and resume some or all of its undo registration. It is up to you to decide how to handle this. Certainly, it would work if those thousands of undo registrations did happen, but it would be a tremendous waste of memory to have to remember all those intermediate rectangles when you will never have to restore one of those intermediate states.
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