Undo

4 important aspects that are discussed:

1. Undoable Actions: what actions should (or can) be undone?
2. State restoration: what part of UI is restored after undo?
3. Granularity: how much should be undone at a time?
4. Scope: is undo global, local, or someplace in between?

Recommendations for Undo Granularity

• Ignore direct manipulation intermediate states
  • e.g. ignore mousemove during object resize or move states
• Delimit chunks on discrete “input breaks“
  • e.g. words in text
• Chunk all changes resulting from a single interface event
  • e.g. find and replace multiple words

Forward Undo

Forward Undo

• Start from base document, then maintain of list of changes to compute current document
• Undo by removing last change from list when computing current document

Reverse Undo

• Apply change to update document, but also save “reverse” change
• Undo by applying reverse change to document

How to implement reverse change? 2 options:

Option 1: Command pattern

• save command and “reverse command” to change state

Option 2: Memento pattern

• save snapshots of each document state
• could be complete state or difference from “last” state

The stacks of Forward and Reverse Undo