Undo

• Enables exploratory learning
  • Try the unknowns
• Allows you to recover from errors
  • Input errors (human) and interpretation errors (computer)
• Allows you to evaluate modification
  • Fast undo/redo cycle

Undo Design Choices

Undoable Actions

• Which actions should/can be undone

Possible actions

• Some actions may be omitted from udo
  • Change to selection, window resizing etc.
• Some actions are destructive and not easily undone
  • e.g. equitting program with unsaved data
• Some actions can't be undone
  • e.g. printing

In general

• All change sto the model should be undoable
• Changes to the view, or docuemnt's interface state should be undoable if they are extremely tedious or require significant effort
• Prompt the user before performing destructive actions

State Restoration

• User interface state should be meaningful after undo/redo action
  • Change selection (focus) to object(s) changed as a result of undo/redo
  • Scroll to show selection, if neccessary
  • Given focus to the control that is hosting the changed state

Granularity

Define undoable "chunk"

• The conceptual change from one document state to another

• In our drawing example

  • MousePress to start line
  • MouseDrag to define line path
  • MouseRelease to end line
  • MousePress + MouseDrag + MouseRelease = 1 chunk

• Ignore direct manipulation intermediate states

  • e.g. the intermediate states when you drag the shape to anothe position on canvas

• Chunk all changes resulting from an interface event

• Delimit on discrete input breaks

Scope

• Where does undo happen?
  • System level
  • Application level
  • Document level
  • Widget level

Implementing Undo

• Implement an undo stack

Forward Undo

1. Save complete baseline document state at some past point
2. Save change records to transform baseline docment into current document state
3. To do last action, don't apply last change record

Reverse Undo

1. Save complete current document state
2. Save reverse change records to return to previous state
3. To undo last action, apply last reverse change records

Change Record Implementation

Memento pattern

• Save snapshot of each documents tate
• Could be complete state or difference from "last" state

Command pattern

• Save commands to execute to change state

• Java platform uses reverse undo with command pattern

Implementation

• User issues command
  • Execute command to create a new current document state
  • Push reverse command onto undo stack
  • Clear redo stack
• Undo
  • Pop reverse command from undo stack and execute it to create new current document state
  • Push original command on redo stack

• Redo

  • Pop command off redo stack and execute it to create new current document state
  • Push reverse command on undo stack

• See slides for sample code

Destructive Commands

1. Use forward command undo

2. Use reverse command undo, but un-execute command stores previous state for "destructive" commands

   1. That's Memento, which may require a lot of memory