Undo - Undo Models

Undo Models

Undo models can be categorized according to two criteria: linear vs non-linear, and single-user vs multi-user. Linear undo requires the user to revert the latest action before undoing earlier ones. With non-linear undo, the action to be reverted can be freely picked from the action history list.

Non-linear undo can be further subcategorized into direct selective undo, script interpretation undo, and cascading undo.

When multiple users can edit the same document simultaneously, a multi-user undo is needed. Global multi-user undo reverts the latest action made to the document, regardless of who performed the edit. Local multi-user undo only reverts actions done by the local user. Local multi-user undo usually requires a non-linear undo implementation.

The number of previous actions that can be undone varies by program. For example, the stack size ranges from twenty in Photoshop (customizable) to three edits in MS Paint. Simplistic, single-edit undo features sometimes do away with "redo" by treating the undo command itself as an action that can be undone. This is known as the flip undo model, because the user can flip between two program states using the undo command.