Undo Framework

The Undo Framework is a component of the Enthought Tool Suite that provides developers with an API that implements the standard pattern for do/undo/redo commands.

The framework is completely configurable. Alternate implementations of all major components can be provided if necessary.

Framework Concepts

The following are the concepts supported by the framework.

• Command

  A command is an application defined operation that can be done (i.e. executed), undone (i.e. reverted) and redone (i.e. repeated).

  A command operates on some data and maintains sufficient state to allow it to revert or repeat a change to the data.

  Commands may be merged so that potentially long sequences of similar commands (e.g. to add a character to some text) can be collapsed into a single command (e.g. to add a word to some text).

• Macro

  A macro is a sequence of commands that is treated as a single command when being undone or redone.

• Command Stack

  A command is done by pushing it onto a command stack. The last command can be undone and redone by calling appropriate command stack methods. It is also possible to move the stack’s position to any point and the command stack will ensure that commands are undone or redone as required.

  A command stack maintains a clean state which is updated as commands are done and undone. It may be explicitly set, for example when the data being manipulated by the commands is saved to disk.

  PyFace actions are provided as wrappers around command stack methods to implement common menu items.

• Undo Manager

  An undo manager is responsible for one or more command stacks and maintains a reference to the currently active stack. It provides convenience undo and redo methods that operate on the currently active stack.

  An undo manager ensures that each command execution is allocated a unique sequence number, irrespective of which command stack it is pushed to. Using this it is possible to synchronise multiple command stacks and restore them to a particular point in time.

  An undo manager will generate an event whenever the clean state of the active stack changes. This can be used to maintain some sort of GUI status indicator to tell the user that their data has been modified since it was last saved.

Typically an application will have one undo manager and one undo stack for each data type that can be edited. However this is not a requirement: how the command stack’s in particular are organised and linked (with the user manager’s sequence number) can need careful thought so as not to confuse the user - particularly in a plugin based application that may have many editors.

To support this typical usage the PyFace Workbench class has an undo_manager trait and the PyFace Editor class has a command_stack trait. Both are lazy loaded so can be completely ignored if they are not used.