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Overview of Qt's Undo Framework

Introduction

Qt's Undo Framework is an implementation of the Command pattern, for implementing undo/redo functionality in applications.

The Command pattern is based on the idea that all editing in an application is done by creating instances of command objects. Command objects apply changes to the document and are stored on a command stack. Furthermore, each command knows how to undo its changes to bring the document back to its previous state. As long as the application only uses command objects to change the state of the document, it is possible to undo a sequence of commands by traversing the stack downwards and calling undo on each command in turn. It is also possible to redo a sequence of commands by traversing the stack upwards and calling redo on each command.

Classes

The framework consists of four classes:

Concepts

The following concepts are supported by the framework:

QUndoStack provides convenient undo and redo QAction objects that can be inserted into a menu or a toolbar. The text properties of these actions always reflect what command will be undone or redone when they are triggered. Similarly, QUndoGroup provides undo and redo actions that always behave like the undo and redo actions of the active stack.


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Qt 4.6.3