10.5. The Command Pattern

10.5. The Command Pattern
	Chapter 10. MainWindows and Actions

[ fromfile: commandpattern.xml id: commandpattern ]

Providing reversal actions (undo) in your software reduces anxiety and encourages experimentation in your users. This can be a better choice than a confirmation dialog. Qt provides classes that make it easier to write your application with undo capability.

Command Pattern

The Command pattern, as described in [Gamma95], encapsulates operations as objects with a common execution interface.
This can make it possible to place operations in a queue, log operations, and undo the results of an already executed operation.
A successful implementation of this pattern may also provide a common location for handling errors or unexpected conditions.

Qt classes that may be considered to implement this pattern to some degree are QUndoCommand, QRunnable, and QAction.

Implementing the Command Pattern is particularly easy with Qt.
- You can create several commands and queue them in an appropriate container (e.g., QQueue).
- A QUndoCommand can be executed by pushing it onto a QUndoStack.
- If commands need to be executed concurrently, you can derive them from QRunnable and schedule them in a thread pool with QtConcurrent::Run().^[49]
- You might serialize commands to a file and load them again later (perhaps on another machine) for batch or distributed execution ^[50]

^[49]Section 17.2 discusses threads.

^[50]Section 7.4.1 discusses the Serializer Pattern.


10.4. Clipboard and Data Transfer Operations		10.5.1. QUndoCommand and Image Manipulation