I am in the design process of an application, and I would like to use the command pattern for undo/redo purposes. I did some research into the command pattern but the only thing I don't get is: Should a command have the undo and redo methods, or should I make two separate commands, one for undo and one for redo, and call those from the main command itself?
+7
A:
The command object itself should implement the undo / redo functionality.
The commands are generally pushed and popped from a stack maintained by a command manager to implement multi level undo. When commands are executed they are pushed onto the stack and when they are undone they are popped from the stack.
The memento pattern would be used in conjunction with the command pattern, it is not a replacement to the usage of the command pattern. It would be used to maintain the state required for the undo operation.
David
2010-02-06 21:49:03
I'd add that a better solution isn't just a simple stack. Instead, you want a list and a reference to the current item. Undoing walks the reference back, and *redoing* walks it forward. Using a stack doesn't let you redo.
munificent
2010-02-08 23:34:53
+2
A:
You might also want to consider the memento pattern for this, we use it and it works brilliant for undo.
Burt
2010-02-06 23:08:14
Will the memento pattern be less time consuming to implement then the command pattern when i only want it for undoing ?
slayerIQ
2010-02-06 23:29:44
@slayerIQ The memento pattern would be used in conjunction with the command pattern, it is not a replacement to the usage of the command pattern.
David
2010-02-07 00:06:21
So the memento would hold the undo and redo stacks, And the stacks will contain the commands am i right ?
slayerIQ
2010-02-07 00:36:57
@slayerIQ The memento would hold the objects previous state so that it could be undone.
David
2010-02-07 00:43:13
The memento pattern can eat a lot of memory if your objects are large. Using the command pattern basically means storing just the difference between the object's previous and current state, where a memento stores the entire state.
munificent
2010-02-08 23:35:49