How to implement undo functionality?

Question

In my application I want to provide the user with a small undo functionality. There aren't many actions than can be undone by the user. Particularly the actions are:

• Add notes to an object
• Color an object
• Tag a objcet with a string

Now I thought about how to implement this. I first thought of a Action Class that is the abstract base class for the 3 different actions that can be taken by the user. Every time the user takes on of these actions, a new appropriate instance of a subclass of this abstract Action class is created and inserted into a list that contains all actions.

Whenever the user wants to undo something, the list is displayed to the user and he can choose which action he want to undo.

Now I was thinking what has to be stored in such an action object:

• the state of the object before the action
• the actual action that was taken (e.g. the string that was added to a object's notes)

I'm not sure if this is enough. I also thought about something like a chronological ordering, but this should be necessary since the list can be maintained chronologically correct.

Are there any other things I should consider?

Answer 1

Undo/redo is commonly implemented with the Command Pattern. The Action class can be used as the basis for this, but you need a 'do' action and an 'undo' action within each command. Here is an example of this in practice. You should probably store the commands executed in a stack as it makes it much easier to implement and much easier for the user to follow.