Design Pattern for Undo Engine
I'm writing a structural modeling tool for a civil enginering application. I have one huge model class representing the entire building, which include collections of nodes, line elements, loads, etc. which are also custom classes.
I have already coded an undo engine which saves a deep-copy after each modification to the model. Now I started thinking if I could have coded differently. Instead of saving the deep-copies, I could perhaps save a list of each modifier action with a corresponding reverse modifier. So that I could apply the reverse modifiers to the current model to undo, or the modifiers to redo.
I can imagine how you would carry out simple commands that change object properties, etc. But how about complex commands? Like inserting new node objects to the model and adding some line objects which keep references to the new nodes.
How would one go about implementing that?
Answer
Most examples I've seen use a variant of the Command-Pattern for this. Every user-action that's undoable gets its own command instance with all the information to execute the action and roll it back. You can then maintain a list of all the commands that have been executed and you can roll them back one by one.