how to implement undo/redo operation without major

Question

Hi I'm about to add new functionality to application which I'm currently writting. I need to write a undo/redo fnctionality. However 90% of our application is ready and I don't know what is the best way to implementing this functionality without affectig(too much ) code which has been already created.

Answer 1

As Reed Copsey says the most common pattern to implementing do/redo is Command Pattern. The basic idea is to implement actions as commands that implemets some interface like this:

public interface ICommand  { public void Execute(); public void Undo(); }

Then You have a class (Control) that executes all the commands as a whole, such class must be composed by a group of commands, the when you execute the commands each command is pushed inside a Stack (by means the push() method), in the case you want to undo the actions then you take every element from the Stack (by means of pop() ) an executes its Undo() method.

public class Control {  
    private ArrayList<ICommand> commands  = new ArrayList<ICommand>();  
    private Stack<ICommand> stack = new Stack<ICommand>();  

    public Control() {      
        commands.add(new Command1());
        commands.add(new Command2());       
        commands.add(new Command3());       
    }

    public void Execute() {
        for(int index=0; index<=command.size(); index++) { command.Execute(); stack.push(command);}
    }
    public void Undo() 
    {
        while (!stack.empty()) {
            ICommand command = (ICommand)stack.pop();
            if (command != null) { command.Undo(); }        
        }       
    }   
}

Note: this is a very simple code, only for trying to clarify the ideas behind the Command Pattern.

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Answer 2

Couple of questions: What does the undo do? Does it undo all changes or only the last one?

In any case, you would first store the state of the input fields (initial state, or last desired position), then the undo button would take that state and apply it to the field.

Redo would be the same concept, but it would store the state previous to the undo.

Answer 3

You'll need an undo/redo stack, and major changes to make everything aware of this stack.. I'm sorry, but magic doesn't exist :/

Answer 4

There's a free library written by Rockford Lhotka for Business Objects that includes built in undo/redo functionality. I think he even implements it through an interface, so it should be "minimally invasive".

Answer 5

There aren't many details here. However, Undo/Redo functionality is typically handled via some variation of the Command Pattern. Depending on your architecture, this could be a simple reworking of your basic functionality into "commands", or a major overhaul.