how to implement undo/redo operation without major changes in program

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

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

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.

Answer 3

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".