How to implement undo/redo in Java for MVC model?

Question

I am having trouble understanding the undo/redo functions using UndoManager, and integrating it with the MVC model.

I am not sure where to put the various methods(in model, view or control) and I am still not sure how to use the undo manager.

My control class implements UndoableEditListener It creates:

private UndoManager manager = new UndoManager();

and in:

public void actionPerformed(ActionEvent e) {
   if (e.getActionCommand().equals("Undo")) {    
      try {
          manager.undo();   
      } catch (CannotUndoException ex) { 
          ex.printStackTrace(); 
      }
   }
}

---

I understand up to here, but the rest I am not sure what to do. I know I will have to add more in the model and view class, but not sure where. DO I have to have the following classes?

public class UndoAction extends AbstractAction {}

public void undoableEditHappened(UndoableEditEvent e) {

I am simply placing an integer number in a textfield, and then I want to be able to undo this.I set the number in the textfield in the view class.I want to acheive this the simplest way possible, no fancy coding! This is a minor part of my assg but I just cant get it working!!

========================================================== Here is a more detailed description of my code, maybe it will help:

I have a model, view and control package.

Contol has: ButtonGUIControl.java, which implements both

    ActionListener and 
    UndoableEditListener.


    final UndoManager manager = new UndoManager();

In the actionPerformed method, it calls

    if (e.getActionCommand().equals("Undo")){

             try {

                    manager.undo();
                }

and in:

    public void undoableEditHappened(UndoableEditEvent evt) {
          manager.addEdit(evt.getEdit());

}

In the View: Grid.java , which extends JTextField will add the following, wherever it needs to display a number on the GUI:(model is simply an instance of my Model class)

    getDocument().addUndoableEditListener(new ButtonGUIControl(model));

Could it be because the UndoManager is being created in a different package? I really have no idea how to debug this anymore!!

I could post my entire code if that helps. I guess Im not sure how to integrate this with my mvc model structure.

Answer 1

Take a step back for a second. The whole idea here is that a user will use your app and will make a series of changes to something. A text editor is a good example. You can insert characters and lines, delete them again, replace text with other text, scroll the text, etc. In order to support this with MVC you have a model that holds state and a View that displays it.

Your first instinct might be to have the view directly access the model, and then refresh the view every time the user makes a change, but it's very hard to undo those changes with that implementation. Instead, you encode every kind of change the user can make in classes that are able to perform that change and can later undo that change.

For example, an action that inserts text would be implemented by a class that knows the character offset of the insertion point and the string of characters that is to be inserted. The perform action would insert the string at the offset and the undo action would remove the right number of characters after that insertion point. You'd have a different class that would handle deletion, another to handle scrolling etc.

Every time the user takes some action, the view would construct one of these UndoableEdit classes and would tell the instance to run itself (redo()). Once executed, you put that UndoableEdit at the end of a list of UndoableEdit instances that represent all of the actions the user has taken so far. This list makes it very easy to support any sequence of undo requests, redo requests and actual edit actions (resulting in more UndoableEdit's being put on the list).

So back to your question. If your app needs to support undo and redo, then you'll need to implement an UndoManager which simply manages the list of UndoableEdit's and performs undo and redo as necessary. You also have to implement a whole bunch of UndoableEdits, one for each kind of thing your user will do against the UI. As for a listener, I can't see that you really need to do that one.

Answer 2

1. If you need only simple undo/redo, you can use UndoManager as it is, you don't need to subclass or customize it in any way.
2. JTextField (more specifically its model, the Document) has some built-in support for undo, which means you don't need to write UndoableEdit implementations either, the UndoableEdit objects will be automagically created for you (actually AbstractDocument.DefaultDocumentEvent implements UndoableEdit).
3. Full simple working example is here