Java Swing, 2nd Edition

Extending UndoManager

Now that we’ve looked at all of the classes and interfaces in the undo framework, we’ll look at a few ideas for extending the functionality it provides.

In this example, we’ll extend UndoManager to add a few extra features. The first thing we’ll add is the ability to get a list of the edits stored in the manager. This is a simple task of returning the contents of the edits vector inherited from CompoundEdit. We also provide access to an array of significant undoable edits and an array of significant redoable edits. These might be useful in a game like chess, in which we want to provide a list of past moves.

The next major feature we add is support for listeners. At this writing, the current UndoManager does not have any way of notifying you when it receives edits. As we saw in an earlier example, this means that you have to listen to each edit-generating component if you want to update the user interface to reflect new undoable or redoable edits as they occur. In our manager, we simply add the ability to add and remove undoable edit listeners to the undo manager itself. Each time an edit is added, the undo manager fires an UndoableEditEvent to any registered listeners. This way, we can just add the undo manager as a listener to each edit-generating component and then add a single listener to the undo manager to update the UI.

The methods of our new undo manager can be divided into two groups, each supporting one of the two features we’re adding. We’ll split the ...