Implementing undo and redo operations in a client-side Java app can be a little trickier than you might expect. It can be especially tricky when building a graphical editor like the one I built for Loadster.

This project provides a simple framework for implementing your own undoable actions (HistoryAction, MementoAction) and chaining them together (SimpleHistoryChain, SwappingHistoryChain).

A HistoryAction has to implement 3 methods: run, undo, and redo.

The run method works just like a java.util.Runnable -- implement it to perform the action you want.

The undo method is responsible for restoring the domain model to its previous state. Keep in mind actions will normally be chained together, so it's important that this method not make any assumptions about what external changes could have happened to the domain model.

The redo method re-applies the changes after they have been undone. For simple cases, this might just call the run method but not always.

Here's a really simple example of a HistoryAction that adds/subtracts a random amount from a random index of an array. See it in action in HistoryChainTest.java.

public class RandomOperationAction implements HistoryAction {
	int index = (int) (Math.random() * testData.length);
	int shift = (int) (Math.random() * 6) - 3;

	public void run() {
		workingData[index] += shift;
	}

	public void undo() {
		workingData[index] -= shift;
	}

	public void redo() {
		workingData[index] += shift;
	}
}

For more complicated UIs, with lots of possible user actions, crafting a HistoryAction for every operation gets really tedious and error-prone. For those situations there is a MementoAction that basically freezes a copy of your model before and after every change, and can thaw it out later to restore the state. You just have to implement two methods, captureState and restoreState.

Check out HistoryChainTest.java for a simple example of a MementoAction. This particular example uses Java serialization to store the model state, but you could certainly use whatever approach makes sense for your situation.

If you are implementing many different actions, you'll want to create your own abstract implementation of MementoAction that takes care of capturing and restoring the state, and then just override the runInternal method for each unique operation.

public abstract class MyMementoAction extends MementoAction {
    protected abstract void runInternal();

    protected byte[] captureState() throws IOException {
        ByteArrayOutputStream state = new ByteArrayOutputStream();
        ObjectOutputStream output = new ObjectOutputStream(state);

        output.writeObject(workingData);
        output.close();

        return state.toByteArray();
    }

    protected void restoreState(byte[] state) throws IOException {
        try {
            ObjectInputStream input = new ObjectInputStream(new ByteArrayInputStream(state));
            int[] cannedData = (int[]) input.readObject();

            for (int i = 0; i < workingData.length; i++) {
                workingData[i] = cannedData[i];
            }

            input.close();
        } catch (Exception e) {
            throw new IOException(e);
        }
    }
}

public class ShuffleAction extends MyMementoAction {
    protected void runInternal() {
        Collections.shuffle(Arrays.asList(workingData));
    }
}

public class SortAction extends MyMementoAction {
    protected void runInternal() {
        Collections.sort(Arrays.asList(workingData));
    }
}

While using Java serialization for a simple byte array is overkill, it works great for storing temporary copies of a more complicated object-oriented domain model.

Once you've implemented your actions, chain them together with a history chain. This makes it easy to hook the undo/redo operations in with your user interface.

HistoryChain chain = new SimpleHistoryChain();

// Run an action
chain.run(new SortAction());

// Run a different action
chain.run(new ShuffleAction());

// Undo the action
chain.undo();

// Undo another action
chain.undo();

// Redo the action you just undid
chain.redo();

You'll also want to call the canUndo and canRedo methods from your UI, so it knows when to enable or disable those menu options and shortcuts.

If you find you're using a lot of MementoAction and your serialized domain model is kind of big, take a look at SwappingHistoryChain. It can automatically store the serialized mementos in files when the stack of undoable actions gets too big. Make sure to call clear() on the history chain when you're done with it so the files are properly deleted.

Copyright (c) 2013 Andy Hawkes

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