Thread: SwingWorker implemented - how to do additional GUI processing?

In a previous question I asked how I could implement processor-intensive work, which was called by a Swing component, without the GUI locking up. I was then pointed to the SwingWorker class. I implemented this and it works! But...

I am writing a program that solves sudoku puzzles. It has a Swing GUI with a "solve" button. I have written an actionListener class with an actionPerformed method that is invoked when the user presses this button. It is the following:

        public void actionPerformed(ActionEvent event) {
            try {
                worker = new SudokuSolveWorker();
                worker.addPropertyChangeListener(this);
                worker.execute();
                SolveFrame sf = new SolveFrame();
            } catch (Exception exc) {
                System.out.println(exc.getMessage());
                exc.printStackTrace();
            }
        }

The code creates a worker, a new instance of SudokuSolveWorker. The worker.execute() statement causes the doInBackground() method of this class to be called. This solves the Sudoku. A property change listener "picks up" the result:

        public void propertyChange(PropertyChangeEvent event) {
            try {
                SudokuModel solvedModel = (SudokuModel) worker.get();
                if (solvedModel.solved) {
                    model = solvedModel;
                    repaint();
                } else {
                    JOptionPane.showMessageDialog(null, "Sudoku cannot be solved");
                }
            } catch (Exception exc) {
                System.out.println(exc.getMessage());
                exc.printStackTrace();
            }
        }

As I wrote, this works without freezing up the user interface in the sense that the program seems unstable. However, the user interface does not respond to "commands" (mouse clicks) anymore until the worker thread has finished.

In the first code fragment I create an instance of SolveFrame. This is a simple frame with a "cancel" button. It is drawn on the screen, even though it is called after the worker.execute() statement. I'd like the user to be able to click this "cancel" button, after which the solving of the sudoku puzzle should be stopped. However, since the program does not respond to mouse clicks anymore, the "cancel" button cannot be pressed.

How can I solve this?

I fire off the GUI as follows:

        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                ViewController_Graphics view = new ViewController_Graphics(model);

Every code I showed above is a method of (a subclass of) ViewController_Graphics.

SolveFrame is a JFrame. Here is the code:

    public class SolveFrame extends JFrame {
        SolveFrame() {
            super("Solving");
            setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            JPanel mainPanel = new JPanel();
            JPanel topPanel = new JPanel();
            JPanel bottomPanel = new JPanel();
            topPanel.setLayout(new FlowLayout(FlowLayout.CENTER));
            bottomPanel.setLayout(new FlowLayout(FlowLayout.CENTER));
            mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.Y_AXIS));
            mainPanel.add(topPanel);
            mainPanel.add(bottomPanel);
            topPanel.add(new JLabel("Solving sudoku..."));
            JButton cancelButton = new JButton("Cancel");
            bottomPanel.add(cancelButton);
            add(mainPanel);
            setSize(new Dimension(170,120));
            setVisible(true);
        }
    }

But I don't think the problem is this SolveFrame. Also if I don't create this frame, the user interface does not respond to user input.

--- Update ---

I think I may have found a clue to an answer. I thought that the propertyChange listener was only invoked when the worker thread ended. However, I just found out that it gets called right after the worker thread is started. And then one of the first lines executed is the worker.get statement. And this is blocking until the worker thread has finished.

That would explain why the GUI becomes unresponsive.

However, what I don't understand then is why the propertyChange listener is invoked before the worker thread ends. According to the docs I have read it should only be invoked then. But apparently my docs are not correct. Who can tell me under what conditions the propertyChange listener is invoked? And how do I make something (a listener or so...) that gets invoked only when the thread has ended?