当我释放鼠标按钮时，MouseEvent未注册发布

Question

public void mousePressed(MouseEvent e) {
    //Invoked when a mouse button has been pressed on a component.
    if (e.getButton() == MouseEvent.BUTTON1) {
        isDown = true;
        System.out.println("isDown is now true");
    }
    if (e.getButton() == MouseEvent.BUTTON3) {
        isDown2 = true;
        System.out.println("isDown2 is now true");
    }
    do {
        Point location = MouseInfo.getPointerInfo().getLocation(); 
        int x = location.x - (drawingPanel.getLocationOnScreen()).x;
        int y = location.y - (drawingPanel.getLocationOnScreen()).y;
        drawingPanel.paint(drawingPanel.getGraphics(), (x - (x % 20) - 1), (y - (y % 20) - 1), 19, 19);
    } while (isDown);
    System.out.println("Mouse has been pressed down.");
}

public void mouseReleased(MouseEvent e) {
    //Invoked when a mouse button has been released on a component.
    if (e.getButton() == MouseEvent.BUTTON1) {
        isDown = false;
        System.out.println("isDown is now false");
    }
    if (e.getButton() == MouseEvent.BUTTON3) {
        isDown2 = false;
        System.out.println("isDown2 is now false");
    }
    System.out.println("Mouse has been released.");
}

This is what I have so far. My original intentions were to design the code so that the boolean isDown would be set to true when the mouse was pressed down and then I would have the while loop run while isDown is true. If the mouse button is released, I would set isDown to false in order to terminate the while loop.

What I am messing up here? Is it not possible for two MouseEvent methods to be running at the same time? The change in the isDown boolean variable is not being registered and I have an infinite while loop on my hands.

Answer 1

This is a classic violation of the Event Dispatching Thread.

All UI code is run from within a single thread. All events are dispatched the UI from this same thread, meaning that should you block this thread (using a loop or other blocking operation), no events will be dispatched. This will make your program look like it's hung.

Take a look at Concurrency in Swing for more details.

What you really should be doing is using a MouseMoitionListener to track drag events instead. Check out How to use Mouse Listeners for more details.

This drawingPanel.paint(drawingPanel.getGraphics(), (x - (x % 20) - 1), (y - (y % 20) - 1), 19, 19); also worries me to no end.

You should never be using getGraphics to perform custom painting. getGraphics can return null and is only a snap shot of the last paint cycle. Any painting done using this method will be removed/cleaned when another repaint occurs.

You should be creating a custom component (such as a JPanel ) and overriding it's paintComponent method and performing any painting you need in it. Check out Performing Custom Painting for more details

Example

public class MouseDraggedTest {

    public static void main(String[] args) {
        new MouseDraggedTest();
    }

    public MouseDraggedTest() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception ex) {
                }

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private Map<Point, List<Point>> mapPoints;
        private Point currentPoint;

        public TestPane() {
            mapPoints = new HashMap<>(25);
            MouseAdapter mouseListener = new MouseAdapter() {
                @Override
                public void mousePressed(MouseEvent e) {
                    currentPoint = e.getPoint();
                    mapPoints.put(currentPoint, new ArrayList<Point>(25));
                }

                @Override
                public void mouseReleased(MouseEvent e) {
                    List<Point> points = mapPoints.get(currentPoint);
                    if (points.isEmpty()) {
                        mapPoints.remove(currentPoint);
                    }
                    currentPoint = null;
                }

                @Override
                public void mouseDragged(MouseEvent me) {
                    List<Point> points = mapPoints.get(currentPoint);
                    points.add(me.getPoint());
                    repaint();
                }
            };
            addMouseListener(mouseListener);
            addMouseMotionListener(mouseListener);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            for (Point startPoint : mapPoints.keySet()) {
                List<Point> points = mapPoints.get(startPoint);
                for (Point p : points) {
                    if (startPoint != null) {
                        g.drawLine(startPoint.x, startPoint.y, p.x, p.y);
                    }
                    startPoint = p;
                }
            }
        }
    }
}

Answer 2

Your are doing the while loop inside your mousePressed() method. It means you are blocking the Swing Event Dispatch Thread! As this method will not return as long as isDown is true, then the execution will not come back to the listener handler, which will never call your mouseReleased() listener.

As a general rule, you should never launch a long running operation in a listener, because it makes your GUI not respond to any event as long as it runs. Which in this case means forever! Any listener should not do more than set a few flags, and then return. It is necessary for your application to stay responsive.

One typical solution is to launch a new thread to do the long running work. It would free the Event Dispatching Thread and your mouseReleased() listener will be called.

The problem with this solution is that your work is painting on a Swing component. All graphics should be done in an overriden paintComponent method. As @MadProgrammer is already explaining you that, I won't go into details about it.