MouseMotionListener in Java Swing, using it with components inside components etc

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Problem :

I am working on a Touch User interface in Swing. While I know this isn’t optimal, I am on a short deadline and don’t have time to Touch-screen specific GUI packages (if there are any).

I want my users to be able to ‘swipe’ their finger across the screen, and the view of a special JScrollPane I made moves with it.
The code is very simple –

    public class PanScrollPane extends JScrollPane implements MouseMotionListener{  
public PanScrollPane() {
    super();        
    this.addMouseMotionListener(this);      
}
@Override
public void mouseDragged(MouseEvent arg0) {
    System.out.println("Mouse Dragged!");       
}
@Override
public void mouseMoved(MouseEvent arg0) {
    System.out.println("Mouse Moved!");     
}

The problem I’m having is that the JScrollPane is a container for all sorts of JComponents. When I first started working on this, I figured the MouseMovedEvent and MouseDraggedEvent would propagate up the ‘GUI tree’, untill they encountered a Component with a listener specifically for that event. Now it seems that any component I add to the panScrollPane blocks any of these MouseMotion events, leaving me unable to pan.

    panScrollPane.add(new JButton("This thing blocks any mouse motion events"));

I figured propagating the MouseEvent by hand (adding listeners to every single component and then having them send the event to their parent) would work. This, however, is a very time-intensive undertaking and as I would rather spend my time working on other things, I was wondering if any of you know any work-around for this problem.

Thanks for reading, and hopefully thanks for answering! 🙂

edit: To make my intentions clearer. I only want the mousemotion events to be caught by the panPanel, any other event (like MouseClick, MouseRelease) should be processed normally

Solution :

This ad hoc approach leverages the existing JScrollPane actions that are usually used in key bindings. You’ll have to tune N to your implementation of Scrollable.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.Action;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JViewport;
import javax.swing.Timer;

/** @see http://stackoverflow.com/questions/7201509 */
public class ScrollAction extends JPanel {

    private static final int TILE = 64;
    private static final int DELTA = 16;

    public ScrollAction() {
        this.setOpaque(false);
        this.setFocusable(true);
        this.setPreferredSize(new Dimension(50 * TILE, 50 * TILE));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.lightGray);
        int w = this.getWidth() / TILE + 1;
        int h = this.getHeight() / TILE + 1;
        for (int row = 0; row < h; row++) {
            for (int col = 0; col < w; col++) {
                if ((row + col) % 2 == 0) {
                    g.fillRect(col * TILE, row * TILE, TILE, TILE);
                }
            }
        }
    }

    private void display() {
        JFrame f = new JFrame("ScrollAction");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        final JScrollPane scrollPane = new JScrollPane(this);
        final ScrollTimer left = new ScrollTimer(scrollPane, "scrollLeft");
        final ScrollTimer right = new ScrollTimer(scrollPane, "scrollRight");
        final ScrollTimer up = new ScrollTimer(scrollPane, "scrollUp");
        final ScrollTimer down = new ScrollTimer(scrollPane, "scrollDown");
        final JViewport viewPort = scrollPane.getViewport();
        viewPort.setPreferredSize(new Dimension(5 * TILE, 5 * TILE));
        viewPort.addMouseMotionListener(new MouseAdapter() {

            @Override
            public void mouseMoved(MouseEvent e) {
                left.stop();
                if (e.getX() < DELTA) {
                    left.start();
                }
                right.stop();
                if (e.getX() > viewPort.getWidth() - DELTA) {
                    right.start();
                }
                up.stop();
                if (e.getY() < DELTA) {
                    up.start();
                }
                down.stop();
                if (e.getY() > viewPort.getHeight() - DELTA) {
                    down.start();
                }
            }
        });
        f.add(scrollPane);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    private static final class ScrollTimer implements ActionListener {

        private static int N = 10;
        private static int DELAY = 100;
        private String cmd;
        private Timer timer;
        private Action action;
        private JScrollPane scrollPane;
        private int count;

        public ScrollTimer(JScrollPane scrollPane, String action) {
            this.cmd = action;
            this.timer = new Timer(DELAY, this);
            this.action = scrollPane.getActionMap().get(action);
            this.scrollPane = scrollPane;
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            if (count++ < N) {
                action.actionPerformed(new ActionEvent(scrollPane, 0, cmd));
            } else {
                timer.stop();
            }
        }

        public void start() {
            count = 0;
            timer.start();
        }

        public void stop() {
            timer.stop();
            count = 0;
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new ScrollAction().display();
            }
        });
    }
}

How about using a GlassPane? I think its meant to address exactly these types of situations.

Getting mouseEvents for a component and all its children is … tricky to get right. You might consider to rely on stable (and extensively tested 🙂 code around. The jdk7 way of doing it is to use a JLayer (which internally registers an AWTEventListener as it has all priviledges). For earlier versions, you can use its predecessor JXLayer

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