How to Effectively Use MouseMotionListener in Java Swing with Nested Components?

MouseMotionListener in Java Swing: Overcoming Component Interception

In developing a touch user interface within Swing, one may encounter the challenge of using the MouseMotionListener interface effectively when nested components obstruct event propagation.

The MouseMovedEvent and MouseDraggedEvent are intended to propagate up the GUI hierarchy, but they can become blocked by components within the container. For instance, when adding a JButton to a JScrollPane, events might不再trigger the JScrollPane's MouseMotionListener.

To overcome this issue, we present an ad hoc approach that utilizes the JScrollPane's built-in actions, typically used for key bindings. By adjusting the N variable to align with your implementation, this solution enables event propagation without the need for extensive manual event forwarding.

Here's a working example:

<code class="java">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;

public class ScrollAction extends JFrame {

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

    public ScrollAction() {
        setupAndDisplay();
    }

    private void setupAndDisplay() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        JPanel panel = new JPanel() {
            @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  viewport.getWidth() - DELTA) {
            right.start();
        }
        up.stop();
        if (e.getY()  viewport.getHeight() - DELTA) {
            down.start();
        }
    }

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

            @Override
            public void run() {
                new ScrollAction().setVisible(true);
                pack();
            }
        });
    }
}</code>

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