Why JScrollPane does not react to mouse wheel events?

Question

I have a JScrollPane containing a panel with a BoxLayout (PAGE AXIS).

My problem is that the JScrollPane does not react to mouse wheel events. To make it scroll using the mouse wheel i need to be on the JScrollBar.

I found this thread and i have no MouseMotionListener or MouseWheelListener, only a MouseListener. I think my problem come from the fact that my JScrollPane act on a JPanel that contains other panels itself. So when the mouse is on a panel within the JScrollPane it seems that the event is consumed by this panel i never seen by the scroll pane.

Is there a correct way to make the events caught by the children of the scroll pane visible to this scroll pane?

SSCCE:

Here a simple test case trying to show when i try to do in my Swing application.

The frame:

public class NewJFrame extends javax.swing.JFrame {

    public NewJFrame() {
        initComponents();
        for (int i = 0; i < 50; i++) {
            jPanel1.add(new TestPanel());
        }
    }

private void initComponents() {
        jScrollPane1 = new javax.swing.JScrollPane();
        jPanel1 = new javax.swing.JPanel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jPanel1.setLayout(new javax.swing.BoxLayout(jPanel1,    javax.swing.BoxLayout.PAGE_AXIS));
        jScrollPane1.setViewportView(jPanel1);

        getContentPane().add(jScrollPane1, java.awt.BorderLayout.CENTER);

        pack();
    }

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
           @Override
            public void run() {
                new NewJFrame().setVisible(true);
           }
        });
    }
}

And the TestPanel definition:

public class TestPanel extends javax.swing.JPanel {

    public TestPanel() {
        initComponents();
    }

    private void initComponents() {

        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        jScrollPane1 = new javax.swing.JScrollPane();
        jTextArea1 = new javax.swing.JTextArea();

        jLabel1.setText("jLabel1");

        setBackground(new java.awt.Color(255, 51, 51));
        setLayout(new java.awt.BorderLayout());

        jLabel2.setText("TEST LABEL");
        jLabel2.setHorizontalTextPosition(javax.swing.SwingConstants.CENTER);
        add(jLabel2, java.awt.BorderLayout.PAGE_START);

        jTextArea1.setEditable(false);
        jTextArea1.setColumns(20);
        jTextArea1.setRows(5);
        jTextArea1.setFocusable(false);
        jScrollPane1.setViewportView(jTextArea1);

        add(jScrollPane1, java.awt.BorderLayout.CENTER);
   }
}

The JTextArea seems to consume the event since when the mouse cursor is inside it, the scrolling using wheel does not work. I have to put the mouse cursor outside the text area to make it works again.

Noelnoelani

please show an SSCCE that demonstrates the problem – Tincher

Could you post the code you're using to generate these components? I can't reproduce your issue... – Swarth

+1 for the near-SSCCE (it's not compilable as you forgot the field declarations :-) – Tincher

@Tincher to be honest it my first attempt at writing an SSCE. I made a mistake since i intentionally removed the field declarations because i though they would be useless. After reading again the SSCCE HOWTO, i realized i was wrong : D – Noelnoelani

Simplest solution without affecting/hacking any other components: enter link description here – Esquire

Answer 1

Walter beat me to analysing the issue :-)

Adding a bit of detail:

It's correct that a JScrollPane supports mouseWheelHandling. According to the rules of mouseEvent dispatching, the top-most (in z-order) component gets the event, and that's the scrollPane around the textArea. So if wheeling the textarea is not required, a simple solution might be to disable the wheel-support in its scrollPane. And JScrollPane even has api for doing it:

scrollPane.setWheelScrollingEnabled(false); 

Unfortunately, that doesn't work. Reason it's not working is that this property has no effect in the event dispatch chain which ultimately calls into eventTypeEnabled:

case MouseEvent.MOUSE_WHEEL:
      if ((eventMask & AWTEvent.MOUSE_WHEEL_EVENT_MASK) != 0 ||
          mouseWheelListener != null) {
          return true;
      }

This returns true if a mouseWheelListener is installed - which is done unconditionally by BasicScrollPaneUI, and not removed when the wheelEnabled property is changed (the ui doesn't even listen to that property ...) Plus the listener simply does nothing if the property is false. At least one of those facts is a bug, the ui should

• either remove/add the listener depending on wheelEnabled
• or: implement the listener such that it dispatches the event up the chain (as Walter does in his example)

The first option can be handled by application code:

scrollPane = new JScrollPane();
scrollPane.removeMouseWheelListener(scrollPane.getMouseWheelListeners()[0]);

it's a bit of a hack (as bug-workarounds always are :-), production code would have to listen to the wheelEnable to re-install if needed plus listen to LAF changes to update/re-remove the listeners installed by the ui.

Implementing the second option in slight modification (as to Walter's dispatching) by subclassing the JScrollPane and dispatch the event to parent if the wheelEnabled is false:

scrollPane = new JScrollPane() {

    @Override
    protected void processMouseWheelEvent(MouseWheelEvent e) {
        if (!isWheelScrollingEnabled()) {
            if (getParent() != null) 
                getParent().dispatchEvent(
                        SwingUtilities.convertMouseEvent(this, e, getParent()));
            return;
        }
        super.processMouseWheelEvent(e);
    }

};
scrollPane.setWheelScrollingEnabled(false); 

Tincher

Answer 2

The mouse wheel event gets consumed by the scroll pane around the text area. You can try to manually pass the event to the parent scroll pane like this:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class TestScrollPane2 {
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                // might want to use a http://tips4java.wordpress.com/2009/12/20/scrollable-panel/
                JPanel panel = new JPanel(new GridLayout(0, 1));
                for (int i = 0; i < 10; i++) {
                    panel.add(new JScrollPane(new JTextArea(3, 40)) {
                         @Override
                        protected void processMouseWheelEvent(MouseWheelEvent e) {
                            Point oldPosition = getViewport().getViewPosition();
                            super.processMouseWheelEvent(e);

                            if(getViewport().getViewPosition().y == oldPosition.y) {
                                delegateToParent(e);
                            }
                        }

                        private void delegateToParent(MouseWheelEvent e) {
                            // even with scroll bar set to never the event doesn't reach the parent scroll frame
                            JScrollPane ancestor = (JScrollPane) SwingUtilities.getAncestorOfClass(
                                    JScrollPane.class, this);
                            if (ancestor != null) {
                                MouseWheelEvent converted = null;
                                for (MouseWheelListener listener : ancestor
                                        .getMouseWheelListeners()) {
                                    listener.mouseWheelMoved(converted != null ? converted
                                            : (converted = (MouseWheelEvent) SwingUtilities
                                                    .convertMouseEvent(this, e, ancestor)));
                                }
                            }
                        }
                    });
                }
                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                frame.getContentPane().add(new JScrollPane(panel));
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

Mofette

Personally I wouldn't bother with messaging parent's listeners - a simply dispatch should be enough (once the scrollPane decided to not handle it itself) – Tincher