JScrollPane adding JPanels at the top and keeping current scroll view

[John Jacobs]

I have a JScrollPane that contains a vertical Box. I'm inserting new JPanel's at the top of Box. If I use the scrollbar to scroll down I'd like for the current view to remain where I scrolled down to. For example, if I have 50 panels in the box and use the scrollbar to view panel 20, I'd like the view to remain on box 20 even though other boxes are added on top. Additionally, if I use the scrollbar to scroll back up to the top I'd like the view to display new panels as they are added. Any idea how to do this?

BTW, it isn't necessary to use a JScrollPane or a Box. The example code is just to help explain what I am trying to do.

Example code:

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

public class TestScrollPane extends JFrame {

    JScrollPane scrollPane;
    Box box;
    private static int panelCount = 0;

    public TestScrollPane() {
        setPreferredSize(new Dimension(200, 400));
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        scrollPane = new JScrollPane();
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        scrollPane.getVerticalScrollBar().setUnitIncrement(15);
        box = Box.createVerticalBox();
        scrollPane.getViewport().add(box);
        
        this.add(scrollPane);
        this.pack();
        this.setLocationRelativeTo(null);
        this.setVisible(true);

        Timer t = new Timer(500, new ActionListener() {

            public void actionPerformed(ActionEvent ae) {
                box.add(new TestPanel(), 0);
                scrollPane.validate();
            }
        });
        t.setRepeats(true);
        t.start();
    }

    public class TestPanel extends JPanel {

        int myId = panelCount++;

        public TestPanel() {
            this.setLayout(new GridBagLayout());
            this.setBorder(BorderFactory.createBevelBorder(1));
            JLabel label = new JLabel("" + myId);
            label.setHorizontalAlignment(JLabel.CENTER);
            label.setVerticalAlignment(JLabel.CENTER);

            this.setMaximumSize(new Dimension(100, 100));
            this.setPreferredSize(new Dimension(100, 100));

            this.add(label);
        }
    }

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

            public void run() {
                TestScrollPane testScrollPane = new TestScrollPane();
            }
        });
    }
}

[Rob Spoor]

Welcome to the Ranch!

I'll move this thread to our GUI forum.

[Rob Camick]

Cross posted: http://stackoverflow.com/questions/5132802/jscrollpane-adding-jpanels-at-the-top-and-keeping-current-scroll-view/5134255#5134255

Be Forthright When Cross Posting To Other Sites

[John Jacobs]

Rob Camick wrote:Cross posted: http://stackoverflow.com/questions/5132802/jscrollpane-adding-jpanels-at-the-top-and-keeping-current-scroll-view/5134255#5134255

Be Forthright When Cross Posting To Other Sites

Sorry, I forgot. Anyways, the folks at that site where able to provide the solution. Pretty obvious and simple in hindsight.

            public void actionPerformed(ActionEvent ae) {
                Point view = scrollPane.getViewport().getViewPosition();
                TestPanel panel = new TestPanel();
                box.add(panel, 0);
                scrollPane.validate();
                if (view.y != 0) {
                    view.y += panel.getHeight();
                    scrollPane.getViewport().setViewPosition(view);
                }
            }