JList JScrollPane problem

[Mike Rane]

Hello,

I have a JList with a VERTICAL_WRAP layout orientation. The list is in a JScrollPane. Everything is fine if I use the scrollbar buttons/arrows or click on the scrollbar's track. The scrolling is "savvy" i.e., it scrolls a row at a time. The weirdness occurs when I drag the scrollbar's knob. The knob may be left in a position that doesn't correspond to the value returned by the scrollbar's getValue() method. For example, getValue() might return 0, but the knob is not all the way to the left. I would like the knob to snap to the nearest tick, so to speak/write. How can I do this?

I'm using the windows look and feel. When using the java laf I don't notice the problem, but the scrolling isn't savvy.

Thanks.

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class ListTest
{
    /**
     * adapted from JList api discussion section
     */
    private JScrollPane getScrollPane()
    {
        ListModel model = new AbstractListModel()
        {
            public int getSize() { return 200; };
            public Object getElementAt(int index) { return "index " + index; }
        };
        JList list = new JList(model);
        list.setLayoutOrientation(JList.VERTICAL_WRAP);
        list.setPrototypeCellValue("index " + 200);
        list.setVisibleRowCount(18);
        JScrollPane scrollPane = new JScrollPane(list);
        scrollPane.getHorizontalScrollBar().addAdjustmentListener(
                              new SnapToListener(list));
        return scrollPane;
    }
 
    public static void main(String[] args)
    {
        ListTest test = new ListTest();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test.getScrollPane());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class SnapToListener implements AdjustmentListener
{
    JList list;
    int colWidth;
 
    public SnapToListener(JList list)
    {
        this.list = list;
        // the 'fixedCellWidth' property can be set directly
        // or is set by/in the 'setPrototypeCellValue' method
        if(list.getFixedCellWidth() != -1)
            colWidth = list.getFixedCellWidth();
        else
        {
            int length = list.getModel().getSize();
            int rows = list.getVisibleRowCount();
            int cols = length/rows + (length % rows == 0 ? 0 : 1);
            colWidth = list.getPreferredSize().width / cols;
        }
        System.out.println("colWidth = " + colWidth);
    }
 
    public void adjustmentValueChanged(AdjustmentEvent e)
    {
        if(!e.getValueIsAdjusting())
            snapToColumns(e.getAdjustable());
    }
 
    private void snapToColumns(Adjustable adjustable)
    {
        int value = adjustable.getValue();
        int partialExposure = value % colWidth;
        System.out.println("value = " + value + "\t" +
                           "partialExposure = " + partialExposure);
        if(partialExposure > 0)
            adjustable.setValue(value - partialExposure);
    }
}

[Mike Rane]

Craig,

Thanks for the response. Your code works perfectly when I use the default look and feel. However, when I use the windows look and feel I'm not getting the snap behavior. For some reason the number returned by getValue() doesn't correspond to the position of the knob.

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class WinScroll
{
    private JScrollPane getScrollPane()
    {
        ListModel model = new AbstractListModel()
        {
            public int getSize() { return 200; };
            public Object getElementAt(int index) { return "index " + index; }
        };
        JList list = new JList(model);
        list.setLayoutOrientation(JList.VERTICAL_WRAP);
        list.setPrototypeCellValue("index " + 200);
        list.setVisibleRowCount(18);
        JScrollPane scrollPane = new JScrollPane(list);
        scrollPane.getHorizontalScrollBar().addAdjustmentListener(
                           new ImprovedSnapToListener(list));
        return scrollPane;
    }
 
    public static void main(String[] args)
    {
        // PLAF default value of "List.lockToPositionOnScroll" property:
        // javax.swing.plaf.metal.MetalLookAndFeel                      false
        // com.sun.java.swing.plaf.motif.MotifLookAndFeel               false
        // com.sun.java.swing.plaf.windows.WindowsLookAndFeel           true
        // com.sun.java.swing.plaf.windows.WindowsClassicLookAndFeel    true
        String className = UIManager.getSystemLookAndFeelClassName();
        try
        {
            UIManager.setLookAndFeel(className);
        }
        catch(ClassNotFoundException cnfe)
        {
            System.err.println("class not found: " + cnfe.getMessage());
        }
        catch(InstantiationException ie)
        {
            System.err.println("instantiation: " + ie.getMessage());
        }
        catch(IllegalAccessException iae)
        {
            System.err.println("illegal access: " + iae.getMessage());
        }
        catch(UnsupportedLookAndFeelException ulafe)
        {
            System.err.println("unsupported laf: " + ulafe.getMessage());
        }
 
        System.out.println("default lockToPositionOnScroll = " +
                           UIManager.getBoolean("List.lockToPositionOnScroll"));
        UIManager.put("List.lockToPositionOnScroll", Boolean.FALSE);
 
        WinScroll test = new WinScroll();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test.getScrollPane());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class ImprovedSnapToListener implements AdjustmentListener
{
    JList list;
    int colWidth;
 
    public ImprovedSnapToListener(JList list)
    {
        this.list = list;
        // the 'fixedCellWidth' property can be set directly
        // or is set by/in the 'setPrototypeCellValue' method
        if(list.getFixedCellWidth() != -1)
            colWidth = list.getFixedCellWidth();
        else
        {
            int length = list.getModel().getSize();
            int rows = list.getVisibleRowCount();
            int cols = length/rows + (length % rows == 0 ? 0 : 1);
            colWidth = list.getPreferredSize().width / cols;
        }
        System.out.println("colWidth = " + colWidth);
    }
 
    public void adjustmentValueChanged(AdjustmentEvent e)
    {
        if(!e.getValueIsAdjusting())
            snapToColumns(e.getAdjustable());
    }
 
    private void snapToColumns(Adjustable adjustable)
    {
        int value = adjustable.getValue();
        int partialExposure = value % colWidth;
        System.out.println("value = " + value + "\t" +
                           "partialExposure = " + partialExposure);
        int max = adjustable.getMaximum() - adjustable.getVisibleAmount();
        if(partialExposure > 0 && value < max)
            adjustable.setValue(value - partialExposure);
    }
}

[Mike Rane]

Perfect

Thank you