JScrollPane and JList auto scroll

Question

I've got the next code:

    listModel = new DefaultListModel();
    listModel.addElement(dateFormat.format(new Date()) + ": Msg1");
    messageList = new JList(listModel);
    messageList.setLayoutOrientation(JList.VERTICAL);

    messageScrollList = new JScrollPane(messageList);
    messageScrollList.setPreferredSize(new Dimension(500, 200));

    messageScrollList.getVerticalScrollBar().addAdjustmentListener(new AdjustmentListener() {  
        public void adjustmentValueChanged(AdjustmentEvent e) {  
            e.getAdjustable().setValue(e.getAdjustable().getMaximum());  
        }
    }); 

It auto scrolls down. But, if I try to scroll back up to re-read a message, it forces a scroll down. How can I fix this?

Answer 1

To auto scroll I am using a very simple concept of static variable and list.makeVisible(int index)

    public class autoScrollExample
    {
     static int index=0;
     private void someFunction()
     /*   
     * 
     */
     list1.add("element1");
     list1.makeVisible(index++);
     /*
     *
     */

     private void someOtherFunction()
     /*   
     * 
     */
     list1.add("element2");
     list1.makeVisible(index++);
     /*
     *
     */


    }

Answer 2

I used JScrollPane with JTextArea. I avoided force scroll down by scrolling only when JScrollBar max value changed:

        JScrollPane scrollPane = new JScrollPane(jTextArea);

        verticalScrollBarMaximumValue = scrollPane.getVerticalScrollBar().getMaximum();
        scrollPane.getVerticalScrollBar().addAdjustmentListener(
                e -> {
                    if ((verticalScrollBarMaximumValue - e.getAdjustable().getMaximum()) == 0)
                        return;
                    e.getAdjustable().setValue(e.getAdjustable().getMaximum());
                    verticalScrollBarMaximumValue = scrollPane.getVerticalScrollBar().getMaximum();
                });

I suppose, there is better solution to filter events without extra variables, and would appreciate if somebody post it.

Answer 3

@question enquirer. Please change your code from

messageScrollList.getVerticalScrollBar().addAdjustmentListener(new AdjustmentListener() {  
        public void adjustmentValueChanged(AdjustmentEvent e) {  
            e.getAdjustable().setValue(e.getAdjustable().getMaximum());  
        }
    });

to

messageScrollList.getVerticalScrollBar().addAdjustmentListener(new AdjustmentListener() {  
        public void adjustmentValueChanged(AdjustmentEvent e) {  
            e.getAdjustable().setValue(e.getAdjustable().getValue());  
        }
    }); 

That is change getMaximum() to getValue() Then it works as required. getMaximum() takes the scroller to its maximum; while getValue() takes it wherever event happens.

You can avoid using this piece of code altogether if you put one line

messageScrollList.setViewportView(messageList);

That works like add(component) for jList and gets its inherent behaviour.

Answer 4

I found this really useful: http://forums.sun.com/thread.jspa?threadID=623669 (post by 'inopia')
It works perfectly

As he says: "The problem here is that it can become a bit difficult to find an event that fires after both the ListModel, JList and JScrollPane have been updated."

Answer 5

When adding a new message, invoke scrollRectToVisible() on the JList using a Rectangle having the same dimensions as your message pane's preferred size. Given a vertical orientation, it may be convenient to make the preferred size of the JScrollPane's JViewport an integral multiple of the message pane's height. See also: How to Use Scroll Panes.

Addendum: This compelling discussion of Text Area Scrolling may be helpful, too.

Answer 6

this.list = blah blah... 
this.list.setSelectedValue(whatever);   
final JScrollPane sp = new JScrollPane(this.list); // needs to be after the parent is the sp 
this.list.ensureIndexIsVisible(this.list.getSelectedIndex());