Smart JScrollPane autoscrolling

Question

I am attempting to implmement smart autoscrolling on a JScrollPane containing a JTextPane. The JTextPane is used for logging my app in color. However I'm running into a wall trying to do smart autoscrolling. By smart autoscrolling I don't mean blindly autoscrolling every time something changes, I mean checking to see if your scrolled all the way down, then autoscroll. However no matter what I do it either always autoscrolls or doesn't at all

As a test script, here's the setup (the JFrame has been left out)

final JTextPane textPane = new JTextPane();
textPane.setEditable(false);
final JScrollPane contentPane = new JScrollPane(textPane);
contentPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

And here's the ugly auto add test loop

while (true)
    try {
        Thread.sleep(1000);
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    JScrollBar scrollBar = scroll;
                    boolean preCheck = ((scrollBar.getVisibleAmount() != scrollBar.getMaximum()) && (scrollBar.getValue() + scrollBar.getVisibleAmount() == scrollBar.getMaximum()));
                    System.out.println("Value: " + scroll.getValue()
                                    + " | Visible: " + scrollBar.getVisibleAmount()
                                    + " | Maximum: " + scrollBar.getMaximum()
                                    + " | Combined: " + (scrollBar.getValue() + scrollBar.getVisibleAmount())
                                    + " | Vis!=Max : " + (scrollBar.getVisibleAmount() != scrollBar.getMaximum())
                                    + " | Comb=Max: " + (scrollBar.getValue() + scrollBar.getVisibleAmount() == scrollBar.getMaximum())
                                    + " | Eval: " + preCheck);
                    StyledDocument doc = textPane.getStyledDocument();
                    doc.insertString(doc.getLength(), "FAGAHSIDFNJASDKFJSD\n", doc.getStyle(""));
                    if (!preCheck)
                            textPane.setCaretPosition(doc.getLength());
                } catch (BadLocationException ex) {
                            ex.printStackTrace();
                }
            }
        });
    } catch (Exception e) {
        e.printStackTrace();
    }

Its not pretty, but it gets the job done.

Here's though the relevant check

 boolean preCheck = ((scrollBar.getVisibleAmount() != scrollBar.getMaximum()) && (scrollBar.getValue() + scrollBar.getVisibleAmount() == scrollBar.getMaximum()));
 if (preCheck)
     textPane.setCaretPosition(doc.getLength());

Thats the part thats been giving me trouble. There is first a check to see if the bar is visible but unusable (not enough text, making the bar the full length), then if the bottom of the bar is equal to the maximum. In theory, that should work. However nothing, including moving the check around, has gotten the results I would like.

Any suggestions?

NOT A DUPLICATE of this or this, as they are wanting it to always scroll, not just sometimes.

Answer 1

Edit:

I replaced the following code with a more flexible version that will work on any component in a JScrollPane. Check out: Smart Scrolling.

import java.awt.*;
import java.awt.event.*;
import java.util.Date;
import javax.swing.*;
import javax.swing.text.*;

public class ScrollControl implements AdjustmentListener
{
    private JScrollBar scrollBar;
    private JTextComponent textComponent;
    private int previousExtent = -1;

    public ScrollControl(JScrollPane scrollPane)
    {
        Component view = scrollPane.getViewport().getView();

        if (! (view instanceof JTextComponent))
            throw new IllegalArgumentException("Scrollpane must contain a JTextComponent");

        textComponent = (JTextComponent)view;

        scrollBar = scrollPane.getVerticalScrollBar();
        scrollBar.addAdjustmentListener( this );
    }

    @Override
    public void adjustmentValueChanged(final AdjustmentEvent e)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                checkScrollBar(e);
            }
        });
    }

    private void checkScrollBar(AdjustmentEvent e)
    {
        //  The scroll bar model contains information needed to determine the
        //  caret update policy.

        JScrollBar scrollBar = (JScrollBar)e.getSource();
        BoundedRangeModel model = scrollBar.getModel();
        int value = model.getValue();
        int extent = model.getExtent();
        int maximum = model.getMaximum();
        DefaultCaret caret = (DefaultCaret)textComponent.getCaret();

        //  When the size of the viewport changes there is no need to change the
        //  caret update policy.

        if (previousExtent != extent)
        {
            //  When the height of a scrollpane is decreased the scrollbar is
            //  moved up from the bottom for some reason. Reposition the
            //  scrollbar at the bottom

            if (extent < previousExtent
            &&  caret.getUpdatePolicy() == DefaultCaret.UPDATE_WHEN_ON_EDT)
            {
                scrollBar.setValue( maximum );
            }

            previousExtent = extent;
            return;
        }

        //  Text components will not scroll to the bottom of a scroll pane when
        //  a bottom inset is used. Therefore the location of the scrollbar,
        //  the height of the viewport, and the bottom inset value must be
        //  considered when determining if the scrollbar is at the bottom.

        int bottom = textComponent.getInsets().bottom;

        if (value + extent + bottom < maximum)
        {
            if (caret.getUpdatePolicy() != DefaultCaret.NEVER_UPDATE)
                caret.setUpdatePolicy(DefaultCaret.NEVER_UPDATE);
        }
        else
        {
            if (caret.getUpdatePolicy() != DefaultCaret.UPDATE_WHEN_ON_EDT)
            {
                caret.setDot(textComponent.getDocument().getLength());
                caret.setUpdatePolicy(DefaultCaret.UPDATE_WHEN_ON_EDT);
            }
        }
    }

    private static void createAndShowUI()
    {
        JPanel center = new JPanel( new GridLayout(1, 2) );
        String text = "1\n2\n3\n4\n5\n6\n7\n8\n9\n0\n";

        final JTextArea textArea = new JTextArea();
        textArea.setText( text );
        textArea.setEditable( false );
        center.add( createScrollPane( textArea ) );
        System.out.println(textArea.getInsets());

        final JTextPane textPane = new JTextPane();
        textPane.setText( text );
        textPane.setEditable( false );
        center.add( createScrollPane( textPane )  );
        textPane.setMargin( new Insets(5, 3, 7, 3) );
        System.out.println(textPane.getInsets());

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(center, BorderLayout.CENTER);
        frame.setSize(500, 200);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);

        Timer timer = new Timer(2000, new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                try
                {
                    Date now = new Date();
                    textArea.getDocument().insertString(textArea.getDocument().getLength(), "\n" + now.toString(), null);
                    textPane.getDocument().insertString(textPane.getDocument().getLength(), "\n" + now.toString(), null);
                }
                catch (BadLocationException e1) {}
            }
        });
        timer.start();
    }

    private static JComponent createScrollPane(JComponent component)
    {
        JScrollPane scrollPane = new JScrollPane(component);
        new ScrollControl( scrollPane );

        return scrollPane;
    }

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