Question

I’m trying to translate between view and viewport coordinates.
But the JViewport/JScrollpane doesn’t seem to work as documented. JViewport.toViewCoordinates()
thinks the view is always at the top left of the component, even though that’s clearly not the case.

String text = "blahblahblah\nblahblah\nblah";
JFrame frame = new JFrame("title");
JTextArea textArea = new JTextArea(text, 1, 30); // shows only one line
frame.add(new JScrollPane(textArea));
frame.pack();
frame.setVisible(true);
textArea.setCaretPosition(text.length()); // now showing the last line
JViewport viewport = ((JViewport)textArea.getParent());

viewport.getViewRect(); // returns java.awt.Rectangle[x=0,y=0,width=330,height=16]

viewport.getViewPosition(); // returns java.awt.Point[x=0,y=0]

viewport.toViewCoordinates(new Point(0,0)); // returns java.awt.Point[x=0,y=0]

The above is contrived example. My real JTextArea is larger than one line. I don’t need JTextArea “model” coordinate (the offset in the text). I need genuine 2d coordinates.

The view position shouldn’t be (0,0), as the first visible character in the viewport is actually in the 3rd line of the JTextArea.

Any other suggestions on how I can translate between view and component coordinates when using JScrollPane?

— added —

This also fails.

SwingUtilities.convertPoint(viewport,0,0, textArea);
(java.awt.Point) java.awt.Point[x=0,y=0]

— added —

Here is the final working version, based on the answer I received.
it shows java.awt.Point[x=0,y=32] which is what I expected.

@Test
public void test() throws InterruptedException {

    String text = "blahblahblah\nblahblah\nblah";
    JFrame frame = new JFrame("title");
    JTextArea textArea = new JTextArea(text, 1, 30);
    frame.add(new JScrollPane(textArea));
    frame.pack();
    frame.setVisible(true);
    textArea.setCaretPosition(text.length());
    final JViewport viewport = ((JViewport)textArea.getParent());

    SwingUtilities.invokeLater(new Runnable()
    {
        @Override
        public void run() {
            System.out.println(viewport.getViewPosition());
        }
    });

    Thread.sleep(1000);
}

Answer 1

The problem is that the method to get the viewPosition() executes before the viewport has actually been scrolled. This is because sometimes Swing adds code to the end of the event thread for later processing.

Usually this problem can be solved by wrapping your code in a SwingUtilities.invokeLater() so the code is executed after Swing has done all its processing. However in the simple demo below I found I needed to add two invokeLater() methods. I’m not sure why.

Move the caret up/down and you will see the view position change. The second value will contain the correct position:

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

public class Test5
{
    public static void createAndShowGUI()
    {
        String text = "one\ntwo\nthree\nfour\nfive";
        JFrame frame = new JFrame("title");
        JTextArea textArea = new JTextArea(text, 1, 30); // shows only one line
        JScrollPane scrollPane = new JScrollPane( textArea );
        frame.add(scrollPane);
        frame.pack();
        frame.setVisible(true);

        final JViewport viewport = scrollPane.getViewport();

        textArea.addCaretListener( new CaretListener()
        {
            public void caretUpdate(CaretEvent e)
            {
                SwingUtilities.invokeLater(new Runnable()
                {
                    public void run()
                    {
                        System.out.println("First : " + viewport.getViewPosition() );

                        SwingUtilities.invokeLater(new Runnable()
                        {
                            public void run()
                            {
                                System.out.println("Second: " + viewport.getViewPosition() );
                            }
                        });
                    }
                });
            }
        });

        textArea.setCaretPosition(text.length());
    }

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