Having some trouble understanding JViewPorts

Question

I have been trying to understand JViewports and tried to work with one. I have been using this gentleman's class, which just extends the JViewport class.

If I want to set the viewport in the middle of a 6480*4860 object implementing the JPanel Class, how come this code gives me a blank view, even if, upon debugging,I have confirmation that the class implementing JPanel I am using has been added to the view? Am I completely mixed up in my calls to the different methods or in my comprehension of the coordinates used and their meaning? And one more : Does calling

    v.setOpaque(true);

set only the viewport opaque or it will set all its children too? I guess I will quickly know the answer to this one if I figure out how to properly use viewports.

public myProgram(){ 
    ...

    myCustomJPanel = new MyCustonJPanel();
    myCustomJPanel.setBackground(Color.WHITE);
    GrabbableViewport v = new GrabbableViewport();
    v.setViewSize(new Dimension(720,540));
    v.setViewPosition(new Point(720,540));
    v.setView(myCustomJPanel);
    v.setViewPosition(new Point(720,540));
    v.setLocation(43, 5);
    appropriateParentPanel.add(v);
}

Here is the class I am trying to use. It takes much more space for this short question, but formatted text should please you more than plain text! If I am not mistaken, though, the problem should not come from here.

// Copyright (c) 2006 - 2008, Markus Strauch.
// All rights reserved.
// 
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
// 
// * Redistributions of source code must retain the above copyright notice, 
// this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above copyright notice, 
// this list of conditions and the following disclaimer in the documentation 
// and/or other materials provided with the distribution.
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE 
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF 
// THE POSSIBILITY OF SUCH DAMAGE.

package net.sf.sdedit.ui.components;

import java.awt.Component;
import java.awt.Cursor;
import java.awt.Image;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;

import javax.swing.ImageIcon;
import javax.swing.JComponent;
import javax.swing.JViewport;
import javax.swing.event.MouseInputListener;

/**
 * A <tt>GrabbableViewport</tt> is a <tt>JViewport</tt> that scrolls its
 * view when the mouse is dragged. While the mouse is being dragged, its cursor
 * is set to a &quot;grabbing hand&quot;, like in applications such as Acrobat
 * Reader.
 * 
 * @author Markus Strauch
 * 
 */
public class GrabbableViewport extends JViewport implements MouseInputListener {

    private static Cursor HAND = new Cursor(Cursor.MOVE_CURSOR);

    private static Cursor DFLT = new Cursor(Cursor.DEFAULT_CURSOR);

    public static void setHandCursorIcon(ImageIcon icon) {
        Image grabbingHand = icon.getImage();
        HAND = Toolkit.getDefaultToolkit().createCustomCursor(grabbingHand,
                new Point(0, 0), "hand cursor");
    }

    /**
     * Constructor.
     */
    public GrabbableViewport () {
        super ();
    }

    private Rectangle rect; 

    private Point point;    

    private JComponent view;    

    public void setView(Component view) {
        super.setView(view);
        if (this.view != view) {
            if (this.view != null) {
                this.view.removeMouseListener(this);
                this.view.removeMouseMotionListener(this);
            }
            if (view != null) {
                view.addMouseListener(this);
                view.addMouseMotionListener(this);
            }
            this.view = (JComponent) view;
        }
    }

    public void mouseClicked(MouseEvent e) {
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }

    public void mousePressed(MouseEvent e) {
        view.setCursor(HAND);
        //((Component) e.getSource()).setCursor(HAND);
        rect = getViewRect();
        point = screenLocation(e);
    }

    private Point screenLocation(MouseEvent e) {
        Point root = view.getLocationOnScreen();
        Point mouse = e.getPoint();
        if (rect != null && !rect.contains(mouse)) {
            return null;
        }
        Point screenPoint = new Point(root.x + mouse.x, root.y + mouse.y);
        return screenPoint;
    }

    public void mouseReleased(MouseEvent e) {
        view.setCursor(DFLT);
        scrollTo(screenLocation(e));
        clear();
    }

    public void mouseDragged(MouseEvent e) {
        scrollTo(screenLocation(e));
    }

    public void mouseMoved(MouseEvent e) {
    }

    private void scrollTo(Point newPoint) {
        if (point != null && newPoint != null && rect != null) {
            int deltaX = point.x - newPoint.x;
            int deltaY = point.y - newPoint.y;
            rect.x = rect.x + deltaX;
            rect.y = rect.y + deltaY;
            ((JComponent) getView()).scrollRectToVisible(rect);
                point = newPoint;
        }
    }

            private void clear() {
                rect = null;
            point = null;
        }
    }

I could give you a sscce but I think this should be simple enough, although I can piece together some example quickly if you ask.

Answer 1

Because GrabbableViewport extends JViewport, I'd guess that you're supposed to use it in a JScrollPane, as shown here. If not, the example may help you construct an sscce using ScrollPanePaint.MyPanel.

myCustomJPanel = new MyCustonJPanel();
GrabbableViewport v = new GrabbableViewport();
v.setView(myCustomJPanel);
JScrollPane scrollPane = new JScrollPane();
scrollPane.setViewport(v);
enclosingContainer.add(scrollPane);

See also this related example the leverages the existing JScrollPane actions.

Using setOpaque(true) is a promise to paint every pixel, as discussed here.