I am having issues with the java.awt.ScrollPane class's SCROLLBARS_AS_NEEDED display policy handling component resize. Ideally, if I have a ScrollPane that contains a Component that is significantly smaller than the Scrollpane and I shrink the Scrollpane to a size that is still greater than the child component, no scrollbars will appear. However, in practice both scrollbars seem to flicker while the resize operation is occurring and may even persist until the next redraw after the operation completes. Here is a small example to demonstrate what I mean:
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.ScrollPane;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
public class Tester implements Runnable{
/**
* @param args
*/
public static void main(String[] args) {
System.setProperty("sun.awt.noerasebackground", "true");
Tester t = new Tester();
SwingUtilities.invokeLater(t);
}
@SuppressWarnings("serial")
@Override
public void run() {
JFrame frame = new JFrame("Tooltip tester");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(800, 800);
Canvas c = new Canvas(){
@Override
public void paint(Graphics g){
g.setColor(Color.WHITE);
g.fillRect(0, 0, this.getWidth(), this.getHeight());
}
};
c.setPreferredSize(new Dimension(400, 400));
ScrollPane s = new ScrollPane();
s.add(c);
frame.add(s);
frame.setVisible(true);
}
}
Shrinking the window in the application above should cause scrollbars to flash. I believe this is caused by a bug in the implementation of the peer of the ScrollPane. (Source Here) I've copied the method that I believe has the error (line 145 in the link).
Dimension getChildSize() {
ScrollPane sp = (ScrollPane)target;
if (sp.countComponents() > 0) {
Component c = sp.getComponent(0);
return c.size();
} else {
return new Dimension(0, 0);
}
}
To me, it seems like the getChildSize() method should call c.getPreferredSize() rather than c.size(). If the ScrollPane child's size is greater than its current preferred size, it should be able to shrink without scroll bars showing up (in my opinion). To test this theory, I overrode the size() method in my example from above:
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.ScrollPane;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
public class Tester implements Runnable{
/**
* @param args
*/
public static void main(String[] args) {
System.setProperty("sun.awt.noerasebackground", "true");
Tester t = new Tester();
SwingUtilities.invokeLater(t);
}
@SuppressWarnings("serial")
@Override
public void run() {
JFrame frame = new JFrame("Tooltip tester");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(800, 800);
Canvas c = new Canvas(){
private Dimension prefSize = new Dimension();
@Override
public void paint(Graphics g){
g.setColor(Color.WHITE);
g.fillRect(0, 0, this.getWidth(), this.getHeight());
}
@Override
public void setPreferredSize(Dimension preferredSize) {
prefSize = preferredSize;
super.setPreferredSize(preferredSize);
}
@Override
public Dimension size() {
return prefSize;
}
};
c.setPreferredSize(new Dimension(400, 400));
ScrollPane s = new ScrollPane();
s.add(c);
frame.add(s);
frame.setVisible(true);
}
}
This code behaves exactly how I want it to. However, overriding the size() method to do something other than what the documentation says it should do isn't a particularly elegant way to fix this problem. I feel like I must be doing something wrong here. Is it possible that a fairly basic functionality of the ScrollPane peer for X11 is broken?
A few other related notes:
-Use of java.awt.Canvas is non-negotiable for my purposes. In my actual application, I am drawing something very quickly using Java AWT Native Interface.
-While I read the documentation about new improvements to light/heavyweight compatibility in Java 7 and later versions of Java 6, I didn't really see an improvement in either case so I decided to stick with awt.ScrollPane over swing.JScrollPane. Either way, a ScrollPane should work correctly in the simple case I showed above.
