Help with Scroll Bars

[Abion47]

I'm trying to make a game for a class project. In it, I'm making the main game window a JPanel inside a JScrollPane. To optimize, I only want to paint the part of the JPanel that is visible, so I want to get the values for the scroll bar when they change.

Since the values for the scroll bars and the size of the JPanel don't exactly line up, I'm trying to do this proportionally. For that, I need the maximum values of the scroll bars as well as their current ones. The problem is that the max values from getMaximum() don't line up with what is the apparent maximum. For example, take the following code:

public class GameWindow extends AdjustmentListener {
	public GameWindow() {
		canvas = new GameCanvas();
		scr = new JScrollPane(canvas);
		add(scr);
		scr.getHorizontalScrollBar().addAdjustmentListener(this);
	}

	public void adjustmentValueChanged(AdjustmentEvent e) {
		if (e.getSource() == scr.getHorizontalScrollBar()) {
			int maxVal = scr.getHorizontalScrollBar().getMaximum();
			
			System.out.println("Horiz Value: " + e.getValue() + "/" + maxVal);
		}
	}
}

When I drag the scrollbar to the right, the listener prints something to the effect of:

"Horiz Value: 775/1024"

Am I doing something wrong? Is there a better method or even class that I should be using?

[Eranga]

Are you sure that listener pick the scrolling action properly?

[Fubarable]

I think that your problem is that you're not taking the BoundedRangeModel's extent into consideration, which represents the width of the scrollbar's "thumb". e.g.,

package yr2011.m02.e;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.AdjustmentEvent;
import java.awt.event.AdjustmentListener;

import javax.swing.*;

public class ScrollPosition {

   private static void createAndShowUI() {
      JFrame frame = new JFrame("ScrollPosition");
      frame.getContentPane().add(new GameWindow());
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

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

class GameWindow extends JPanel {
   private static final int P_WIDTH = 800;
   private static final int P_HEIGHT = 600;
   private static final int SP_WIDTH = 500;
   private static final int SP_HEIGHT = 400;
   private static final Dimension PANEL_SIZE = new Dimension(P_WIDTH, P_HEIGHT);
   private static final Dimension SCROLL_PANE_SIZE = new Dimension(SP_WIDTH, SP_HEIGHT);

   private GameCanvas canvas;
   private JScrollPane scr;

   public GameWindow() {
      setLayout(new BorderLayout());
      canvas = new GameCanvas();
      scr = new JScrollPane(canvas);
      add(scr);

      canvas.setPreferredSize(PANEL_SIZE);
      scr.setPreferredSize(SCROLL_PANE_SIZE);
      scr.getHorizontalScrollBar().addAdjustmentListener(new AdjustmentListener() {

         public void adjustmentValueChanged(AdjustmentEvent e) {
            if (e.getSource() == scr.getHorizontalScrollBar()) {
               BoundedRangeModel brModel = scr.getHorizontalScrollBar().getModel();

               int maxVal = brModel.getMaximum();
               int extent = brModel.getExtent();

               System.out.println("Horiz Value: " + e.getValue() + "/(" + maxVal + "-" + extent + ")");
            }
         }
      });
   }
}

class GameCanvas extends JPanel {

}

[Abion47]

Dear blimey, that appears to have worked like a charm. Thanks!

[Fubarable]

Dear blimey, that appears to have worked like a charm. Thanks!

You're welcome, but,...

I'm trying to make a game for a class project. In it, I'm making the main game window a JPanel inside a JScrollPane. To optimize, I only want to paint the part of the JPanel that is visible, ....

JScrollPane should do this automatically for you actually. You don't have to do anything special for this feature.

[Abion47]

The problem is that every time something updates the GameCanvas (such as scrolling) the entire thing gets painted. Ultimately the map getting painted on the canvas will be rather huge, so this is to tell my game logic where the current viewport is, so it will only draw in that portion of the canvas.