Painting dissappears during resizing (with mouse)

[ira greenberg]

I am relatively new to java and am building a Swing Graphics app. The gui is a JFrame with nested JPanels using a variety of Box layouts. The main drawng algorithm plots geometry in real time (based on user interaction) using a GeneralPath. The app runs continuously(utilizing a Swing Timer) unless the user hits a stop button. The drawing algorithm is in a class called Poly that extends a JComponent. I have tried implementing in 2 different ways.


1. I add an instance of Poly each iteration, which works fine, except that it is really slow, as each JComponent gets added. After a few hundred iterations, the repaint rate is not acceptable.


2. I only instantiate a single Poly object and call some drawing methods. The repaint rate is really fast, but whenever I resize the main application window or change the backgroun of the JPanel, the drawing dissappears. I have removed all paint/paintcomponent methods in all my classes and am using getGraphics() locally in the drawing algoirthm.


This is really, really frustrating and help would be greatly appreciated.


Thanks, Ira

[Craig Wood]

The getGraphics method is a one–time deal. It just borrows the current graphics context for query or the occasional use. Once java decides to repaint the component (triggered by minimize/restore, maximize, restore events or after a partial/full covering/uncovering by another window) all is lost.

One solution is keep a reference to the objects you have created for rendering and arrange for the component to re–render them each time java (AWT or Swing) calls for the component to repaint itself.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.*;
import java.util.List;
import javax.swing.*;
import javax.swing.Timer;
 
public class HoldingState
{
    public static void main(String[] args)
    {
        StatefulPanel panel = new StatefulPanel();
        DiscMaker maker = new DiscMaker(panel);
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(panel);
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class DiscMaker
{
    StatefulPanel statefulPanel;
    Random seed;
 
    public DiscMaker(StatefulPanel sp)
    {
        statefulPanel = sp;
        seed = new Random();
        new Timer(1000, new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                Color bg = getNewColor();
                Color border = getNewColor();
                Point p = getNewLocation();
                List list = statefulPanel.getDiscList();
                list.add(new Disc(bg, border, p));
                statefulPanel.repaint();
            }
        }).start();
    }
 
    private Color getNewColor()
    {
        return new Color(seed.nextInt(0x1000000));
    }
 
    private Point getNewLocation()
    {
        int w = statefulPanel.getWidth();
        int h = statefulPanel.getHeight();
        int x = Disc.DIA/2 + seed.nextInt(w - Disc.DIA);
        int y = Disc.DIA/2 + seed.nextInt(h - Disc.DIA);
        return new Point(x, y);
    }
}
 
class StatefulPanel extends JPanel
{
    List discs;
 
    public StatefulPanel()
    {
        discs = new ArrayList();
        setBackground(Color.white);
    }
 
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        for(int i = 0; i < discs.size(); i++)
        {
            Disc disc = (Disc)discs.get(i);
            disc.draw(g2);
        }
    }
 
    public List getDiscList()
    {
        return discs;
    }
}
 
class Disc
{
    final static int DIA = 20;
    Color bgColor, borderColor;
    Point point;
    Ellipse2D e;
 
    public Disc(Color bg, Color border, Point p)
    {
        bgColor = bg;
        borderColor = border;
        point = p;
        e = new Ellipse2D.Double(p.x - DIA/2, p.y - DIA/2, DIA, DIA);
    }
 
    public void draw(Graphics2D g2)
    {
        g2.setPaint(bgColor);
        g2.fill(e);
        g2.setPaint(borderColor);
        g2.draw(e);
    }
}