Repaint JPanel only after window resize is finished

Question

I have a JPanel on which I've painted four rectangles. The color for each of these rectangles is selected at random. The colors should change only when the user clicks on a particular rectangle.

The problem is, while a user is resizing the window, everything on the JPanel is 'repainted' repeatedly. This causes the rectangles to rapidly change color.

Ideally, I would need the colors of the rectangles to stay the same during a resize. Otherwise, I could also manage with a solution where the JPanel is repainted only once after a resize has been completed.

Do you have any general ideas on how I could implement this? I feel like it would have been a lot easier if there was a onStartResize and onFinishResize callback method in ComponentListener.

Thanks!

score 6 · Accepted Answer · edited May 23 '17 at 12:24

This example may serve to illustrate the violation adduced by @kleopatra. As a component is resized, the event dispatch mechanism helpfully invokes repaint() for you. If you change the state of what you're rendering, say in paintComponent(), you'll see it cycle rapidly. In the example below, the bottom row flickers as you resize, while the top row remains unchanged.

Addendum: AnimationTest is a related example that takes advantage of this effect to perform animation in a ComponentAdapter.

ResizeMe

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

/** @https://stackoverflow.com/questions/7735774 */
public class ResizeMe extends JPanel {

    private static final int N = 4;
    private static final int SIZE = 100;
    private static final Random rnd = new Random();
    private final List<JLabel> list = new ArrayList<JLabel>();
    private boolean randomize;

    public ResizeMe(boolean randomize) {
        this.randomize = randomize;
        this.setLayout(new GridLayout(1, 0));
        for (int i = 0; i < N; i++) {
            JLabel label = new JLabel();
            label.setPreferredSize(new Dimension(SIZE, SIZE));
            label.setOpaque(true);
            list.add(label);
            this.add(label);
        }
        initColors();
        this.addComponentListener(new ComponentAdapter() {

            @Override
            public void componentResized(ComponentEvent e) {
                System.out.println(e);
            }
        });
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (randomize) {
            initColors();
        }
    }

    private void initColors() {
        for (JLabel label : list) {
            label.setBackground(new Color(rnd.nextInt()));
        }
    }

    private static void display() {
        JFrame f = new JFrame("ResizeMe");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLayout(new GridLayout(0, 1));
        f.add(new ResizeMe(false), BorderLayout.NORTH);
        f.add(new ResizeMe(true), BorderLayout.SOUTH);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

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

            @Override
            public void run() {
                display();
            }
        });
    }
}

was about to giving a five (nice screenshot :-) - until I saw your code: changing state in paintComponent ... tsssee. Don't, it can create nasty loops! — kleopatra, Oct 12 '11 at 08:24
Exactly right; it's meant to serve as an example of what _not_ to do. — trashgod, Oct 12 '11 at 08:34

score 5 · Answer 2 · answered Oct 12 '11 at 08:17

5

you are probably getting the painting logic incorrectly (guessing of course, without seeing any code :) Looks like you're changing the color in the paintComponent method. Which violates the general rule: Dont change any state of the component while painting.

Instead think of the rectangle having fixed colors until they are clicked, then change the color and repaint. Resizing doesn't come into the play anywhere.

answered Oct 12 '11 at 08:17

kleopatra

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+1 I've added an [example](http://stackoverflow.com/questions/7735774/repaint-jpanel-only-after-window-resize-is-finished/7737098#7737098) that illustrates the violation. – trashgod Oct 12 '11 at 08:23

score 3 · Answer 3 · answered Oct 12 '11 at 06:37

3

Initialize an array of Colors. On click on the rect refill the array with new random colors. If the array is empty also fill it with random Colors. Store the array together with the Rectangles.

answered Oct 12 '11 at 06:37

StanislavL

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score 1 · Answer 4 · answered Oct 12 '11 at 07:18

that right the safiest way is implemnts ComponentListener, its method componentResized(ComponentEvent e), inside this method start javax.swing.Timer with small dealy 350-500ms, if resize still continue only Timer#restart(),

but This causes the rectangles to rapidly change color. indicate another problem, how is that possible, because MouseListener has nothing to do with Resizing,

score -1 · Answer 5 · answered Oct 12 '11 at 06:05

-1

I assume you're using paintAll(Graphics g). Try creating a method called redoRectangles(), which randomly changes the colors of the rectangles. Call it at initialization and as a mouse event.

If you're not using paintAll, I have no idea.

answered Oct 12 '11 at 06:05

personak

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1

wrong assumption (except the OP got the painting _completely_ wrong): nobody _ever_ touches paintAll ... – kleopatra Oct 12 '11 at 08:19

Repaint JPanel only after window resize is finished

5 Answers5