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!

Answer 1

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.

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();
            }
        });
    }
}