Is there a simple way to have a fade/animation change from JButton Icon to Rollover Icon?

Question

Bellow is one of my buttons code

    continueGame = new JButton("");
    continueGame.setIcon(new ImageIcon("resources/buttonUI/continueGame.png"));
    continueGame.setRolloverEnabled(true);
    continueGame.setRolloverIcon(new ImageIcon("resources/buttonUI/continueGameOnHover.png"));
    continueGame.setBorderPainted(false);
    continueGame.setContentAreaFilled(false);
    continueGame.setBounds(frameSize.x/2-343/2, (int)((frameSize.y)*0.28), 343, 85);
    continueGame.addActionListener(this);

This works perfectly fine, but when I hover the button, it instantly changes from the base icon, to the RolloverIcon. Is there a way to have a fade between them or a slow change?

Answer 1

"Simple" is subjective

The "biggest" issue is trying to figure out "where" the painting should occur.

The icon and roll over icon are painted by the UI delegate. This is kind of painful as you really don't want to design a UI delegate for every possible look and feel you might want to use it.

So, instead, you need to take over a lot of the functionality yourself.

The following approach basically uses a delegate pattern to off load the blending of the "normal" and "roll over" icons and sets the button's icon property accordingly (it's a cheat)

The basic functionality is controller by a Swing Timer, see How to Use Swing Timers for more details.

It is time based, that is, rather then running until a value is achieved, it run's over a specified period of time. This generally produces a better animation effect.

import java.awt.AlphaComposite;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ButtonModel;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class JavaApplication155 {

    public static void main(String[] args) {
        new JavaApplication155();
    }

    public JavaApplication155() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    JFrame frame = new JFrame();
                    frame.add(new TestPane());
                    frame.pack();
                    frame.setLocationRelativeTo(null);
                    frame.setVisible(true);
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() throws IOException {
            JButton btn = new JButton();
            BufferedImage cat = ImageIO.read(getClass().getResource("cat.png"));
            BufferedImage dog = ImageIO.read(getClass().getResource("dog.png"));

            btn.setIcon(new ImageIcon(cat));
            btn.setRolloverEnabled(true);

            RolloverAnimator animator = new RolloverAnimator(btn, cat, dog);

            setLayout(new GridBagLayout());
            add(btn);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

    }

    public class RolloverAnimator {

        private JButton btn;
        private BufferedImage normalImage;
        private BufferedImage rolloverImage;

        private double progress;

        private Timer timer;

        private int totalAnimationTime = 300;
        private int animationTime = totalAnimationTime;
        private Long startedAt = null;

        private boolean wasRolledOver = false;

        public RolloverAnimator(JButton btn, BufferedImage normalImage, BufferedImage rolloverImage) {
            this.btn = btn;
            this.normalImage = normalImage;
            this.rolloverImage = rolloverImage;
            ButtonModel model = btn.getModel();
            model.addChangeListener(new ChangeListener() {
                @Override
                public void stateChanged(ChangeEvent e) {
                    if (model.isArmed()) {
                        return;
                    }
                    if (model.isRollover() && !wasRolledOver) {
                        wasRolledOver = true;
                        prepare();
                    } else if (!model.isRollover() && wasRolledOver) {
                        wasRolledOver = false;
                        prepare();
                    }
                }
            });

            timer = new Timer(5, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    if (startedAt == null) {
                        startedAt = System.currentTimeMillis();
                    }
                    long duration = System.currentTimeMillis() - startedAt;
                    progress = (double) duration / (double) animationTime;
                    if (progress > 1.0) {
                        progress = 1.0;
                        startedAt = null;
                        timer.stop();
                    }
                    BufferedImage target = transition(normalImage, rolloverImage, progress);
                    btn.setIcon(new ImageIcon(target));

                    if (!timer.isRunning()) {
                        progress = 0.0;
                    }
                }
            });
        }

        protected void prepare() {
            if (timer.isRunning()) {
                timer.stop();
                animationTime = (int) ((double) totalAnimationTime * progress);
                startedAt = System.currentTimeMillis() - (totalAnimationTime - (long)animationTime);
            } else {
                animationTime = totalAnimationTime;
                startedAt = null;
            }
            progress = 0.0;

            timer.start();
        }

        public BufferedImage transition(BufferedImage from, BufferedImage to, double progress) {
            int width = Math.max(from.getWidth(), to.getWidth());
            int height = Math.max(from.getHeight(), to.getHeight());
            BufferedImage target = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
            Graphics2D g2d = target.createGraphics();

            float normalAlpha = wasRolledOver ? 1.0f - (float) progress : (float) progress;
            float rollOverAlpha = wasRolledOver ? (float) progress : 1.0f - (float) progress;

            g2d.setComposite(AlphaComposite.SrcOver.derive(normalAlpha));
            draw(from, target, g2d);
            g2d.setComposite(AlphaComposite.SrcOver.derive(rollOverAlpha));
            draw(to, target, g2d);
            g2d.dispose();
            return target;
        }

        protected void draw(BufferedImage img, BufferedImage on, Graphics2D g2d) {
            int xPos = (img.getWidth() - on.getWidth()) / 2;
            int yPos = (img.getHeight() - on.getHeight()) / 2;
            g2d.drawImage(img, xPos, yPos, btn);
        }
    }

}