Is it possible to make some items in the menu to f

问题:

I have a JMenu of 16 JMenuItems, of which I want 3 items to be displayed upfront and the rest 13 items to fade in with a 500 ms delay. Is there a way to do this animation in Java?

回答1:

This is not as easy as it sounds.

Basically I originally thought "I'll attach a popup listener to the popup menu that the menu items are added to"...but apparently this doesn't work so well. The menu popup is built dynamically on demand. Makes sense, but it's still a pain.

So instead, I've found that if I wait for addNotify I can simply start the animation engine.

The animation engine is a simple concept. It has a javax.swing.Timer that ticks at a regular interval. Coupled with a start time and a duration, we can calculate the progress of the animation and generate the alpha value as required.

The only thing left is then to notify all the interested parties that the animation has changed and voila...

import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.NumberFormat;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JPopupMenu;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.PopupMenuEvent;
import javax.swing.event.PopupMenuListener;

public class FadeMenu {

    private AnimationEngine engine;

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

    public FadeMenu() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                engine = new AnimationEngine();

                JMenuBar mb = new JMenuBar();
                JMenu flip = new JMenu("Flip");

                flip.add("Static 1");
                flip.add("Static 2");
                flip.add("Static 3");

                flip.add(new FadeMenuItem("Fade 1"));
                flip.add(new FadeMenuItem("Fade 2"));
                flip.add(new FadeMenuItem("Fade 3"));
                flip.add(new FadeMenuItem("Fade 4"));

                mb.add(flip);

                JFrame frame = new JFrame("Testing");
                frame.setJMenuBar(mb);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
        }

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

    public class FadeMenuItem extends JMenuItem {

        public FadeMenuItem(String text) {
            super(text);
            engine.addTimingListener(new TimingListener() {
                @Override
                public void timingEvent() {
                    repaint();
                }
            });
        }

        @Override
        protected void paintComponent(Graphics g) {
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setComposite(AlphaComposite.SrcOver.derive(engine.getAlpha()));
            super.paintComponent(g2d);
            g2d.dispose();
        }

        @Override
        public void removeNotify() {
            Container parent = getParent();
            if (parent instanceof JPopupMenu) {
                JPopupMenu menu = (JPopupMenu) parent;
                engine.stop();
            }
            super.removeNotify(); 
        }

        @Override
        public void addNotify() {
            super.addNotify();
            Container parent = getParent();
            if (parent instanceof JPopupMenu) {
                JPopupMenu menu = (JPopupMenu) parent;
                engine.restart();
            }
        }
    }

    public interface TimingListener {

        public void timingEvent();
    }

    public class AnimationEngine {

        private Timer fade;
        private float alpha;
        private long startTime;
        private long duration = 1000;
        private List<TimingListener> listeners;

        public AnimationEngine() {
            listeners = new ArrayList<>(5);
            fade = new Timer(40, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    long elapsed = System.currentTimeMillis() - startTime;
                    if (elapsed >= duration) {
                        ((Timer) e.getSource()).stop();
                        alpha = 1f;
                    } else {
                        alpha = (float) elapsed / (float) duration;
                    }
                    fireTimingEvent();
                }
            });
            fade.setRepeats(true);
            fade.setCoalesce(true);
            fade.setInitialDelay(500);
        }

        public void addTimingListener(TimingListener listener) {
            listeners.add(listener);
        }

        public void removeTimingListener(TimingListener listener) {
            listeners.add(listener);
        }

        protected void fireTimingEvent() {
            for (TimingListener listener : listeners) {
                listener.timingEvent();
            }
        }

        public void restart() {
            fade.stop();
            alpha = 0;
            fireTimingEvent();
            startTime = System.currentTimeMillis();
            fade.start();
        }

        public float getAlpha() {
            return alpha;
        }

        public void stop() {
            fade.stop();
        }
    }
}

While this works on Windows, I'd be concerned that it might not work on other platforms, as the means by which the menus are generated are controlled (in part) by the UI delegate. This could become very messy, very quickly

回答2:

start a timer to fire an event to fade in