JMenuBar SelectionModel ChangeListener only fires once

Question

I am trying to get my JMenuBar to model the behavior of the menu bars of Firefox and iTunes. The behavior: the menu bar is initially hidden. But, when you press Alt, the menu bar appears (with the first item selected) and when you don't have a menu item selected, the menu bar disappears. My idea was to listen for selection changes to the JMenuBar via a ChangeListener on its SelectionModel.

However, the behavior of the attached SSCCE is not as desired. When the frame loads, the JMenuBar is not visible. When you press Alt, the menu bar appears with the first menu selected (thanks to the WindowsLookAndFeel). However, every subsequent Alt pressed fires no ChangeEvents. I can't figure out why...

Anyone have light to shed?

public class MenuBarTest extends javax.swing.JFrame {

    public MenuBarTest() {
        initComponents();
        jMenuBar1.setVisible(false);
        jMenuBar1.getSelectionModel().addChangeListener(new ChangeListener() {
            @Override
            public void stateChanged(ChangeEvent e) {
                System.out.println(e.toString());
                jMenuBar1.setVisible(jMenuBar1.isSelected());
                System.out.println(jMenuBar1.isSelected());
                System.out.println(jMenuBar1.getSelectionModel().isSelected());
            }
        });
    }

    private void initComponents() {

        jMenuBar1 = new javax.swing.JMenuBar();
        jMenu1 = new javax.swing.JMenu();
        jMenuItem1 = new javax.swing.JMenuItem();
        jMenu2 = new javax.swing.JMenu();
        jMenuItem2 = new javax.swing.JMenuItem();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jMenu1.setText("File");
        jMenuItem1.setText("jMenuItem1");
        jMenu1.add(jMenuItem1);
        jMenuBar1.add(jMenu1);
        jMenu2.setText("Edit");
        jMenuItem2.setText("jMenuItem2");
        jMenu2.add(jMenuItem2);
        jMenuBar1.add(jMenu2);
        setJMenuBar(jMenuBar1);
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
                layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGap(0, 400, Short.MAX_VALUE));
        layout.setVerticalGroup(
                layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGap(0, 279, Short.MAX_VALUE));

        pack();
    }

    public static void main(String args[]) {
        try {
            UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | javax.swing.UnsupportedLookAndFeelException ex) {
            ex.printStackTrace();
        }
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewClass().setVisible(true);
            }
        });
    }
    private javax.swing.JMenu jMenu1;
    private javax.swing.JMenu jMenu2;
    private javax.swing.JMenuBar jMenuBar1;
    private javax.swing.JMenuItem jMenuItem1;
    private javax.swing.JMenuItem jMenuItem2;
}

Answer 1

Looks like the menubar is never unselected, once it had been selected. Not sure if that's a bug or not.

Could be a better idea to listen directly to the MenuSelectionManager as that's where you are notified about all changes to menu selection anywhere. Needs some logic to filter out those unrelated to the menuBar, something similar to:

ChangeListener listener = new ChangeListener() {
    @Override
    public void stateChanged(ChangeEvent e) {
        MenuElement[] elements = MenuSelectionManager.defaultManager().getSelectedPath();
        jMenuBar1.setVisible(elements.length > 0 && elements[0] == jMenuBar1);
    }
};
MenuSelectionManager.defaultManager().addChangeListener(listener);

Update

A hefty drawback of hiding the menubar is that accelerators to its menuItems stop working. The reason is that only componentInputMaps of components which are showing are asked to handle them. This is done deep down in the bowels of the swing package, namely by the package private class KeyboardManager. No way to hook-in a custom manager (which might be implemented to handle menubars that are not showing).

At the other end of the chain, we can interfer, though. Basically two options, both subclassing menubar:

• (extremely dirty trick!) override isShowing to always return true. I've seen this done, but can't really recommend because there might be side-effects that I don't know
• a slightly dirty trick: add a property hidden and implement getPreferredSize to return a 0 height if hidden. The dirtyness is its reliance on RootPaneLayout respecting the pref height ...

The revised ChangeListener:

bar.setHidden(true);
ChangeListener listener = new ChangeListener() {
    @Override
    public void stateChanged(ChangeEvent e) {
        MenuElement[] elements = MenuSelectionManager.defaultManager().getSelectedPath();
        bar.setHidden(!(elements.length >0 && elements[0] == bar));
    }
};
MenuSelectionManager.defaultManager().addChangeListener(listener);

The custom menuBar:

public static class JHideableMenuBar extends JMenuBar {

    private boolean hidden;

    public void setHidden(boolean hidden) {
        if (this.hidden == hidden) return;
        this.hidden = hidden;
        revalidate();
    }

    @Override
    public Dimension getPreferredSize() {
        Dimension pref = super.getPreferredSize();
        if (hidden) {
            pref.height = 0;
        }
        return pref;
    }

}