java - How to prevent JComboBox from becoming unresponsive when using a custom ListCellRenderer

Question

I am making a font chooser using JComboBox and a custom ListCellRenderer. I want the JComboBox to display all available fonts, with each font name displayed in its own font. I am currently using around 500 fonts.

An example of a ListCellRenerer that provides this functionality:

private class ComboBoxRenderer extends JLabel implements ListCellRenderer {

    private JLabel label = new JLabel("Test");

    @Override
    public Component getListCellRendererComponent(JList list, Object value,
            int index, boolean isSelected, boolean cellHasFocus) {

        Font tempFont = label.getFont();
        setFont(new Font((String) value, tempFont.getStyle(),
                tempFont.getSize()));

        setText((String) value);

        return this;
    }
}

The problem is that, when using this renderer, the JComboBox becomes unresponsive during program execution. The first time one clicks on the combobox to reveal the list, it takes a couple of seconds for the list to load. The second time one clicks, the list is instantly displayed.

If one comments the line

setFont(new Font((String) value, tempFont.getStyle(),tempFont.getSize()));

, the combobox works just fine.

How can one prevent this unresponsiveness?

Answer 1

What happens is that the combo's internals try to find the preferred size dynamically. For doing so, it loops through all items in the list, feeds the renderer with the items to measure the rendering component's preferred size.

You can prevent that by setting a prototypeValue for measuring, then the size is measured once using that prototype

 comboBox.setPrototypeDisplayValue(sampleFont);

Edit: as @Boro detected, that's not enough - it sets the prototype for the comboBox itself only, not for the list in the popup (as it should, how crazily buggy can that ... possibly be). To hack around, we have to manually set it, here's a code snippet to play with

public class ComboWithPrototype {

    private JComponent createContent() {
        final Font[] systemFonts = GraphicsEnvironment
                .getLocalGraphicsEnvironment().getAllFonts();

        final JComboBox box = new JComboBox();
        box.setRenderer(new ComboBoxRenderer());
        box.setPrototypeDisplayValue(systemFonts[0]);
        Accessible a = box.getUI().getAccessibleChild(box, 0);
        if (a instanceof javax.swing.plaf.basic.ComboPopup) {
            JList popupList = ((javax.swing.plaf.basic.ComboPopup) a).getList();
            // route the comboBox' prototype to the list
            // should happen in BasicComboxBoxUI
            popupList.setPrototypeCellValue(box.getPrototypeDisplayValue());
        }
        Action action = new AbstractAction("set model") {

            @Override
            public void actionPerformed(ActionEvent e) {
                box.setModel(new DefaultComboBoxModel(systemFonts));
            }
        };
        JComponent panel = new JPanel(new BorderLayout());
        panel.add(box);
        panel.add(new JButton(action), BorderLayout.SOUTH);
        return panel;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame("");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new ComboWithPrototype().createContent());
                frame.setLocationRelativeTo(null);
                frame.pack();
                frame.setVisible(true);
            }
        });
    }

Custom ListCellRenderer (slightly changed, to expect items of type Font)

private class ComboBoxRenderer extends DefaultListCellRenderer {

    private Font baseFont = new JLabel("Test").getFont();

    @Override
    public Component getListCellRendererComponent(JList list, Object value,
            int index, boolean isSelected, boolean cellHasFocus) {

        super.getListCellRendererComponent(list, value, index, isSelected,
                cellHasFocus);
        if (value instanceof Font) {

            Font font = (Font) value;
            setFont(new Font(font.getName(), baseFont.getStyle(), baseFont.getSize())); 
            setText(font.getName());
        }

        return this;
    }
}