如何防止 JComboBox 在使用自定义 ListCellRenderer 时变得无响应

Question

我正在使用 JComboBox 和自定义 ListCellRenderer 制作字体选择器。我想 JComboBox 显示所有可用字体，每个字体名称都以其自己的字体显示。我目前使用大约 500 种字体。

提供此功能的 ListCellRenerer 示例：

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;
    }
}

问题是，使用此呈现器时，JComboBox 在程序执行期间变得无响应。第一次单击组合框以显示列表时，加载列表需要几秒钟的时间。第二次点击时，列表会立即显示。

如果有人评论这一行

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

，组合框就可以正常工作。

如何防止这种无反应现象呢？

Answer 1

所发生的情况是组合的内部尝试动态地找到首选大小。为此，它循环遍历列表中的所有项目，向渲染器提供项目以测量渲染组件的首选大小。

您可以通过设置用于测量的原型值来防止这种情况，然后使用该原型测量一次大小

 comboBox.setPrototypeDisplayValue(sampleFont);

编辑：正如@Boro检测到的那样，这还不够 - 它仅为组合框本身设置原型，而不为弹出窗口中的列表设置原型（如应该的，那是多么疯狂的越野车……可能是）。为了解决这个问题，我们必须手动设置它，这是一个使用

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 的代码片段（稍作更改，以期望 Font 类型的项目）

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;
    }
}

Answer 2

@kleopatra 比你注意到我，但 setPrototypeDisplayValue 看起来像是懒惰的选择，我的修正

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.GraphicsEnvironment;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import javax.swing.DefaultListCellRenderer;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JPopupMenu;
import javax.swing.JScrollPane;
import javax.swing.JViewport;
import javax.swing.plaf.basic.BasicComboBoxRenderer;

public class SystemFontDisplayer extends JFrame {

    private static final long serialVersionUID = 1L;
    private JComboBox fontsBox;

    public SystemFontDisplayer() {

        GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
        String[] fontFamilyNames = ge.getAvailableFontFamilyNames();
        fontsBox = new JComboBox(fontFamilyNames);
        fontsBox.setSelectedItem(0);
        fontsBox.setRenderer(new ComboRenderer(fontsBox));
        fontsBox.addItemListener(new ItemListener() {

            @Override
            public void itemStateChanged(ItemEvent e) {
                if (e.getStateChange() == ItemEvent.SELECTED) {
                    final String fontName = fontsBox.getSelectedItem().toString();
                    fontsBox.setFont(new Font(fontName, Font.PLAIN, 16));
                }
            }
        });
        fontsBox.setSelectedItem(0);
        fontsBox.getEditor().selectAll();
        add(fontsBox, BorderLayout.CENTER);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(400, 60));
        setLocation(200, 105);
        pack();

        java.awt.EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                fontsBox.setPopupVisible(true);
                fontsBox.setPopupVisible(false);
            }
        });
        setVisible(true);
    }

    public static void main(String arg[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                SystemFontDisplayer systemFontDisplayer = new SystemFontDisplayer();
            }
        });
    }

    private class ComboRenderer extends BasicComboBoxRenderer {

        private static final long serialVersionUID = 1L;
        private JComboBox comboBox;
        final DefaultListCellRenderer defaultRenderer = new DefaultListCellRenderer();
        private int row;

        private ComboRenderer(JComboBox fontsBox) {
            comboBox = fontsBox;
        }

        private void manItemInCombo() {
            if (comboBox.getItemCount() > 0) {
                final Object comp = comboBox.getUI().getAccessibleChild(comboBox, 0);
                if ((comp instanceof JPopupMenu)) {
                    final JList list = new JList(comboBox.getModel());
                    final JPopupMenu popup = (JPopupMenu) comp;
                    final JScrollPane scrollPane = (JScrollPane) popup.getComponent(0);
                    final JViewport viewport = scrollPane.getViewport();
                    final Rectangle rect = popup.getVisibleRect();
                    final Point pt = viewport.getViewPosition();
                    row = list.locationToIndex(pt);
                }
            }
        }

        @Override
        public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
            super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
            if (list.getModel().getSize() > 0) {
                manItemInCombo();
            }
            final JLabel renderer = (JLabel) defaultRenderer.getListCellRendererComponent(list, value, row, isSelected, cellHasFocus);
            final Object fntObj = value;
            final String fontFamilyName = (String) fntObj;
            setFont(new Font(fontFamilyName, Font.PLAIN, 16));
            return this;
        }
    }
}