java - JCombobox focusLost 没有触发——为什么？

Question

我的代码中有一个JCombobox。我已经添加了FocusLost event. 但无论如何它并没有被解雇。我已经尝试了很多时间，但没有找到解决方案。

jcbItemType.addFocusListener(new java.awt.event.FocusAdapter() {
    public void focusLost(java.awt.event.FocusEvent evt) {
        jcbItemTypeFocusLost(evt);
    }
});

private void jcbItemTypeFocusLost(java.awt.event.FocusEvent evt)                                      
    {                                          
        // TODO add your handling code here:
        System.out.println("name=" + ((Component) evt.getSource()).getName());
        System.out.println("index=" + jcbItemType.getSelectedIndex());
    }

但是控制台中没有打印任何内容。请告诉我我做错了什么。

Answer 1

我找到了一种非常简单的方法来解决这个问题。

JComboBox 默认编辑器有一个内部类 BasicComboBoxEditor$BorderlessTextField，它是获取和失去焦点的组件。

它可以通过简单地访问

Component component = comboBox.getEditor().getEditorComponent();  
if (component instanceof JTextField) 
    JTextField borderlesstextfield = (JTextField) borderless;

然后像对任何 JTextField 一样添加焦点侦听器

borderlesstextfield.addFocusListener(new FocusListener() 
{
   public void focusGained(FocusEvent e) 
     {
     }
   public void focusLost(FocusEvent e) 
     {
     }
}});

现在您有一个 FocusListener 将按预期响应 ComboBox 获得和失去焦点

Answer 2

• FocusListener不是JComboBox的正确侦听器，与另一个侦听器一起可以创建无限循环（尤其是可编辑的 JComboBox），

• FocusListener是异步的，有时很难捕获事件是正确的命令，尤其是在 JComponents 也添加了另一个 Listener(s) 的情况下

示例如何从派生中监听FocusJTextField / JFormattedTextField

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ComboBoxTwo extends JFrame implements ItemListener {

    private static final long serialVersionUID = 1L;
    private JComboBox mainComboBox;
    private JComboBox subComboBox;

    public ComboBoxTwo() {
        String[] items = {"Select Item", "Color", "Shape", "Fruit"};
        String[] subItems1 = {"Select Color", "Red", "Blue", "Green"};
        mainComboBox = new JComboBox(items);
        mainComboBox.addItemListener(this);
        mainComboBox.addFocusListener(fcsListener);
        add(mainComboBox, BorderLayout.WEST);
        subComboBox = new JComboBox(subItems1);
        subComboBox.setPrototypeDisplayValue("XXXXXXXXXXXXXXXXXXX");
        subComboBox.addItemListener(this);
        add(subComboBox, BorderLayout.EAST);
    }

    @Override
    public void itemStateChanged(ItemEvent e) {
        if (e.getStateChange() == ItemEvent.SELECTED) {
            if (e.getSource() == mainComboBox) {
                System.out.println("Source  : mainComboBox");
            } else if (e.getSource() == subComboBox) {
                System.out.println("Source  : subComboBox");
            }
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame frame = new ComboBoxTwo();
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
//
    private FocusListener fcsListener = new FocusListener() {

        @Override
        public void focusGained(FocusEvent e) {
            dumpInfo(e);
        }

        @Override
        public void focusLost(FocusEvent e) {
            dumpInfo(e);
        }

        private void dumpInfo(FocusEvent e) {
            System.out.println("Source  : " + name(e.getComponent()));
            System.out.println("Opposite : " + name(e.getOppositeComponent()));
            System.out.println("Temporary: " + e.isTemporary());
            final Component c = e.getComponent();//works for editable JComboBox too
            if (c instanceof JFormattedTextField) {
                SwingUtilities.invokeLater(new Runnable() {

                    @Override
                    public void run() {
                        ((JFormattedTextField) c).selectAll();
                    }
                });
            } else if (c instanceof JTextField) {
                SwingUtilities.invokeLater(new Runnable() {

                    @Override
                    public void run() {
                        ((JTextField) c).selectAll();
                    }
                });
            }
        }

        private String name(Component c) {
            return (c == null) ? null : c.getName();
        }
    };
}