Java Swing - JDialog 默认焦点

Question

我找到了 in the Internet这样的信息:

“当 JDialog(或 JFrame)可见时，焦点默认放在第一个可聚焦的组件上。”

让我们考虑这样的代码:

public class MyDialog extends JDialog{
    // Dialog's components:
    private JLabel dialogLabel1 = new JLabel("Hello");
    private JLabel dialogLabel2 = new JLabel("Message");
    private JButton dialogBtn = new JButton("Sample Btn text");

    public MyDialog(JFrame parent, String title, ModalityType modality){
        super(parent, title, modality);
        dialogBtn.setName("Button");    //
        dialogLabel1.setName("Label1"); // - setting names
        dialogLabel2.setName("Label2"); //
        setTitle(title);
        setModalityType(modality);
        setSize(300, 100);
        setLocation(200, 200);
        // adding comps to contentPane
        getContentPane().add(dialogLabel1, BorderLayout.PAGE_START);
        getContentPane().add(dialogBtn, BorderLayout.CENTER);
        getContentPane().add(dialogLabel2, BorderLayout.PAGE_END);
        pack();
    }

    public void showDialog(){
        setVisible(true);
        listComps(rootPane.getComponents());
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    }

    /*
     * Itereates through all subcomps recursively and displays some relevant info
     * OUTPUT FORM:  ComponentName | isFocusable | hasFocus
     */
    private void listComps(Component[] comps){
        if(comps.length == 0) return;  
        for(Component c : comps){       
            JComponent jC = (JComponent)c;
            System.out.println(jC.getName() + " | " + jC.isFocusable() +" | " + jC.hasFocus());
            listComps(jC.getComponents());
        }    
    }

    public static void main(String[] args) {
        final JFrame frame = new JFrame();
        frame.setPreferredSize(new Dimension(300, 400));
        frame.setVisible(true);
        JButton btn = new JButton("Show dialog");
        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                    MyDialog dialog = new MyDialog(frame, "Sample title", ModalityType.APPLICATION_MODAL);
                    dialog.showDialog();
            }
        });
        frame.add(btn, BorderLayout.CENTER);
        frame.pack();
    }
}

输出是: 运行:

null.glassPane | true | false
null.layeredPane | true | false
null.contentPane | true | false
Label1 | true | false
Button | true | true
Label2 | true | false

为什么焦点设置到 JButton？它不是第一个可聚焦的组件! 当我删除 JButton 时，焦点未获得任何组件。为什么？默认情况下，所有组合都是可聚焦的...

Answer 1

It is not the first focusable component! When I've removed JButton, focus wasn't gained to any component. Why? All compos are focusable by default.

要回答原因:这是 FocusTraversalPolicy 的决定，尤其是 DefaultFocusTraversalPolicy 中的 accept(..) 最终会回落到虚拟 NullComponentPeer(默认情况下不可聚焦，因为它实际上并不存在 :-)

从您的评论看来，真正的问题可能是“如果 rootPane 没有可聚焦的子项，如何实现 keyBindings”——如果是这样，选项是

• 使用rootPane的componentInputMap，即WHEN_IN_FOCUSED_WINDOW
• 强制 rootPane 本身成为可聚焦的(最好只有在没有可聚焦的子级时才这样做)