无法将焦点转移到未显示的CardLayout JPanel

Question

Why can't I shift focus to an unshown JPanel card in a CardLayout ?

I'm implementing a CardLayout -based interface that needs to be keyboard navigation friendly. When a user finishes tabbing through all the fields on one card, the user needs to be able to tab to the next card.

I've already implemented a FocusTraversalPolicy that produces the right Component at each point in the process, and a FocusAdapter to pop up any cards newly tabbed to, but something is eating the messages and preventing focus change.

I can uncleanly pass the CardLayout to the FocusTraversalPolicy to change the card— though any of its functions are called several times in Swing's many threads and leads to strange behavior. Besides, that way's just dirty.

I do not want to use key bindings b/c that would require reimplementing all of the focus work Java already does for me, and is also really unclean.

Basically: Java dislikes shifting focus to unshown cards in CardLayout s— how can I override this?

Answer 1

It sounds like you have a wizard-like UI. If so, add a "Next" button as the last field on each card.

The action of the Next button would be to flip to the next card as set focus to the first entry field.

The last entry field on each card would transfer focus to the Next button, who could then be "pressed" with a strike of the spacebar when it receives focus (which is the default behavior of a JButton), keeping it keyboard-friendly.

This would alleviate the need for special KeyBindings or FocusTraversalPolicies.

EDIT:

Try this, using FocusListeners for the JTextFields. Tab thorugh the fields and the cards will flip to the next one automatically once you tab out of the last field. You could use ActionListeners instead if you wish:

EDIT 2: Added hack for panels that only have 1 field.

import java.awt.*;
import java.awt.event.*;
import java.beans.*;

import javax.swing.*;

public class CardLayoutDemo2 implements Runnable
{
  final static String CARD1 = "One";
  final static String CARD2 = "Two";
  final static String CARD3 = "Three";

  JPanel cards;
  CardLayout cardLayout;
  JTextField tf1, tf2, tf3, tf4, tf5;
  JButton dummy;

  public static void main(String[] args)
  {
    SwingUtilities.invokeLater(new CardLayoutDemo2());
  }

  public void run()
  {
    tf1 = new JTextField(10);
    tf2 = new JTextField(10);
    tf2.addFocusListener(new CardFlipper(CARD2));

    tf3 = new JTextField(10);
    tf4 = new JTextField(10);
    tf4.addFocusListener(new CardFlipper(CARD3));

    tf5 = new JTextField(10);
    tf5.addFocusListener(new CardFlipper(CARD1));

    dummy = new JButton()
    {
      @Override
      public Dimension getPreferredSize()
      {
        return new Dimension(0,0);
      }
    };

    dummy.addFocusListener(new FocusAdapter()
    {
      @Override
      public void focusGained(FocusEvent e)
      {
        dummy.transferFocus();
      }
    });

    JPanel card1 = new JPanel();
    card1.add(new JLabel("One"));
    card1.add(tf1);
    card1.add(new JLabel("Two"));
    card1.add(tf2);

    JPanel card2 = new JPanel();
    card2.add(new JLabel("Three"));
    card2.add(tf3);
    card2.add(new JLabel("Four"));
    card2.add(tf4);

    JPanel card3 = new JPanel();
    card3.add(dummy);
    card3.add(new JLabel("Five"));
    card3.add(tf5);    

    cardLayout = new CardLayout();
    cards = new JPanel(cardLayout);
    cards.add(card1, CARD1);
    cards.add(card2, CARD2);
    cards.add(card3, CARD3);

    JFrame f = new JFrame("CardLayout Demo");
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.add(cards, BorderLayout.CENTER);
    f.setSize(180, 200);
    f.setLocationRelativeTo(null);
    f.setVisible(true);
  }

  private class CardFlipper extends FocusAdapter
  {
    private String nextCard;

    CardFlipper(String cardName)
    {
      this.nextCard = cardName;
    }

    @Override
    public void focusLost(FocusEvent e)
    {
      cardLayout.show(cards, nextCard);
    }
  } 
}

Answer 2

I want to keep the program compartmentalized, as it runs in distinct steps.

This does not prevent you from creating a long scrolling form?

You can still create individual panels the way you are doing now. Then instead of adding these panels to a CardLayout where you need to swap panels, you can add the panels to a panel using a BoxLayout (or GridBagLayout).

This would even give more flexibility since each panel can be of a different size without impacting the size of every individual panel.

However, forms do not currently scroll automatically in a JScrollPane, so you may want to check out Scrolling a Form for a class this will do this for you.