当另一个类调用Singleton.instance.show时，具有CardLayout的Singleton不会显示卡片

分类：编程问答

如何解决当另一个类调用Singleton.instance.show时，具有CardLayout的Singleton不会显示卡片

它将建议一个参考问题。public static MainWindow instance = new MainWindow();看起来很可疑，因为您必须先创建一个MainWindowfirst 实例才能进行初始化，这建议您现在有两个实例MainWindow，一个在屏幕上，另一个不在屏幕上。

static以这种方式使用不是一个好主意，因为它会导致类似这样的问题。相反，您应该将控制器的引用传递给页面。控制器将定义每个页面可以执行的操作（如果操作正确，则将其定义为interface）

另外，您可以将导航与页面分开，形成单独的机制，这意味着页面不在乎，可以简单地以您想要的任何顺序显示或在其他地方重复使用

本示例定义了一个简单的控制器，页面可以调用该控制器来实现页面导航

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.EventQueue;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class CardLayoutExample {

    public static void main(String[] args) {
        new CardLayoutExample();
    }

    public CardLayoutExample() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new Wizard());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public interface NavigationController {

        public void nextPage();

        public void previousPage();

        public void lastPage();

        public void firstPage();

    }

    public interface Page {

        public NavigationController getNavigationController();

        public JComponent getView();
        public String getName();

    }

    public class Wizard extends JPanel implements NavigationController {

        private List<Page> pages;
        private Page currentPage;

        private CardLayout cardLayout;

        public Wizard() {
            cardLayout = new CardLayout();
            pages = new ArrayList<>(25);
            setLayout(cardLayout);
            pages.add(new FirstPage("Page01", this));
            pages.add(new SecondPage("Page02", this));
            pages.add(new ThirdPage("Page03", this));

            for (Page page : pages) {
                add(page.getView(), page.getName());
            }

            firstPage();
        }

        @Override
        public void nextPage() {
            int index = pages.indexOf(currentPage);
            index++;
            if (index < pages.size()) {
                cardLayout.next(this);
                currentPage = pages.get(index);
            }
        }

        @Override
        public void previousPage() {
            int index = pages.indexOf(currentPage);
            index--;
            if (index >= 0) {
                cardLayout.previous(this);
                currentPage = pages.get(index);
            }
        }

        @Override
        public void lastPage() {
            Page page = pages.get(pages.size() - 1);
            showPage(page);
        }

        @Override
        public void firstPage() {
            Page page = pages.get(0);
            showPage(page);
        }

        protected void showPage(Page page) {
            cardLayout.show(this, page.getName());
            currentPage = page;
        }

    }

    public abstract class AbstractPage extends JPanel implements Page, ActionListener {

        private NavigationController navigationController;
        private JPanel buttons;
        private String name;

        public AbstractPage(String name, NavigationController navigationController) {
            this.name = name;
            this.navigationController = navigationController;
            setLayout(new BorderLayout());
            buttons = new JPanel(new FlowLayout(FlowLayout.RIGHT));
            add(buttons, BorderLayout.SOUTH);
        }

        protected void insertButton(JButton button) {
            button.addActionListener(this);
            buttons.add(button);
        }

        @Override
        public NavigationController getNavigationController() {
            return navigationController;
        }

        @Override
        public JComponent getView() {
            return this;
        }

        @Override
        public String getName() {
            return super.getName(); 
        }

    }

    public class FirstPage extends AbstractPage implements Page {

        private JButton next = new JButton("Next >");

        public FirstPage(String name, NavigationController controller) {
            super(name, controller);
            JLabel label = new JLabel("First page");
            label.setHorizontalAlignment(JLabel.CENTER);
            add(label);
            insertButton(next);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == next) {
                getNavigationController().nextPage();
            }
        }

    }

    public class SecondPage extends AbstractPage implements Page {

        private JButton next = new JButton("Next >");
        private JButton previous = new JButton("< Previous");

        public SecondPage(String name, NavigationController controller) {
            super(name, controller);
            JLabel label = new JLabel("Second page");
            label.setHorizontalAlignment(JLabel.CENTER);
            add(label);
            insertButton(previous);
            insertButton(next);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == next) {
                getNavigationController().nextPage();
            } else if (e.getSource() == previous) {
                getNavigationController().previousPage();
            }
        }

    }

    public class ThirdPage extends AbstractPage implements Page {

        private JButton previous = new JButton("< Previous");

        public ThirdPage(String name, NavigationController controller) {
            super(name, controller);
            JLabel label = new JLabel("Third page");
            label.setHorizontalAlignment(JLabel.CENTER);
            add(label);
            insertButton(previous);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == previous) {
                getNavigationController().previousPage();
            }
        }

    }
}

此示例将控制器与页面分开，以便按钮不属于页面本身。这样可以释放页面/视图，使其成为您需要的任何内容

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Component;
import java.awt.EventQueue;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.EmptyBorder;

public class CardLayoutExample2 {

    public static void main(String[] args) {
        new CardLayoutExample2();
    }

    public CardLayoutExample2() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new WizardPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class WizardPane extends JPanel {

        private List<String> pages;
        private String currentPage;

        private JButton first;
        private JButton previous;
        private JButton next;
        private JButton last;

        private CardLayout cardLayout;
        private JPanel contentPane;

        public WizardPane() {
            setLayout(new BorderLayout());
            cardLayout = new CardLayout();
            pages = new ArrayList<>(3);

            contentPane = new JPanel(cardLayout);
            contentPane.setBorder(new EmptyBorder(4, 4, 4, 4));

            pages.add("Page01");
            pages.add("Page02");
            pages.add("Page03");

            contentPane.add(new FirstPage(), "Page01");
            contentPane.add(new SecondPage(), "Page02");
            contentPane.add(new ThirdPage(), "Page03");

            JPanel actionsPane = new JPanel(new GridBagLayout());
            actionsPane.setBorder(new EmptyBorder(4, 4, 4, 4));
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridx = 0;
            gbc.gridy = 0;
            actionsPane.add((first = new JButton("<< First")), gbc);
            gbc.gridx++;
            gbc.weightx = 1;
            gbc.anchor = GridBagConstraints.WEST;
            actionsPane.add((previous = new JButton("< Previous")), gbc);

            gbc.gridx++;
            gbc.anchor = GridBagConstraints.EAST;
            actionsPane.add((next = new JButton("Next >")), gbc);
            gbc.gridx++;
            gbc.weightx = 0;
            actionsPane.add((last = new JButton("Last >>")), gbc);

            add(contentPane);
            add(actionsPane, BorderLayout.SOUTH);

            NavigationHandler handler = new NavigationHandler();
            first.addActionListener(handler);
            previous.addActionListener(handler);
            next.addActionListener(handler);
            last.addActionListener(handler);

            gotoFirstPage();
        }

        protected void gotoFirstPage() {
            currentPage = pages.get(0);
            cardLayout.show(contentPane, currentPage);
        }

        protected void gotoPreviousPage() {
            int index = pages.indexOf(currentPage);
            index--;
            if (index >= 0) {
                currentPage = pages.get(index);
                cardLayout.show(contentPane, currentPage);
            }
        }

        protected void gotoNextPage() {
            int index = pages.indexOf(currentPage);
            index++;
            if (index < pages.size()) {
                currentPage = pages.get(index);
                cardLayout.show(contentPane, currentPage);
            }
        }

        protected void gotoLastPage() {
            currentPage = pages.get(pages.size() - 1);
            cardLayout.show(contentPane, currentPage);
        }

        protected class NavigationHandler implements ActionListener {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (e.getSource() == first) {
                    gotoFirstPage();
                } else if (e.getSource() == previous) {
                    gotoPreviousPage();
                } else if (e.getSource() == next) {
                    gotoNextPage();
                } else if (e.getSource() == last) {
                    gotoLastPage();
                }
            }
        }

    }

    public class FirstPage extends JPanel {

        public FirstPage() {
            setLayout(new BorderLayout());
            JLabel label = new JLabel("Page One");
            label.setHorizontalAlignment(JLabel.CENTER);
            add(label);
        }

    }

    public class SecondPage extends JPanel {

        public SecondPage() {
            setLayout(new BorderLayout());
            JLabel label = new JLabel("Page Two");
            label.setHorizontalAlignment(JLabel.CENTER);
            add(label);
        }

    }

    public class ThirdPage extends JPanel {

        public ThirdPage() {
            setLayout(new BorderLayout());
            JLabel label = new JLabel("Page Three");
            label.setHorizontalAlignment(JLabel.CENTER);
            add(label);
        }

    }
}

或者，您可以使用基于模型的方法（可能更可取），该方法定义了组件的显示顺序。例如

解决方法

public class MainWindow extends JPanel {

public static MainWindow instance = new MainWindow();

private CardLayout cards = new CardLayout();

public MainWindow() {
    setLayout(cards);
    add(new FirstPage(),Pages.FIRST.toString());
    add(new SecondPage(),Pages.SECOND.toString());
    add(new ThirdPage(),Pages.THIRD.toString());
}

public void showPage(Pages page) {
    cards.show(this,page.toString());
}

}

showPage(page);如果我在的构造函数中调用该方法，则该方法可以正常工作MainWindow。但是，当我尝试MainWindow.instance.showPage(Pages.SECOND);从ActionListener
调用时，FirstPage什么也没有发生。我检查了该showPage(page)方法是否正常工作。我检查了是否触发了ActionEvent并输入正确的if
/ else子句。我做错了什么，为什么我的第二页没有显示？

public class FirstPage extends JPanel {

    private JButton showSecond = new JButton("Show Second");
    private JButton showThird = new JButton("Show Third");

    public FirstPage() {
        insertButton(showSecond);
        insertButton(showThird);
    }

    private void insertButton(JButton button) {
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (e.getSource() == showSecond) {
                    MainWindow.instance.showPage(Pages.SECOND);
                } else {
                    MainWindow.instance.showPage(Pages.THIRD);
                }
            }
        });
        this.add(button);
    }
}

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