java - 为什么 ActionListener 在 Controller 中不起作用？

Question

这篇文章是this post.的继续基本上我的软件是基于CardLayout面板。现在我想从 cardLayout 外部更改面板。在提供的链接中，我得到了帮助来做到这一点并且它有效。现在，我在软件中添加了一个 Controller ，位于内部面板 ( CardLayout ) 和外部面板之间。在外部面板(leftBar)中，我有一个按钮，如果它是actionListener位于该类内部，它可以正常工作并打开 CardLayout 所选择的内部面板。但如果我采取 ActionListener进入 Controller ，它就是不起作用。没有出现错误。

这是我的示例代码:基础:

public class Base {
        JFrame frame = new JFrame("Panel");
        BorderLayout bl = new BorderLayout();

    public Base(){
        MainPanel mainPanel = new MainPanel();
        LeftBar leftBar = new LeftBar(mainPanel);
        frame.setLayout(bl);
        frame.setSize(800, 600);
        frame.add(leftBar, BorderLayout.WEST);
        frame.add(mainPanel, BorderLayout.CENTER);

        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    public static void main(String[] args) throws IOException {
        new Base();
    }
}

MainPanel(CardLAyout 基础)

public class MainPanel extends JPanel {
    private CardLayout cl = new CardLayout();
    private JPanel panelHolder = new JPanel(cl);

    public MainPanel(){
        FirstPage firstPage = new FirstPage(this);
        SecondPage secondPage = new SecondPage(this);
        LeftBar leftBar = new LeftBar(this);

        Controller controller = new Controller(secondPage, leftBar, this);
        setLayout(new GridLayout(0,1));

        panelHolder.add(firstPage, "firstPage");
        panelHolder.add(secondPage, "secondPage");

        cl.show(panelHolder, "firstPage");
        add(panelHolder);

    }
    public void showPanel(String panelIdentifier){
        cl.show(panelHolder, panelIdentifier);
    }
}

左栏

public class LeftBar extends JPanel{
    private JButton button;
    private MainPanel mainPanel;

    public LeftBar(MainPanel mainPanel){
        this.mainPanel = mainPanel;

        setPreferredSize(new Dimension(200, 40));
        setLayout(new BorderLayout());
        setBackground(Color.black);

        button = new JButton("Show Second Page");


       add(button, BorderLayout.NORTH);
    }
    public void addPageListener(ActionListener listenForButton){
        button.addActionListener(listenForButton);
    }
}

第二页:

public class SecondPage extends JPanel{
    MainPanel mainPanel;
    JButton button;
    public SecondPage(MainPanel mainPanel){
       this.mainPanel = mainPanel;
        setBackground(Color.white);
       add(new JLabel("This is second page"));
    }
}

首页:

public class FirstPage extends JPanel {
    MainPanel mainPanel;
    JButton button;

    public FirstPage(MainPanel mainPanel) {
       this.mainPanel = mainPanel;
       setBackground(Color.GRAY);

       button = new JButton("Show page");
       button.addActionListener(new ActionListener(){
           @Override
           public void actionPerformed(ActionEvent ae) {
               mainPanel.showPanel("secondPage");
           }

       });

       add(button);
    }
}

和 Controller :

public class Controller {
    private SecondPage secondPage;
    private LeftBar leftBar;
    private MainPanel mainPanel;
    public Controller(SecondPage secondPage, LeftBar leftBar, MainPanel mainPanel){
        this.secondPage=secondPage;
        this.leftBar=leftBar;
        this.mainPanel=mainPanel;

        this.leftBar.addPageListener(new ButtonListener());
    }

    class ButtonListener implements ActionListener{
        @Override
        public void actionPerformed(ActionEvent ae) {
            System.out.println("Works");
            mainPanel.showPanel("secondPage");
        }
    }
}

正如您在 LeftBar 中看到的那样我有一个按钮的方法，用作 actionListener 。我在 Controller 中调用该方法并给它一个类。但这不起作用。然而ACtionListener如果它位于 LeftBar 类(内联)中，则有效。

知道如何修复它吗？

Answer 1

您正在创建 LeftBar 的两个实例，其中一个位于 Base 构造函数中，该实例将添加到屏幕上...

public Base() {
    MainPanel mainPanel = new MainPanel();
    LeftBar leftBar = new LeftBar(mainPanel);
    frame.setLayout(bl);
    frame.setSize(800, 600);
    frame.add(leftBar, BorderLayout.WEST);
    frame.add(mainPanel, BorderLayout.CENTER);

还有一个在 MainPanel 构造函数中传递给 Controller ​​...

public class MainPanel extends JPanel {

    private CardLayout cl = new CardLayout();
    private JPanel panelHolder = new JPanel(cl);

    public MainPanel() {
        FirstPage firstPage = new FirstPage(this);
        SecondPage secondPage = new SecondPage(this);
        LeftBar leftBar = new LeftBar(this);

这意味着 Controller 将一个 ActionListener 附加到屏幕上永远不可见的按钮

我要做的第一件事是开始解耦你的代码，而不是传递类的实例，你需要建立一系列 Controller 和 View 可以同意的契约，例如，你的 MainPanel 应该提供一种切换 View 的方法，您的 LeftBar 应该在用户想要切换 View 时提供通知

例如...

public interface Pageable {

    public void showView(String name);

}

public interface Navigatable {

    public void addActionListener(ActionListener listener);

}

然后你的主视图将实现Pageable接口(interface)

public class MainPanel extends JPanel implements Pageable {
    //...
    @Override
    public void showView(String name) {
        cl.show(panelHolder, name);
    }

并且您的LeftBar将实现Navigatable接口(interface)

public class LeftBar extends JPanel implements Navigatable {
    //...
    @Override
    public void addActionListener(ActionListener listener) {
        button.addActionListener(listener);
    }

这只是定义了任何实现的契约(Contract)功能，您的 Controller 不应该关心任何其他事情(也不应该允许它做不该做的事情，比如从 View 中删除所有组件，顽皮的 Controller )

然后你的 Controller 就负责管理接口(interface)之间的契约......

public class Controller {

    private final Navigatable navigatable;
    private final Pageable pageable;

    public Controller(Navigatable navigatable, Pageable pageable) {
        this.navigatable = navigatable;
        this.pageable = pageable;

        navigatable.addActionListener(new ButtonListener());
    }

    class ButtonListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent ae) {
            pageable.showView("secondPage");
        }
    }
}

例如...

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.IOException;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Base {

    JFrame frame = new JFrame("Panel");
    BorderLayout bl = new BorderLayout();

    public Base() {
        MainPanel mainPanel = new MainPanel();
        LeftBar leftBar = new LeftBar(mainPanel);
        frame.setLayout(bl);
        frame.setSize(800, 600);
        frame.add(leftBar, BorderLayout.WEST);
        frame.add(mainPanel, BorderLayout.CENTER);

        Controller controller = new Controller(leftBar, mainPanel);

        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    public static void main(String[] args) throws IOException {
        new Base();
    }

    public interface Pageable {

        public void showView(String name);

    }

    public interface Navigatable {

        public void addActionListener(ActionListener listener);

    }

    public class MainPanel extends JPanel implements Pageable {

        private CardLayout cl = new CardLayout();
        private JPanel panelHolder = new JPanel(cl);

        public MainPanel() {
            FirstPage firstPage = new FirstPage(this);
            SecondPage secondPage = new SecondPage(this);
            setLayout(new GridLayout(0, 1));

            panelHolder.add(firstPage, "firstPage");
            panelHolder.add(secondPage, "secondPage");

            cl.show(panelHolder, "firstPage");
            add(panelHolder);

        }

        public void showPanel(String panelIdentifier) {
            cl.show(panelHolder, panelIdentifier);
        }

        @Override
        public void showView(String name) {
            cl.show(panelHolder, name);
        }
    }

    public class LeftBar extends JPanel implements Navigatable {

        private JButton button;
        private MainPanel mainPanel;

        public LeftBar(MainPanel mainPanel) {
            this.mainPanel = mainPanel;

            setPreferredSize(new Dimension(200, 40));
            setLayout(new BorderLayout());
            setBackground(Color.black);

            button = new JButton("Show Second Page");

            add(button, BorderLayout.NORTH);
        }

        @Override
        public void addActionListener(ActionListener listener) {
            button.addActionListener(listener);
        }
    }

    public class SecondPage extends JPanel {

        MainPanel mainPanel;
        JButton button;

        public SecondPage(MainPanel mainPanel) {
            this.mainPanel = mainPanel;
            setBackground(Color.white);
            add(new JLabel("This is second page"));
        }
    }

    public class FirstPage extends JPanel {

        MainPanel mainPanel;
        JButton button;

        public FirstPage(MainPanel mainPanel) {
            this.mainPanel = mainPanel;
            setBackground(Color.GRAY);

            button = new JButton("Show page");
            button.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent ae) {
                    mainPanel.showPanel("secondPage");
                }

            });

            add(button);
        }
    }

    public class Controller {

        private final Navigatable navigatable;
        private final Pageable pageable;

        public Controller(Navigatable navigatable, Pageable pageable) {
            this.navigatable = navigatable;
            this.pageable = pageable;

            navigatable.addActionListener(new ButtonListener());
        }

        class ButtonListener implements ActionListener {

            @Override
            public void actionPerformed(ActionEvent ae) {
                pageable.showView("secondPage");
            }
        }
    }
}

现在就我个人而言，这只是对您的代码进行一些黑客攻击，就我个人而言，我更喜欢有一个与单个 View /合约交互的 Controller ，它使它变得如此简单......