java - MVC 上特定领域的观察者模式

Question

在 MVC 模式上，这是模型通知 View 的最佳选择(如果这首先是正确的方法)，其中模型存储的所有数据字段中只有几个已更新。特别是当我们只想更新 View 的特定字段时。

我目前正在使用带有观察者/订阅者 (JAVA Swing) 的 MVC 模式，如下所述:https://stackoverflow.com/a/6963529但是当模型更新时，它会在调用 update() 函数时更改 View 中的所有内容，因此无法确定模型中的哪个字段已更改以便仅更新 View 中的必需字段。

我阅读了这个主题:https://softwareengineering.stackexchange.com/a/359008还有这个:https://stackoverflow.com/a/9815189我认为这很有用，但对于后来的人来说，我不太明白如何在变量(int、float 等)上设置 propertyChangeListener。也与此相关:https://stackoverflow.com/a/9815189

软件开始运行的主类:

public class Main {
    public static void main(String[] args) {
        Model m = new Model();
        View v = new View(m);
        Controller c = new Controller(m, v);
        c.initController();
    }
}

所以我在模型上的代码是这样的:

public class Model extends Observable {
   //...
   private float speed;
   private int batteryPercentage;

   public float getSpeed() {
       return speed;
   }
   public void setSpeed(float speed) {
       this.speed = speed;
       setChanged();
       notifyObservers();
   }

    public int getBatteryPercentage() {
        return batteryPercentage;
    }
    public void setBatteryPercentage(int batteryPercentage) {
        this.batteryPercentage = batteryPercentage;
        setChanged();
        notifyObservers();
    }
}

View 知道模型:

public class View implements Observer {
    private Model model;
    private JTextField txtFldSpeed;
    private JTextField txtFldBattery;
    private JFrame mainWindow;

    public View(Model m) {
        this.model = m;
        initialize();
    }
    private void initialize() {
        mainWindow = new JFrame();
        mainWindow.setTitle("New Window");
        mainWindow.setMinimumSize(new Dimension(1280, 720));
        mainWindow.setBounds(100, 100, 1280, 720);
        mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel tPanel1 = new JPanel();
        tPanel1.setBorder(new LineBorder(new Color(0, 0, 0)));
        tPanel1.setLayout(null);
        mainWindow.getContentPane().add(tPanel1);

        mainWindow.getContentPane().add(tPanel1);
        txtFldSpeed = new JTextField();
        txtFldSpeed.setEditable(false);
        txtFldSpeed.setBounds(182, 11, 116, 22);
        tPanel1.add(txtFldSpeed);

        txtFldBattery = new JTextField();
        txtFldBattery.setEditable(false);
        txtFldBattery.setBounds(182, 43, 116, 22);
        tPanel1.add(txtFldBattery);

        mainWindow.setVisible(true);
    }
    @Override
    public void update(Observable o, Object arg) {
        txtFldSpeed.setText(Float.toString(model.getSpeed()) + " kn");
        txtFldBattery.setText(Integer.toString(model.getBatteryPercentage()) + " %");
    }
}

Controller 将 View 添加为模型的观察者:

public class Controller {
    private Model model;
    private View view;

    public Controller(Model m, View v) {
        this.model = m;
        this.view = v;
    }

    public void initController() {    
        model.addObserver(view);
        model.setSpeed(10);
    }
}

我期待的是，当模型更新时，比方说，函数 setSpeed() 被调用， View 被告知她需要在那个特定字段上更新自己，而不是每个“可变”字段(如 txtFldBattery。

我想这样做是因为在 View 上，每秒更新几次字段，并且因为我需要更新 View 上的所有内容，所以不需要 JComboBox经常更新，在尝试选择一个选项时一直关闭。

Answer 1

我会使用 SwingPropertyChangeSupport，使模型的每个状态字段成为“绑定(bind)属性”，以便可以单独收听每个状态字段。

例如，假设您有一个如下所示的模型:

public class MvcModel {
    public static final String SPEED = "speed";
    public static final String BATTERY = "battery";
    public static final int MAX_SPEED = 40;
    private float speed;
    private int batteryPercentage;
    private SwingPropertyChangeSupport pcSupport = new SwingPropertyChangeSupport(this);

    public float getSpeed() {
        return speed;
    }

    public void setSpeed(float speed) {
        float oldValue = this.speed;
        float newValue = speed;
        this.speed = speed;
        pcSupport.firePropertyChange(SPEED, oldValue, newValue);
    }

    public int getBatteryPercentage() {
        return batteryPercentage;
    }

    public void setBatteryPercentage(int batteryPercentage) {
        int oldValue = this.batteryPercentage;
        int newValue = batteryPercentage;
        this.batteryPercentage = batteryPercentage;
        pcSupport.firePropertyChange(BATTERY, oldValue, newValue);
    }

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        pcSupport.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        pcSupport.removePropertyChangeListener(listener);
    }

    public void addPropertyChangeListener(String name, PropertyChangeListener listener) {
        pcSupport.addPropertyChangeListener(name, listener);
    }

    public void removePropertyChangeListener(String name, PropertyChangeListener listener) {
        pcSupport.removePropertyChangeListener(name, listener);
    }

}

speed 和 batteryPercent 字段都是“绑定(bind)字段”，因为对这些字段的任何更改都将触发属性更改支持对象向已向支持对象注册的任何监听器发送通知消息，如 public void setXxxx(...) 方法。

这样 Controller 就可以在模型上为它想要监听的任何属性注册监听器，然后将任何更改通知 View 。例如:

class SpeedListener implements PropertyChangeListener {
    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        float speed = model.getSpeed();
        view.setSpeed(speed);
    }
}

设置可能类似于:

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.GridLayout;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;

import javax.swing.*;
import javax.swing.event.SwingPropertyChangeSupport;

public class MVC2 {

    private static void createAndShowGui() {
        MvcModel model = new MvcModel();
        MvcView view = new MvcView();
        MvcController controller = new MvcController(model, view);
        controller.init();

        JFrame frame = new JFrame("MVC2");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(view.getMainDisplay());
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

class MvcView {
    private JPanel mainPanel = new JPanel();
    private JSlider speedSlider = new JSlider(0, MvcModel.MAX_SPEED);
    private JSlider batterySlider = new JSlider(0, 100);
    private JProgressBar speedBar = new JProgressBar(0, MvcModel.MAX_SPEED);
    private JProgressBar batteryPercentBar = new JProgressBar(0, 100);

    public MvcView() {
        speedSlider.setMajorTickSpacing(5);
        speedSlider.setMinorTickSpacing(1);
        speedSlider.setPaintTicks(true);
        speedSlider.setPaintLabels(true);
        speedSlider.setPaintTrack(true);

        batterySlider.setMajorTickSpacing(20);
        batterySlider.setMinorTickSpacing(5);
        batterySlider.setPaintTicks(true);
        batterySlider.setPaintLabels(true);
        batterySlider.setPaintTrack(true);

        speedBar.setStringPainted(true);
        batteryPercentBar.setStringPainted(true);

        JPanel inputPanel = new JPanel(new GridLayout(0, 1));
        inputPanel.add(createTitledPanel("Speed", speedSlider));
        inputPanel.add(createTitledPanel("Battery %", batterySlider));

        JPanel displayPanel = new JPanel(new GridLayout(0, 1));
        displayPanel.add(createTitledPanel("Speed", speedBar));
        displayPanel.add(createTitledPanel("Battery %", batteryPercentBar));

        mainPanel.setLayout(new GridLayout(1, 0));
        mainPanel.add(createTitledPanel("Input", inputPanel));
        mainPanel.add(createTitledPanel("Display", displayPanel));
    }

    private JComponent createTitledPanel(String title, JComponent component) {
        JPanel titledPanel = new JPanel(new BorderLayout());
        titledPanel.setBorder(BorderFactory.createTitledBorder(title));
        titledPanel.add(component);
        return titledPanel;
    }


    public JComponent getMainDisplay() {
        return mainPanel;
    }


    public void setSpeed(float speed) {
        speedBar.setValue((int) speed);
    }


    public void setBatteryPercent(int batteryPercent) {
        batteryPercentBar.setValue(batteryPercent);
    }


    public JSlider getSpeedSlider() {
        return speedSlider;
    }

    public JSlider getBatterySlider() {
        return batterySlider;
    }

}

class MvcController {
    private MvcModel model;
    private MvcView view;

    public MvcController(MvcModel model, MvcView view) {
        this.model = model;
        this.view = view;

        model.addPropertyChangeListener(MvcModel.BATTERY, new BatteryListener());
        model.addPropertyChangeListener(MvcModel.SPEED, new SpeedListener());

        view.getSpeedSlider().addChangeListener(chngEvent -> {
            int value = view.getSpeedSlider().getValue();
            model.setSpeed(value);
        });

        view.getBatterySlider().addChangeListener(chngEvent -> {
            int value = view.getBatterySlider().getValue();
            model.setBatteryPercentage(value);
        });
    }

    public void init() {
        view.getSpeedSlider().setValue(10);
        view.getBatterySlider().setValue(100);

        model.setSpeed(10);
        model.setBatteryPercentage(100);
    }

    class SpeedListener implements PropertyChangeListener {
        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            float speed = model.getSpeed();
            view.setSpeed(speed);
        }
    }

    class BatteryListener implements PropertyChangeListener {
        @Override
        public void propertyChange(PropertyChangeEvent evt) {
            int batteryPercent = model.getBatteryPercentage();
            view.setBatteryPercent(batteryPercent);
        }
    }    
}

---

旁注:Observer 和 Observable 在最新版本的 Java 中已被弃用，因此应该避免使用它们。