簡單的 MVP 示例

為了說明 MVP 模式的簡單示例用法，請考慮以下程式碼，該程式碼僅使用按鈕和標籤建立簡單 UI。單擊該按鈕時，標籤會更新按鈕被單擊的次數。

我們有 5 個類：

• 模型 - 保持狀態的 POJO（MVP 中的 M）
• View - 具有 UI 程式碼的類（MVP 中的 V）
• ViewListener - 提供響應檢視中操作的方法的介面
• 演示者 - 響應輸入並更新檢視（MV 中的 P）
• 應用程式 - 將所有內容組合在一起並啟動應用程式的主要類

一個最小的模型類，只保留一個 count 變數。

/**
 * A minimal class to maintain some state 
 */
public class Model {
    private int count = 0;

    public void addOneToCount() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

通知偵聽器的最小介面：

/**
 * Provides methods to notify on user interaction
 */
public interface ViewListener {
    public void onButtonClicked();
}

檢視類構造所有 UI 元素。檢視，只認為，應該有參照 UI 元素（即沒有按鈕，文字欄位等在演示者或其他類）。

/**
 * Provides the UI elements
 */

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.WindowConstants;

public class View {
    // A list of listeners subscribed to this view
    private final ArrayList<ViewListener> listeners;
    private final JLabel label;
    
    public View() {
        final JFrame frame = new JFrame();
        frame.setSize(200, 100);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setLayout(new GridLayout());

        final JButton button = new JButton("Hello, world!");

        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(final ActionEvent e) {
                notifyListenersOnButtonClicked();
            }
        });
        frame.add(button);

        label = new JLabel();
        frame.add(label);

        this.listeners = new ArrayList<ViewListener>();

        frame.setVisible(true);
    }

    // Iterate through the list, notifying each listner individualy 
    private void notifyListenersOnButtonClicked() {
        for (final ViewListener listener : listeners) {
            listener.onButtonClicked();
        }
    }

    // Subscribe a listener
    public void addListener(final ViewListener listener) {
        listeners.add(listener);
    }

    public void setLabelText(final String text) {
        label.setText(text);
    }
}

通知邏輯也可以在 Java8 中這樣編碼：

        ...
        final Button button = new Button("Hello, world!");
        // In order to do so, our interface must be changed to accept the event parametre
        button.addActionListener((event) -> {
            notifyListeners(ViewListener::onButtonClicked, event);
            // Example of calling methodThatTakesALong, would be the same as callying:
            // notifyListeners((listener, long)->listener.methodThatTakesALong(long), 10L)
            notifyListeners(ViewListener::methodThatTakesALong, 10L);
        });
        frame.add(button);
        ...

/**
 * Iterates through the subscribed listeneres notifying each listener individually.
 * Note: the {@literal '<T>' in private <T> void} is a Bounded Type Parametre. 
 *
 * @param <T>      Any Reference Type (basically a class).
 * 
 * @param consumer A method with two parameters and no return, 
 *                 the 1st parametre is a ViewListner, 
 *                 the 2nd parametre is value of type T.
 * 
 * @param data     The value used as parametre for the second argument of the
 *                 method described by the parametre consumer.
 */
private <T> void notifyListeners(final BiConsumer<ViewListener, T> consumer, final T data) {
    // Iterate through the list, notifying each listener, java8 style 
    listeners.forEach((listener) -> {

        // Calls the funcion described by the object consumer.
        consumer.accept(listener, data);

        // When this method is called using ViewListener::onButtonClicked
        // the line: consumer.accept(listener,data); can be read as:
        // void accept(ViewListener listener, ActionEvent data) {
        //     listener.onButtonClicked(data);
        // }
        
    });
}

必須重構介面才能將 ActionEvent 作為引數：

public interface ViewListener {
    public void onButtonClicked(ActionEvent evt);
    // Example of methodThatTakesALong signature
    public void methodThatTakesALong(long );
}

這裡只需要一個 notify-method，實際的 listener 方法及其引數作為引數傳遞。在需要的情況下，這也可以用於比實際事件處理稍微不那麼漂亮的東西，只要介面中有方法，它都可以工作，例如：

        notifyListeners(ViewListener::methodThatTakesALong, -1L);

演示者可以接受檢視並將其自身新增為偵聽器。在檢視中單擊按鈕時，檢視會通知所有偵聽器（包括演示者）。既然通知了演示者，它可以採取適當的操作來更新模型（即應用程式的狀態），然後相應地更新檢視。

/**
 * Responsible to responding to user interaction and updating the view
 */
public class Presenter implements ViewListener {
    private final View view;
    private final Model model;

    public Presenter(final View view, final Model model) {
        this.view = view;
        view.addListener(this);
        this.model = model;
    }

    @Override
    public void onButtonClicked() {
        // Update the model (ie. the state of the application)
        model.addOneToCount();
        // Update the view
        view.setLabelText(String.valueOf(model.getCount()));
    }
}

要將所有內容放在一起，可以建立檢視並將其注入演示者。類似地，可以建立並注入初始模型。雖然兩者都可以在演示者中建立，但將它們注入建構函式可以實現更簡單的測試。

public class Application {
    public Application() {
        final View view = new View();
        final Model model = new Model();
        new Presenter(view, model);
    }

    public static void main(String... args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Application();
            }
        });
    }
}