Observable Design Pattern ¶

Aliases 📌: None

Description 📚: An observable is a class that is able to notify its observers when an event occurs.

For instance, A class B will register itself as an observer of class B. If B changes, then A will be notified.

Advantages ✅

• Simplified communication

Disadvantages 🚫

• ???

Notes 📝

• None

For instance, a game class might want to be notified when the player does something. The game will be registered as an Observer.

public class Main {
    public static void main(String[] args) {
        GameManager g = new GameManager();
        Player p = new Player();
        p.addObserver(g);
        p.buy("XXX +9");
    }
}

The game itself may transfer the event to someone else:

public class GameManager implements PropertyChangeListener {
    private final GameMap map = new GameMap();

    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        System.out.println("GameManager, message received");
        // call on each component of the game
        this.map.propertyChange(evt);
        // ...
    }
}

public class GameMap implements PropertyChangeListener {
    @Override // print some random text
    public void propertyChange(PropertyChangeEvent evt) {
        System.out.println("GameMap, message received : " + evt.getNewValue());
    }
}

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;

public class Player {
    private final PropertyChangeSupport support;

    public Player() {
        this.support = new PropertyChangeSupport(this);
    }

    // proxy
    public void addObserver(PropertyChangeListener o) {
        this.support.addPropertyChangeListener(o);
    }

    public void removeObserver(PropertyChangeListener o) {
        this.support.removePropertyChangeListener(o);
    }

    // some method
    public void buy(String item) {
        // call for change
        this.support.firePropertyChange("buy", this.support, item);
        //...
    }
}