Observer Pattern(觀察者模式)

• Definition

The Observer Pattern defines a one-to-many dependency between 
objects so that when one object changes state, all of its 
dependents are notified and updated automatically.

• Class Diagram

• Subject Role

public abstract class Subject {
    private Vector<Observer> obsVector = new Vector<Observer>();
    
    public void addObserver(Observer o) {
        this.obsVector.add(o);
    }
    
    public void delObserverr(Observer o) {
        this.obsVector.remove(o);
    }

    public void notifyObservers() {
        for (Observer o:this.obsVector) {
            o.update();
        }
    }
}

• ConcreteSubject Role

public class ConcreteSubject extends Subject {
    public void doSomething() {
         //do something
        notifyObserver();
    }
}

• Observer Role

public interface Observer {
    public void update();
}

• ConcreteObserver Role

public class ConcreteObserver implements Observer {
    pulic void update() {
        system.put.println("received notification, do somthing");
    }
}

• Client Role

public class Client {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
        Observer obs = new ConcreteObserver();
        subject.addObserver(obs);
        subject.doSomething(); 
    }
}