觀察者模式

• 當一個對象變化時，其它依賴該對象的對象都會收到通知，并且隨著變化！

定義一個接口

public interface Observer {
    public void update();
}

定義兩個實現類，也就是訂閱者

public class Observer1 implements Observer {

    @Override
    public void update() {
        System.out.println("observer1 has received!");
    }
}

public class Observer2 implements Observer {

    @Override
    public void update() {
        System.out.println("observer2 has received!");
    }

}

在定義一個接口

public interface Subject {

    /*增加觀察者*/
    public void add(Observer observer);

    /*刪除觀察者*/
    public void del(Observer observer);

    /*通知所有的觀察者*/
    public void notifyObservers();

    /*自身的操作*/
    public void operation();
}

訂閱一個抽象的類

public abstract class AbstractSubject implements Subject {
    存放所有訂閱的類
    private Vector<Observer> vector = new Vector<Observer>();
    @Override
    public void add(Observer observer) {
        vector.add(observer);
    }

    @Override
    public void del(Observer observer) {
        vector.remove(observer);
    }
    遍歷每個類調用
    @Override
    public void notifyObservers() {
        Enumeration<Observer> enumo = vector.elements();
        while(enumo.hasMoreElements()){
            enumo.nextElement().update();
        }
    }
}

public class MySubject extends AbstractSubject {

    @Override
    public void operation() {
        System.out.println("update self!");
        notifyObservers();
    }

}

測試類

public class ObserverTest {

    public static void main(String[] args) {
        Subject sub = new MySubject();
        sub.add(new Observer1());
        sub.add(new Observer2());

        sub.operation();
    }

}