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

定义一对多的对象间关系，当一个对象的状态发生了改变，通知所有的从属对象

观察者

/**
 * 观察者
 */
public interface IObserver {
    void update(int value);
}

/**
 * 观察者1
 */
public class Observer1 implements IObserver {
    @Override
    public void update(int value) {
        System.out.println("Observer1: myValue in Subject is now: " + value);
    }
}

/**
 * 观察者2
 */
public class Observer2 implements IObserver {
    @Override
    public void update(int value) {
        System.out.println("Observer2: myValue in Subject is now: " + value);
    }
}

被观察者

/**
 * 主题，被观察者
 */
public interface ISubject {
    /**
     * 注册观察者
     * @param o
     */
    void register(IObserver o);

    /**
     * 注销观察者
     * @param o
     */
    void unregister(IObserver o);

    /**
     * 通知观察者
     * @param value
     */
    void notifyObservers(int value);
}

/**
 * 被观察者实现
 */
public class Subject implements ISubject {
    private int value;
    /**
     * 观察者列表
     */
    List<IObserver> observersList = new ArrayList<>();

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
        // 通知所有观察者
        notifyObservers(value);
    }

    @Override
    public void register(IObserver o) {
        observersList.add(o);
    }

    @Override
    public void unregister(IObserver o) {
        observersList.remove(o);
    }

    @Override
    public void notifyObservers(int updatedValue) {
        for (int i = 0; i < observersList.size(); i++) {
            observersList.get(i).update(updatedValue);
        }
    }
}

执行

public class ObserverPatternEx {
    public static void main(String[] args) {
        Subject sub = new Subject();
        Observer1 ob1 = new Observer1();
        Observer2 ob2 = new Observer2();
        sub.register(ob1);
        sub.register(ob2);

        sub.setValue(5);
        System.out.println();
        sub.setValue(25);
        System.out.println();

        //unregister ob1 only
        sub.unregister(ob1);
        //Now only ob2 will observe the change
        sub.setValue(100);
    }
}

类图

observer.png