观察者模式【行为模式】

观察者模式

Define a one-to-many dependency between objects so that when one object change state，
all its dependents are notified and updated automatically.
定义对象间一种一对多的依赖关系，这样，当一的一方状态改变时，所有它的依赖者都会被自动通知。

public class Observer {
    /**
     * 观察者模式：
     * Define a one-to-many dependency between objects so that when one object change state，
     * all its dependents are notified and updated automatically.
     * 定义对象间一种一对多的依赖关系，这样，当一的一方状态改变时，所有它的依赖者都会被自动通知。
     */
    @Test
    public void all() {
        final Student student = Student.builder().name("二狗子").build();
        final Monitor monitor = Monitor.builder().name("monitor").build();
        final Teacher teacher = Teacher.builder().name("teacher").build();

        student.addObserver(monitor);
        student.addObserver(teacher);

        student.play();
    }
}

/**
 * 1）观察者需要观察的事件或行为
 */
interface IObserver {
    void inform(String event);
}

/**
 * 2）被观察者对观察者的操作接口
 */
interface IObserverOperate {
    void addObserver(IObserver observer);

    void removeObserver(IObserver observer);

    void notifyAll(String target);
}

/**
 * 3）具体的观察者实现
 */
@Builder
@Slf4j
class Monitor implements IObserver {
    private final String name;

    @Override
    public void inform(String event) {
        log.info("{} {}", name, event);
    }
}

/**
 * 4）具体的观察者实现
 */
@Builder
@Slf4j
class Teacher implements IObserver {
    private final String name;

    @Override
    public void inform(String event) {
        log.info("{} {}", name, event);
    }
}

/**
 * 4）具体的被观察者实现，实现了观察者管理接口
 */
@Builder
class Student implements IObserverOperate {
    private final String name;
    @Default
    private final List<IObserver> observers = Lists.newArrayList();

    @Override
    public void addObserver(IObserver observer) {
        observers.add(observer);
    }

    @Override
    public void removeObserver(IObserver observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyAll(String target) {
        observers.forEach(observer -> observer.inform(target));
    }

    public void play() {
        /**
         * 一旦主体状态变化，立即通知所有的观察者
         */
        final String event = name + " is plaing";
        notifyAll(event);
    }
}