观察者模式(Observer Pattern)升级版

简介

前面的博客https://goo.gl/DFOz50中已经介绍过基本的观察者模式的代码实现，在实际应用中，我们往往希望代码更加简洁，同时也希望能与现有的框架，比如Spring，结合起来。下面介绍如何做抽象。

简单抽象

前面介绍过观察者模式有Subject，Observer，Client几种角色。在使用面向对象的时候Subject 和Observer都可以被抽象成接口，所以从原来的三种角色可以进化为6个角色。
(1) Subject : 被监控对象，抽象以后我们叫做BUS(总线，所有对被监控对象的操作都由它完成);
(2) concreteSubject：具体的被监控对象
(3)Observer（Listener）：观察者
(4)concreteObserver：具体的观察者
(5)event : 监控对象和被监控对象交互时用到的消息

下面的类图用来解释新的观察者模式：

Note:
（1） 图中引入了泛型T作为观察者，从而使得每一个Bus的具体实现对应一个Listener接口，这样一个Bus最终只对应到一类事件；
（2）Listener和Bus之间交互使用一个单独的类Event实现；
（3）ExecutorService的引入使得监听器的响应与主线程异步进行，提升效率。

如果对上面的图进一步抽象的话就编程了下面这样：

代码实现

Step 1

创建AbstractBus

public abstract class BaseBus<T> {
    private final List<T> listeners = new CopyOnWriteArrayList<T>();
    private final ExecutorService executorService;

    protected BaseBus(ExecutorService executorService) {
        this.executorService = executorService;
    }


    public void iterate(final ListenerCallback<T> callback){
        for(final T t:listeners){
            this.executorService.submit(new Callable<Object>() {
                @Override
                public Object call() throws Exception {
                    callback.doWithListener(t);
                    return true;
                }
            });
        }
    }

    public interface ListenerCallback<T> {
        Object doWithListener(T t);
    }

    public boolean addListener(T listener) {
        return this.listeners.add(listener);
    }

    public List<T> getListeners() {
        return listeners;
    }
    public void setListeners(List<T> listeners) {
        this.listeners.addAll(listeners);
    }
}

Step 2

创建AbstractBus的具体实现：

public class ProfileChangeBus extends BaseBus<ProfileChangeListener> {
    public ProfileChangeBus(){
        super(new ThreadPoolExecutor(
                2, 10,
                0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(100),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.CallerRunsPolicy()));
    }

    public void fireCreate(final ProfileChangeEvent event) {
        event.setType(0);
        iterate(new ListenerCallback<ProfileChangeListener>() {
            @Override
            public Object doWithListener(ProfileChangeListener profileChangeListener) {
                profileChangeListener.onCreate(event);
                return true;
            }
        });
    }

    public void fireUpdate(final ProfileChangeEvent event) {
        event.setType(0);
        this.iterate(new ListenerCallback<ProfileChangeListener>() {
            @Override
            public Object doWithListener(ProfileChangeListener profileChangeListener) {
                profileChangeListener.onUpdate(event);
                return true;
            }
        });
    }

    public void fireDelete(final ProfileChangeEvent event) {
        event.setType(0);
        this.iterate(new ListenerCallback<ProfileChangeListener>() {
            @Override
            public Object doWithListener(ProfileChangeListener profileChangeListener) {
                profileChangeListener.onDelete(event);
                return true;
            }
        });
    }
}

Step 3

创建Listener

public interface ProfileChangeListener {
    void onCreate(ProfileChangeEvent profileChangeEvent);

    void onUpdate(ProfileChangeEvent profileChangeEvent);

    void onDelete(ProfileChangeEvent profileChangeEvent);
}

Step 4

创建Listener的具体实现，这里有3个实现类：
(1)

public class ProfileChangeListenerAdapter implements ProfileChangeListener {
    @Override
    public void onCreate(ProfileChangeEvent profileChangeEvent) {
        System.out.println("trigger create");
    }

    @Override
    public void onUpdate(ProfileChangeEvent profileChangeEvent) {
        System.out.println("trigger update");
    }

    @Override
    public void onDelete(ProfileChangeEvent profileChangeEvent) {
        System.out.println("trigger delete");
    }
}

(2)

public class ProfileChangeListenerAdapter4MQ extends ProfileChangeListenerAdapter {
    @Override
    public void onCreate(ProfileChangeEvent profileChangeEvent) {
        System.out.println("mq trigger create");
    }

    @Override
    public void onUpdate(ProfileChangeEvent profileChangeEvent) {
        System.out.println("mq trigger update");
    }

    @Override
    public void onDelete(ProfileChangeEvent profileChangeEvent) {
        System.out.println("mq trigger delete");
    }
}

(3)

public class ProfileChangeListener4DB implements ProfileChangeListener{
    @Override
    public void onCreate(ProfileChangeEvent profileChangeEvent) {
        System.out.println("db trigger create");
    }

    @Override
    public void onUpdate(ProfileChangeEvent profileChangeEvent) {
        System.out.println("db trigger create");
    }

    @Override
    public void onDelete(ProfileChangeEvent profileChangeEvent) {
        System.out.println("db trigger create");
    }
}

Step 5

创建Event

public class ProfileChangeEvent {
    private int type;
    private String message;

    public ProfileChangeEvent(int type, String message){
        this.type = type;
        this.message = message;
    }

    public int getType() {
        return type;
    }

    public void setType(int type) {
        this.type = type;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

Step 6

将Listener注入到ConcreteBus

  <bean id="profileChangeBus" class="designpattern.observer.ProfileChangeBus">
        <property name="listeners">
            <list>
                <bean class="designpattern.observer.ProfileChangeListener4DB" />
                <bean class="designpattern.observer.ProfileChangeListenerAdapter4MQ"/>
            </list>
        </property>
    </bean>

Step 7

测试：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {
        "classpath:config/*.xml"
})
public class ObserverPatternTest {
    @Autowired
    private ProfileChangeBus profileChangeBus;
    @Test
    public void test(){
        profileChangeBus.fireCreate(new ProfileChangeEvent(1, "1"));
    }
}