Google Guava EventBus

在Google Guava 10版本引入了EventBus, 它主要用来简化我们处理生产/消费者编程模型. 

基本用法 
使用Guava之后, 如果要订阅消息, 就不用再继承指定的接口, 只需要在指定的方法上加上@Subscribe注解即可: 

public class EventListener {

    public int lastMessage = 0;

    @Subscribe
    public void listen(OurTestEvent event) {
        lastMessage = event.getMessage();
    }

    public int getLastMessage() {
        return lastMessage;
    }
}

上面的lastMessage用来接收消息. 

下面定义的类用来对消息进行封装: 

public class OurTestEvent {

    private final int message;

    public OurTestEvent(int message) {
        this.message = message;
    }

    public int getMessage() {
        return message;
    }
}

通过写一个测试来了解EventBus如何工作: 

@Test
public void shouldReceiveEvent() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");
    EventListener listener = new EventListener();

    eventBus.register(listener);

    // when
    eventBus.post(new OurTestEvent(200));

    // then
    assertThat(listener.getLastMessage()).isEqualTo(200);
}

上面的测试是不是很简单? 

MultiListener的使用 

只需要在要订阅消息的方法上加上@Subscribe注解即可实现对多个消息的订阅: 

public class MultipleListener {

    public Integer lastInteger;
    public Long lastLong;

    @Subscribe
    public void listenInteger(Integer event) {
        lastInteger = event;
    }

    @Subscribe
    public void listenLong(Long event) {
        lastLong = event;
    }

    public Integer getLastInteger() {
        return lastInteger;
    }

    public Long getLastLong() {
        return lastLong;
    }
}

下面是对应的测试: 

@Test
public void shouldReceiveMultipleEvents() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");
    MultipleListener multiListener = new MultipleListener();

    eventBus.register(multiListener);

    // when
    eventBus.post(new Integer(100));
    eventBus.post(new Long(800));

    // then
    assertThat(multiListener.getLastInteger()).isEqualTo(100);
    assertThat(multiListener.getLastLong()).isEqualTo(800L);
}

高级用法 

1.Dead Event 

如果EventBus发送的消息都不是订阅者关心的称之为Dead Event. 看下面的例子: 

/**
 * Listener waiting for the event that any message was posted but not delivered to anyone
 */
public class DeadEventListener {

    boolean notDelivered = false;

    @Subscribe
    public void listen(DeadEvent event) {
        notDelivered = true;
    }

    public boolean isNotDelivered() {
        return notDelivered;
    }
}

下面是测试类: 

@Test
public void shouldDetectEventWithoutListeners() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");

    DeadEventListener deadEventListener = new DeadEventListener();
    eventBus.register(deadEventListener);

    // when
    eventBus.post(new OurTestEvent(200));

    assertThat(deadEventListener.isNotDelivered()).isTrue();
}

如果没有消息订阅者监听消息, EventBus将发送DeadEvent消息, 这时我们可以通过log的方式来记录这种状态. 

2.Event的继承 

如果Listener A监听Event A, 而Event A有一个子类Event B, 此时Listener A将同时接收Event A和B消息 

看下面的例子: 

public class NumberListener {

    private Number lastMessage;

    @Subscribe
    public void listen(Number integer) {
        lastMessage = integer;
    }

    public Number getLastMessage() {
        return lastMessage;
    }
}


public class IntegerListener {

    private Integer lastMessage;

    @Subscribe
    public void listen(Integer integer) {
        lastMessage = integer;
    }

    public Integer getLastMessage() {
        return lastMessage;
    }
}

对应的测试类: 

@Test
public void shouldGetEventsFromSubclass() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");
    IntegerListener integerListener = new IntegerListener();
    NumberListener numberListener = new NumberListener();
    eventBus.register(integerListener);
    eventBus.register(numberListener);

    // when
    eventBus.post(new Integer(100));

    // then
    assertThat(integerListener.getLastMessage()).isEqualTo(100);
    assertThat(numberListener.getLastMessage()).isEqualTo(100);

    //when
    eventBus.post(new Long(200L));

    // then
    // this one should has the old value as it listens only for Integers
    assertThat(integerListener.getLastMessage()).isEqualTo(100);
    assertThat(numberListener.getLastMessage()).isEqualTo(200L);
}