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);
- }
