前言
EventBus的核心思想是观察者模式 (生产/消费者编程模型) 。
通过前面的文章我们已经知道，如何使用eventBus了。我们需要先定义一个Observer(前文中的EventListener类)，然后将其注册到eventBus里，通过 @Subscribe 定义消息回调函数。
那我们先看看register(Object object) 和unregister(Object object) 方法。
register (Object object) 解析
public void register(Object object) {
Multimap, EventSubscriber> methodsInListener =
finder.findAllSubscribers(object);
subscribersByTypeLock.writeLock().lock();
try {
subscribersByType.putAll(methodsInListener);
} finally {
subscribersByTypeLock.writeLock().unlock();
}
}
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publicvoidregister(Objectobject){
Multimap,EventSubscriber>methodsInListener=
finder.findAllSubscribers(object);
subscribersByTypeLock.writeLock().lock();
try{
subscribersByType.putAll(methodsInListener);
}finally{
subscribersByTypeLock.writeLock().unlock();
}
}
可以看到是先通过SubscriberFindingStrategy接口里的findAllSubscribers方法获取所有标记了@ Subscribe 注解的方法，其中该接口的具体实现是AnnotatedSubscriberFinder类。放到一个guava里定义的Multimap里。然后是把获取到的methodsInListener放到一个叫subscribersByType的 guava里定义的SetMultimap里 。
public Multimap, EventSubscriber> findAllSubscribers(Object listener) {
Multimap, EventSubscriber> methodsInListener = HashMultimap.create();
Class> clazz = listener.getClass();
for (Method method : getAnnotatedMethods(clazz)) {
Class>[] parameterTypes = method.getParameterTypes();
Class> eventType = parameterTypes[0];
EventSubscriber subscriber = makeSubscriber(listener, method);
methodsInListener.put(eventType, subscriber);
}
return methodsInListener;
}
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publicMultimap,EventSubscriber>findAllSubscribers(Objectlistener){
Multimap,EventSubscriber>methodsInListener=HashMultimap.create();
Class>clazz=listener.getClass();
for(Methodmethod:getAnnotatedMethods(clazz)){
Class>[]parameterTypes=method.getParameterTypes();
Class>eventType=parameterTypes[0];
EventSubscribersubscriber=makeSubscriber(listener,method);
methodsInListener.put(eventType,subscriber);
}
returnmethodsInListener;
}
findAllSubscribers方法里，最重要的是methodsInListener，它的结构可以简单理解为一个map，其中key是eventType，在我前文写的例子中就是com.sww.eventbus.domain.MessageEvent，其中value是subscriber，就是例子中的com.sww.eventbus.listener.EventListener#onMessageEvent。
总之，一句话就是先通过标记找到所有已经注册进来的观察者，然后存放到容器里备用。
那unregister就是从容器删除它们，
unRegister (Object object) 解析
public void unregister(Object object) {
Multimap, EventSubscriber> methodsInListener = finder.findAllSubscribers(object);
for (Entry, Collection> entry :
methodsInListener.asMap().entrySet()) {
Class> eventType = entry.getKey();
Collection eventMethodsInListener = entry.getValue();
subscribersByTypeLock.writeLock().lock();
try {
Set currentSubscribers = subscribersByType.get(eventType);
if (!currentSubscribers.containsAll(eventMethodsInListener)) {
throw new IllegalArgumentException(
"missing event subscriber for an annotated method. Is " + object + " registered?");
}
currentSubscribers.removeAll(eventMethodsInListener);
} finally {
subscribersByTypeLock.writeLock().unlock();
}
}
}
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publicvoidunregister(Objectobject){
Multimap,EventSubscriber>methodsInListener=finder.findAllSubscribers(object);
for(Entry,Collection>entry:
methodsInListener.asMap().entrySet()){
Class>eventType=entry.getKey();
CollectioneventMethodsInListener=entry.getValue();
subscribersByTypeLock.writeLock().lock();
try{
SetcurrentSubscribers=subscribersByType.get(eventType);
if(!currentSubscribers.containsAll(eventMethodsInListener)){
thrownewIllegalArgumentException(
"missing event subscriber for an annotated method. Is "+object+" registered?");
}
currentSubscribers.removeAll(eventMethodsInListener);
}finally{
subscribersByTypeLock.writeLock().unlock();
}
}
}
post( Object event)解析
有了观察者，下面就是发送事件了，阅读过前文会知道是通过eventBus.post(Object event)来发送事件消息。那咱们来看看这个post方法。
public void post(Object event) {
Set> dispatchTypes = flattenHierarchy(event.getClass());
boolean dispatched = false;
for (Class> eventType : dispatchTypes) {
subscribersByTypeLock.readLock().lock();
try {
Set wrappers = subscribersByType.get(eventType);
if (!wrappers.isEmpty()) {
dispatched = true;
for (EventSubscriber wrapper : wrappers) {
enqueueEvent(event, wrapper);
}
}
} finally {
subscribersByTypeLock.readLock().unlock();
}
}
if (!dispatched && !(event instanceof DeadEvent)) {
post(new DeadEvent(this, event));
}
dispatchQueuedEvents();
}
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publicvoidpost(Objectevent){
Set>dispatchTypes=flattenHierarchy(event.getClass());
booleandispatched=false;
for(Class>eventType:dispatchTypes){
subscribersByTypeLock.readLock().lock();
try{
Setwrappers=subscribersByType.get(eventType);
if(!wrappers.isEmpty()){
dispatched=true;
for(EventSubscriberwrapper:wrappers){
enqueueEvent(event,wrapper);
}
}
}finally{
subscribersByTypeLock.readLock().unlock();
}
}
if(!dispatched&&!(eventinstanceofDeadEvent)){
post(newDeadEvent(this,event));
}
dispatchQueuedEvents();
}
该方法就是从之前的容器subscribersByType里获取到eventType对应的观察者，然后组装成EventWithSubscriber放到队列里。
void enqueueEvent(Object event, EventSubscriber subscriber) {
eventsToDispatch.get().offer(new EventWithSubscriber(event, subscriber));
}
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voidenqueueEvent(Objectevent,EventSubscribersubscriber){
eventsToDispatch.get().offer(newEventWithSubscriber(event,subscriber));
}
然后就是最后的dispatchQueuedEvents()，经过一层层深入进去，可以发现wrapper.handleEvent(event)，其中 handleEvent方法就是最终的关键了
public void handleEvent(Object event) throws InvocationTargetException {
checkNotNull(event);
try {
method.invoke(target, new Object[] { event });
} catch (IllegalArgumentException e) {
throw new Error("Method rejected target/argument: " + event, e);
} catch (IllegalAccessException e) {
throw new Error("Method became inaccessible: " + event, e);
} catch (InvocationTargetException e) {
if (e.getCause() instanceof Error) {
throw (Error) e.getCause();
}
throw e;
}
}
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publicvoidhandleEvent(Objectevent)throwsInvocationTargetException{
checkNotNull(event);
try{
method.invoke(target,newObject[]{event});
}catch(IllegalArgumentExceptione){
thrownewError("Method rejected target/argument: "+event,e);
}catch(IllegalAccessExceptione){
thrownewError("Method became inaccessible: "+event,e);
}catch(InvocationTargetExceptione){
if(e.getCause()instanceofError){
throw(Error)e.getCause();
}
throwe;
}
}
就是通过Java的反射机制实现。
需要说明的是，如果没有订阅者注册到要发送的event事件上，并且该event不是DeadEvent，那么它将被包装成DeadEvent中并重新发布。也就是其中这三行代码索要做的
if (!dispatched && !(event instanceof DeadEvent)) {
post(new DeadEvent(this, event));
}
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if(!dispatched&&!(eventinstanceofDeadEvent)){
post(newDeadEvent(this,event));
}
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