EventBus.getDefault()
在注册、解绑、发送事件的时候，我们都需要先获取Eventbus实例，通过getDefault获取。
public static EventBus getDefault() {
EventBus instance = defaultInstance;
if (instance == null) {
synchronized (EventBus.class) {
instance = EventBus.defaultInstance;
if (instance == null) {
instance = EventBus.defaultInstance = new EventBus();
}
}
}
return instance;
}
加锁的懒汉式单例模式获取Eventbus。
构造方法初始化，使用了目前使用广泛的建造者模式：
public EventBus() {
this(DEFAULT_BUILDER);
}
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
backgroundPoster = new BackgroundPoster(this);
asyncPoster = new AsyncPoster(this);
indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
builder.strictMethodVerification, builder.ignoreGeneratedIndex);
logSubscriberExceptions = builder.logSubscriberExceptions;
logNoSubscriberMessages = builder.logNoSubscriberMessages;
sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
throwSubscriberException = builder.throwSubscriberException;
eventInheritance = builder.eventInheritance;
executorService = builder.executorService;
}
注册订阅者以接收事件。
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
通过反射，查找该Subscriber中，通过@Subscribe注解的方法（消费方法）信息，将这些方法信息封装到SubscriberMethod中，封装的内容包括Method对象、ThreadMode、事件类型、优先级、是否粘性等。一个Subscriber可能对应多个事件消费方法，因此他们的关系是1对多。
再调用subscribe方法：
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
if (subscriberMethod.sticky) {
if (eventInheritance) {
Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
for (Map.Entry<Class<?>, Object> entry : entries) {
Class<?> candidateEventType = entry.getKey();
if (eventType.isAssignableFrom(candidateEventType)) {
Object stickyEvent = entry.getValue();
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
1：将注册者和事件消费方法封装起来，这样做的原因是完成二者的绑定关系。
2：就像上述注册者和事件消费方法是1：N的关系。一个Event与注册者之间也是1对多的关系。因为一个Event可能会被不同的Activity注册。也就是说Event、注册者、事件消费方法的关系是：1：N：M（其中M、N均大于等于1）。
3：注册者（比如MainActivity.this）与事件消费方法（SubscriberMethod）的关系，我们封装到了Subscription(s)中了。而Event和Subscription(s)的关系，我们通过HashMap保存，key为event.class，value即Subscription(s)。
4：已注册的不能重复注册。
5：按照优先级保存Subscription。
6：粘性相关的处理。
发布事件：
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
PostingThreadState中，eventQueue就是保存发布到当前线程的Event的。
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<>();
boolean isPosting;
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
最终调用了postToSubscription，将事件发送给订阅者。
线程模式（ThreadMode）在此处发挥了作用，根据ThreadMode方式的不同，有不同的处理策略。
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case POSTING:
invokeSubscriber(subscription, event);
break;
case MAIN:
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
case MAIN_ORDERED:
if (mainThreadPoster != null) {
mainThreadPoster.enqueue(subscription, event);
} else {
// temporary: technically not correct as poster not decoupled from subscriber
invokeSubscriber(subscription, event);
}
break;
case BACKGROUND:
if (isMainThread) {
backgroundPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
case ASYNC:
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}
但是，根据不同的类型切换线程后，最终调用的都是invokeSubscriber(subscription, event)方法：
void invokeSubscriber(Subscription subscription, Object event) {
try {
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
最终用反射去执行了该注册者的注册方法。
注册、发射、接收事件过程总结：
