EventBus源码解析
Posted 许佳佳233
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概述
本篇内容适合对EventBus有兴趣，或者已经对EventBus有一定使用经验的读者。
如果读者此前还没有具体使用过EventBus，推荐阅读下笔者前文：
register
关键逻辑：
-
遍历当前注册的类，获取其中使用了eventBus注解的方法。
-
将这些方法都注册到两个HashMap中，分别是subscriptionsByEventType和typesBySubscriber。通过synchronized加锁，来保证线程安全。
-
subscriptionsByEventType：key是eventType，value是List
typesBySubscriber:key是注册的对象，value是List -
方法执行的时候，会去判断下是否有stcky事件，如果有的话会直接触发。
public void register(Object subscriber)
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this)
for (SubscriberMethod subscriberMethod : subscriberMethods)
subscribe(subscriber, subscriberMethod);
// Must be called in synchronized block
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)
// Existing sticky events of all subclasses of eventType have to be considered.
// Note: Iterating over all events may be inefficient with lots of sticky events,
// thus data structure should be changed to allow a more efficient lookup
// (e.g. an additional map storing sub classes of super classes: Class -> List<Class>).
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);
post
关键逻辑：
- post源码做了两件事，第一件事是将此时的event加入到eventQueue中。第二件事是将当前线程改成posting状态。
- posting状态会去循环处理eventQueue中的event，将其放到对应的subscription中执行。
– 查找subscription的过程是：先找到与event相关的类，然后遍历这些类与其相关的subscription。
public void post(Object event)
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
if (!postingState.isPosting)
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
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;
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance)
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++)
Class<?> clazz = eventTypes.get(h);
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
else
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
if (!subscriptionFound)
if (logNoSubscriberMessages)
Log.d(TAG, "No subscribers registered for event " + eventClass);
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class)
post(new NoSubscriberEvent(this, event));
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass)
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this)
subscriptions = subscriptionsByEventType.get(eventClass);
if (subscriptions != null && !subscriptions.isEmpty())
for (Subscription subscription : subscriptions)
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
finally
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
if (aborted)
break;
return true;
return false;
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass)
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this)
subscriptions = subscriptionsByEventType.get(eventClass);
if (subscriptions != null && !subscriptions.isEmpty())
for (Subscription subscription : subscriptions)
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
finally
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
if (aborted)
break;
return true;
return false;
postSticky
关键逻辑：
- 与post相比，postSticky会先将event加入到stickyEvents这个Map中。
- 在前面register中的逻辑已经讲到，当一个类注册的时候，会去判断是否有stickyEvent，如果有的话会直接触发。
public void postSticky(Object event)
synchronized (stickyEvents)
stickyEvents.put(event.getClass(), event);
// Should be posted after it is putted, in case the subscriber wants to remove immediately
post(event);
多线程的逻辑
关键逻辑：
- 在最终触发subscription时，会在postToSubscription中选择执行的线程。
- POSTING: 直接在当前线程触发
- MAIN：如果当前是主线程，那么直接触发。如果当前不在主线程，那么会通过handler抛到主线程去执行。
- BACKGROUND:如果当前是子线程，那么直接触发。如果当前不在子线程，那么会抛到eventBus的线程池中执行。
- ASYNC：抛到eventBus的线程池中执行。
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 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);
final class BackgroundPoster implements Runnable
private final PendingPostQueue queue;
private final EventBus eventBus;
private volatile boolean executorRunning;
BackgroundPoster(EventBus eventBus)
this.eventBus = eventBus;
queue = new PendingPostQueue();
public void enqueue(Subscription subscription, Object event)
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this)
queue.enqueue(pendingPost);
if (!executorRunning)
executorRunning = true;
eventBus.getExecutorService().execute(this);
@Override
public void run()
try
try
while (true)
PendingPost pendingPost = queue.poll(1000);
if (pendingPost == null)
synchronized (this)
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null)
executorRunning = false;
return;
eventBus.invokeSubscriber(pendingPost);
catch (InterruptedException e)
Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
finally
executorRunning = false;
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