前面一篇文章讲解了EventBus的使用，但是作为开发人员，不能只停留在仅仅会用的层面上，我们还需要弄清楚它的内部实现原理。所以本篇博文将分析EventBus的源码，看看究竟它是如何实现“发布/订阅”功能的。
事件注册
根据前一讲EventBus使用详解我们已经知道EventBus使用首先是需要注册的，注册事件的代码如下：
EventBus.getDefault().register(this);EventBus对外提供了一个register方法来进行事件注册，该方法接收一个Object类型的参数，下面看下register方法的源码:
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
// 判断该类是否是匿名内部类
boolean forceReflection = subscriberClass.isAnonymousClass();
List<SubscriberMethod> subscriberMethods =
subscriberMethodFinder.findSubscriberMethods(subscriberClass, forceReflection);
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}该方法首先获取获取传进来参数的Class对象，然后判断该类是否是匿名内部类。然后根据这两个参数通过subscriberMethodFinder.findSubscriberMethods方法获取所有的事件处理方法。
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, boolean forceReflection) {
String key = subscriberClass.getName();
List<SubscriberMethod> subscriberMethods;
synchronized (METHOD_CACHE) {
subscriberMethods = METHOD_CACHE.get(key);
}
if (subscriberMethods != null) {
//缓存命中，直接返回
return subscriberMethods;
}
if (INDEX != null && !forceReflection) {
// 如果INDEX不为空，并且subscriberClass为非匿名内部类，
// 则通过findSubscriberMethodsWithIndex方法查找事件处理函数
subscriberMethods = findSubscriberMethodsWithIndex(subscriberClass);
if (subscriberMethods.isEmpty()) {
//如果结果为空，则使用findSubscriberMethodsWithReflection方法再查找一次
subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);
}
} else {
//INDEX为空或者subscriberClass未匿名内部类，使用findSubscriberMethodsWithReflection方法查找
subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);
}
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//存入缓存并返回
synchronized (METHOD_CACHE) {
METHOD_CACHE.put(key, subscriberMethods);
}
return subscriberMethods;
}
}通过名字我们就知道这个方法是获取subscriberClass类中所有的事件处理方法（即使用了@Subscribe的方法）。该方法首先会从缓存METHOD_CACHE中去获取事件处理方法，如果缓存中不存在，则需要通过findSubscriberMethodsWithIndex或者findSubscriberMethodsWithReflection方法获取所有事件处理方法，获取到之后先存入缓存再返回。
这个方法里面有个INDEX对象，我们看看它是个什么鬼：
/** Optional generated index without entries from subscribers super classes */
private static final SubscriberIndex INDEX;
static {
SubscriberIndex newIndex = null;
try {
Class<?> clazz = Class.forName("de.greenrobot.event.GeneratedSubscriberIndex");
newIndex = (SubscriberIndex) clazz.newInstance();
} catch (ClassNotFoundException e) {
Log.d(EventBus.TAG, "No subscriber index available, reverting to dynamic look-up");
// Fine
} catch (Exception e) {
Log.w(EventBus.TAG, "Could not init subscriber index, reverting to dynamic look-up", e);
}
INDEX = newIndex;
}由上面代码可以看出EventBus会试图加载一个de.greenrobot.event.GeneratedSubscriberIndex类并创建对象赋值给INDEX，但是EventBus3.0 beta并没有为我们提供该类（可能后续版本会提供）。所以INDEX为null。
我们再返回findSubscriberMethods方法，我们知道INDEX已经为null了，所以必然会调用findSubscriberMethodsWithReflection方法查找所有事件处理函数：
private List<SubscriberMethod> findSubscriberMethodsWithReflection(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = new ArrayList<SubscriberMethod>();
Class<?> clazz = subscriberClass;
HashSet<String> eventTypesFound = new HashSet<String>();
StringBuilder methodKeyBuilder = new StringBuilder();
while (clazz != null) {
String name = clazz.getName();
// 如果查找的类是java、javax或者android包下面的类，则过滤掉
if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
// Skip system classes, this just degrades performance
break;
}
// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
// 通过反射查找所有该类中所有方法
Method[] methods = clazz.getDeclaredMethods();
for (Method method : methods) {
int modifiers = method.getModifiers();
// 事件处理方法必须为public，这里过滤掉所有非public方法
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?>[] parameterTypes = method.getParameterTypes();
// 事件处理方法必须只有一个参数
if (parameterTypes.length == 1) {
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
String methodName = method.getName();
Class<?> eventType = parameterTypes[0];
methodKeyBuilder.setLength(0);
methodKeyBuilder.append(methodName);
methodKeyBuilder.append('>').append(eventType.getName());
String methodKey = methodKeyBuilder.toString();
if (eventTypesFound.add(methodKey)) {
// Only add if not already found in a sub class
// 只有在子类中没有找到，才会添加到subscriberMethods
ThreadMode threadMode = subscribeAnnotation.threadMode();
subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification) {
// 如果某个方法加了@Subscribe注解，并且不是1个参数，则抛出EventBusException异常
if (method.isAnnotationPresent(Subscribe.class)) {
String methodName = name + "." + method.getName();
throw new EventBusException("@Subscribe method " + methodName +
"must have exactly 1 parameter but has " + parameterTypes.length);
}
}
} else if (strictMethodVerification) {
// 如果某个方法加了@Subscribe注解，并且不是public修饰，则抛出EventBusException异常
if (method.isAnnotationPresent(Subscribe.class)) {
String methodName = name + "." + method.getName();
throw new EventBusException(methodName +
" is a illegal @Subscribe method: must be public, non-static, and non-abstract");
}
}
}
// 会继续查找父类的方法
clazz = clazz.getSuperclass();
}
return subscriberMethods;
}该方法主要作用就是找出subscriberClass类以及subscriberClass的父类中所有的事件处理方法（添加了@Subscribe注解，访问修饰符为public并且只有一个参数）。值得注意的是：如果子类与父类中同时存在了相同事件处理函数，则父类中的不会被添加到subscriberMethods。
好了，查找事件处理函数的过程已经完了，我们继续回到register方法中：
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}找到事件处理函数后，会遍历找到的所有事件处理函数并调用subscribe方法将所有事件处理函数注册到EventBus中。
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
// 获取订阅了某种类型数据的 Subscription 。 使用了 CopyOnWriteArrayList ，这个是线程安全的，
// CopyOnWriteArrayList 会在更新的时候，重新生成一份 copy，其他线程使用的是
// copy，不存在什么线程安全性的问题。
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<Subscription>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//如果已经被注册过了，则抛出EventBusException异常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
// subscriberMethod.method.setAccessible(true);
// Got to synchronize to avoid shifted positions when adding/removing concurrently
// 根据优先级将newSubscription查到合适位置
synchronized (subscriptions) {
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;
}
}
}
//将处理事件类型添加到typesBySubscriber
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<Class<?>>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
// 如果该事件处理方法为粘性事件，即设置了“sticky = true”，则需要调用checkPostStickyEventToSubscription
// 判断是否有粘性事件需要处理，如果需要处理则触发一次事件处理函数
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);
}
}
}如果事件处理函数设置了“sticky = true”，则会调用checkPostStickyEventToSubscription处理粘性事件。
private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
if (stickyEvent != null) {
// If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
// --> Strange corner case, which we don't take care of here.
postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
}
}如果存在粘性事件，则立即调用postToSubscription触发该事件的事件处理函数。postToSubscription函数后面讲post时会讲到。
至此，整个register过程就介绍完了。
总结一下，整个过程分为3步：
查找注册的类中所有的事件处理函数（添加了@Subscribe注解且访问修饰符为public的方法）
将所有事件处理函数注册到EventBus
如果有事件处理函数设置了“sticky = true”，则立即处理该事件
post事件
register过程讲完后，我们知道了EventBus如何找到我们定义好的事件处理函数。有了这些事件处理函数，当post相应事件的时候，EventBus就会触发订阅该事件的处理函数。具体post过程是怎样的呢？我们看看代码：
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
if (!postingState.isPosting) {
// 标识post的线程是否是主线程
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
// 循环处理eventQueue中的每一个event对象
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
// 处理完之后重置postingState的一些标识信息
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}currentPostingThreadState是一个ThreadLocal类型，里面存储了PostingThreadState；
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
/** For ThreadLocal, much faster to set (and get multiple values). */
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<Object>();
boolean isPosting;
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
PostingThreadState包含了一个事件队列eventQueue和一些标志信息。eventQueue存放所有待post的事件对象。
我们再回到post方法，首先会将event对象添加到事件队列eventQueue中。然后判断是否有事件正在post，如果没有则会遍历eventQueue中每一个event对象，并且调用postSingleEvent方法post该事件。
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) {
// 如果允许事件继承，则会调用lookupAllEventTypes查找所有的父类和接口类
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) {
// 如果post的事件没有被注册，则post一个NoSubscriberEvent事件
post(new NoSubscriberEvent(this, event));
}
}
}如果允许事件继承，则会调用lookupAllEventTypes查找所有的父类和接口类。
private List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
synchronized (eventTypesCache) {
List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
if (eventTypes == null) {
eventTypes = new ArrayList<Class<?>>();
Class<?> clazz = eventClass;
while (clazz != null) {
eventTypes.add(clazz);
addInterfaces(eventTypes, clazz.getInterfaces());
clazz = clazz.getSuperclass();
}
eventTypesCache.put(eventClass, eventTypes);
}
return eventTypes;
}
}这个方法很简单，就是查找eventClass类的所有父类和接口，并将其保存到eventTypesCache中，方便下次使用。
我们再回到postSingleEvent方法。不管允不允许事件继承，都会执行postSingleEventForEventType方法post事件。
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;
}在postSingleEventForEventType方法中，会已eventClass为key从subscriptionsByEventType对象中获取Subscription列表。在上面讲register的时候我们已经看到EventBus在register的时候会将Subscription列表存储在subscriptionsByEventType中。接下来会遍历subscriptions列表然后调用postToSubscription方法进行下一步处理。
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case PostThread:
// 如果该事件处理函数没有指定线程模型或者线程模型为PostThread
// 则调用invokeSubscriber在post的线程中执行事件处理函数
invokeSubscriber(subscription, event);
break;
case MainThread:
// 如果该事件处理函数指定的线程模型为MainThread
// 并且当前post的线程为主线程，则调用invokeSubscriber在当前线程（主线程）中执行事件处理函数
// 如果post的线程不是主线程，将使用mainThreadPoster.enqueue该事件处理函数添加到主线程的消息队列中
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
case BackgroundThread:
// 如果该事件处理函数指定的线程模型为BackgroundThread
// 并且当前post的线程为主线程，则调用backgroundPoster.enqueue
// 如果post的线程不是主线程，则调用invokeSubscriber在当前线程（非主线程）中执行事件处理函数
if (isMainThread) {
backgroundPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
case Async:
//添加到异步线程队列中
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}该方法主要是根据register注册的事件处理函数的线程模型在指定的线程中触发事件处理函数。在上一讲EventBus使用详解中已经讲过EventBus的线程模型相关概念了，不明白的可以回去看看。
mainThreadPoster、backgroundPoster和asyncPoster分别是HandlerPoster、BackgroundPoster和AsyncPoster的对象，其中HandlerPoster继承自Handle，BackgroundPoster和AsyncPoster继承自Runnable。
我们主要看看HandlerPoster。
mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);在EventBus的构造函数中，我们看到mainThreadPoster初始化的时候，传入的是Looper.getMainLooper()。所以此Handle是运行在主线程中的。
mainThreadPoster.enqueue方法：
void enqueue(Subscription subscription, Object event) {
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
queue.enqueue(pendingPost);
if (!handlerActive) {
handlerActive = true;
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
}
}
}enqueue方法最终会调用sendMessage方法，所以该Handle的handleMessage方法会被调用。
@Override
public void handleMessage(Message msg) {
boolean rescheduled = false;
try {
long started = SystemClock.uptimeMillis();
while (true) {
PendingPost pendingPost = queue.poll();
if (pendingPost == null) {
synchronized (this) {
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null) {
handlerActive = false;
return;
}
}
}
eventBus.invokeSubscriber(pendingPost);
long timeInMethod = SystemClock.uptimeMillis() - started;
if (timeInMethod >= maxMillisInsideHandleMessage) {
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
rescheduled = true;
return;
}
}
} finally {
handlerActive = rescheduled;
}
}在该方法中，最终还是会调用eventBus.invokeSubscriber调用事件处理函数。
BackgroundPoster和AsyncPoster继承自Runnable，并且会在enqueue方法中调用eventBus.getExecutorService().execute(this);具体run方法大家可以自己去看源码，最终都会调用eventBus.invokeSubscriber方法。我们看看eventBus.invokeSubscriber方法的源码：
void invokeSubscriber(PendingPost pendingPost) {
Object event = pendingPost.event;
Subscription subscription = pendingPost.subscription;
PendingPost.releasePendingPost(pendingPost);
if (subscription.active) {
invokeSubscriber(subscription, event);
}
}该方法会调用invokeSubscriber方法进一步处理：
void invokeSubscriber(Subscription subscription, Object event) {
try {
// 通过反射调用事件处理函数
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}该方法最终会通过反射来调用事件处理函数。至此，整个post过程分析完了。
总结一下整个post过程，大致分为3步：
将事件对象添加到事件队列eventQueue中等待处理
遍历eventQueue队列中的事件对象并调用postSingleEvent处理每个事件
找出订阅过该事件的所有事件处理函数，并在相应的线程中执行该事件处理函数
取消事件注册
上面已经分析了EventBus的register和post过程，这两个过程是EventBus的核心。不需要订阅事件时需要取消事件注册：
/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}取消事件注册很简单，只是将register过程注册到EventBus的事件处理函数移除掉。
到这里，EventBus源码我们已经分析完了，如有不对的地方还望指点。
