这篇文章主要是根据我们平时的使用，一步一步的分析EventBus源码流程，因此分为三步：
1、注册订阅者
2、事件发布
3、反注册订阅者
1、register 注册订阅者
在使用eventBus的时候，第一个步骤就是注册订阅者
EventBus.getDefault().register(this);
getDefault方法是一个单例模式的初始化方法，主要就是获取一个实例，源码如下：
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;
}
可以很清楚的看到，这里只是一个DoubleCheck的单例模式，接下来直接看一下构造方法：
/**
* Creates a new EventBus instance; each instance is a separate scope in which events are delivered. To use a
* central bus, consider {@link #getDefault()}.
*/
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;
}
构造方法里面是通过一个Builder模式来对EventBus各项配置进行初始化 在getDefault获取到实例之后，就会调用register方法进行注册，这时候进入register方法看一下注册过程：
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
//获取到订阅者中所有的订阅方法，并储存到list集合中
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
从源码里面看，注册过程只有两步：
a、根据注册时传入的订阅者对象，找到所有的订阅方法；
b、订阅所有的订阅方法
a1)、 首先看一下如何查找到订阅者中所有的订阅方法，即findSubscriberMethods()方法，subscriberMethodFinder是在EventBus构造方法中进行的初始化，进入findSubscriberMethods方法中进行查看：
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//METHOD_CACHE 一个map集合，以订阅者为key，以订阅方法的list集合为value进行缓存
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
//如果当前的对象已经被缓存，则直接获取返回
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {
// 如果忽略索引，就根据反射来获取
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//否则使用索引
subscriberMethods = findUsingInfo(subscriberClass);
}
//如果该订阅者中没有订阅方法，此处会抛出一个异常，提醒你该订阅者和他的父类里面都没有订阅方法（public 修饰并且@Subscribe 进行注解）
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//如果有订阅方法，则把该订阅者和订阅方法进行缓存，并返回订阅方法的list集合
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
a1.1)、 findSubscriberMethods 里面的逻辑已经很清晰了，现在看一下 findUsingInfo（）和findUsingReflection（），一般使用的时候，并没有自定义进行配置，所以一般都是使用索引进行查找
a1.1.1） 这里就先分析一下findUsingInfo（）方法，反射的稍后再看：
//忽略索引时，查找方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
// 创建并初始化FindState对象 FindState是订阅者的一个辅助类，用于获取到订阅方法
FindState findState = prepareFindState();
//关联订阅者
findState.initForSubscriber(subscriberClass);
//此处通过循环，获取到订阅者父类中的订阅方法
while (findState.clazz != null) {
//获取到订阅者的信息
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
//如果可以找到订阅者信息，将订阅方法缓存到 subscriberMethods 中
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
//如果找不到订阅者的信息，则通过反射方法进行获取
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
FindState类是一个辅助类，用来辅助订阅者获取订阅方法，代码比较简单，篇幅原因此处就不在进行源码解析，只要清楚这个类主要用途即可
prepareFindState() 方法则是用来创建并初始化FindState的方法，同时坐着也通过一个缓存池进行了优化
从上面源码中可以看到，在可以直接查找到订阅信息的情况比较简单，直接将查找到的信息进行缓存即可，下面分析一下找不到订阅信息的时候，通过反射方法进行查找：
//通过反射方式，获取到订阅者中所有的订阅方法
private void findUsingReflectionInSingleClass(FindState findState) {
// 1 获取订阅者内所有方法
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
// 2 遍历所有方法，找到所有的订阅方法，并将所有的订阅方法进行缓存到 findState 中
for (Method method : methods) {
int modifiers = method.getModifiers();
//如果是非静态、非抽象、public 修饰的
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?>[] parameterTypes = method.getParameterTypes();
//只有一个参数
if (parameterTypes.length == 1) {
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
// Subscribe 进行注解
if (subscribeAnnotation != null) {
Class<?> eventType = parameterTypes[0];
if (findState.checkAdd(method, eventType)) {
ThreadMode threadMode = subscribeAnnotation.threadMode();
//符合条件的进行缓存
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException("@Subscribe method " + methodName +
"must have exactly 1 parameter but has " + parameterTypes.length);
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException(methodName +
" is a illegal @Subscribe method: must be public, non-static, and non-abstract");
}
}
}
认真看一下每一个步骤，可以发现其实就是通过反射查找到符合条件的订阅方法，然后将订阅方法缓存到findState中，同时对注解的订阅方法进行校验，给出提示，在查找到所有的订阅方法之后，代码执行到了，a1.1.1) findUsingReflectionInSingleClass 执行完毕，然后 findUsingInfo（）在循环查找并缓存订阅者中所有的订阅方法，至此， 在a1.1) findUsingInfo 方法也执行完毕，成功找到了所有的订阅方法，a1)、中忽略索引的情况已经执行完毕 查找完成之后，我们在回过头看一下a1)、findSubscriberMethods 方法中不忽略 索引的方法 findUsingReflection()
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
这段代码就非常简单了，上面已经分析过了，直接使用反射查找到所有的方法，并进行缓存；
在查找到所有的订阅方法之后，将所有的方法进行缓存到 METHOD_CACHE ，至此，注册第一步就完成了；
无论使用哪一种方法，到这里都已经找到所有的订阅方法了，在再次返回到 register（）方法中：
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
这段代码通过循环，将所有的订阅方法和订阅者关联起来，看一下subscribe 方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
//创建订阅者和订阅方法的封装对象
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//由于所有的订阅方法都只有一个参数，并且subscriptionsByEventType 是以 订阅方法的参数的class为key，Subscription集合为value的
// //的map集合，所以，根据eventType 可以找到所有的含有此参数的订阅方法所在的订阅者
// 根据订阅方法的参数类型，查找到所有包含有该Event 的订阅者，Event 即为订阅方法的参数
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);
}
}
//根据优先级，将当前的订阅者插入订阅集合
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;
}
}
//根据订阅者，查找订阅者里面所有的Event 类型，并储存
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//对于粘性事件，则立刻执行
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);
}
}
}
至此，订阅者和订阅事件进行了关联，并且进行了缓存，register（）完成
2、事件发布
事件发布一般通过以下两种方法进行发布，postSticky 为发送粘性事件
EventBus.getDefault().post(new Event("我是测试event数据"));
EventBus.getDefault().postSticky(new Event("我是测试event数据"));
首先先分析一下post 事件
public void post(Object event) {
//获取到ThreadLocal中储存的数据，并将当前post的数据添加到队列中
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
//如果当前没有在post 事件
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
//通过循环取出队列中所有数据，并进行订阅
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;
//此处仅判断是否允许继承，如果允许就会执行父类中的订阅方法，不作为重点关注内容
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) {
logger.log(Level.FINE, "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;
//subscriptionsByEventType，key 为订阅方法的参数类型，value为订阅者和订阅方法封装的list集合
//根据register（）的源码，已经知道在注册的时候，会将订阅者和订阅方法缓存到subscriptionsByEventType里面
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}
//如果post的Event有订阅者接收，循环进行订阅
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;
}
// 订阅方法的执行方法，会根据threadMode将订阅方法在不同的线程进行执行
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 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);
}
}
post方法本身并不复杂，重点关注一下postToSubscription（）方法，在此方法中，根据不同的线程进行处理，执行订阅方法，到这里，事件发布就完成了，订阅方法也已经被执行了，完成了事件的分发
3、反注册订阅者
反注册就非常简单了，仅仅是将缓存的订阅者及其订阅方法移除即可
public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions != null) {
int size = subscriptions.size();
for (int i = 0; i < size; i++) {
Subscription subscription = subscriptions.get(i);
if (subscription.subscriber == subscriber) {
subscription.active = false;
subscriptions.remove(i);
i--;
size--;
}
}
}
}
从代码中可以看出，反注册的时候将注册时候缓存在 typesBySubscriber 和subscriptionsByEventType 中的数据移除，完成了反注册
