如上图所示，简而言之，EventBus是一个用于Android/Java的发布/订阅事件总线，可以简化组件间的数据传输。它的优势在于解耦事件发送者和接受者，在Activities、Fragments以及后台线程和主线程间的数据传输方面表现不错，避免了复杂和容易出错的依赖关系和生命周期问题。
从源码分析的角度来看，EventBus的数据传输机制，主要是依赖于Java反射机制和自定义注解处理器。对于后者，本章暂且按下不表，集中精力分析EventBus是如何通过反射机制传输数据的。
一、EventBus的实例获取
/** Convenience singleton for apps using a process-wide EventBus instance. */
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;
}public EventBus() {
this(DEFAULT_BUILDER);
}private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();从上述代码可知，EventBus的getDefault()方法就是一个采用了单例模式的同步方法，同时分析EventBus的构造器方法可知，EventBus内部的一些参数属性是由EventBusBuilder这个类对象来负责定义的。
public class EventBusBuilder {
private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();
...
MainThreadSupport mainThreadSupport;
...
MainThreadSupport getMainThreadSupport() {
if (mainThreadSupport != null) {
return mainThreadSupport;
} else if (AndroidLogger.isAndroidLogAvailable()) {
Object looperOrNull = getAndroidMainLooperOrNull();
return looperOrNull == null ? null :
new MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull);
} else {
return null;
}
}
//获取主线程Looper
Object getAndroidMainLooperOrNull() {
try {
return Looper.getMainLooper();
} catch (RuntimeException e) {
// Not really a functional Android (e.g. "Stub!" maven dependencies)
return null;
}
}
public EventBus installDefaultEventBus() {
synchronized (EventBus.class) {
if (EventBus.defaultInstance != null) {
throw new EventBusException("Default instance already exists." +
" It may be only set once before it's used the first time to ensure consistent behavior.");
}
EventBus.defaultInstance = build();
return EventBus.defaultInstance;
}
}
public EventBus build() {
return new EventBus(this);
}
}针对EventBusBuilder类，可以简单过一遍，主要是设置了一些属性和生成一些工具，暂时知道它是配合EventBus的构造器方法就好。接着，咱们将注意力转向register方法。
二、EventBus的register方法流程
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); //获取订阅者方法
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); //存储事件类型和订阅对象们
} else {
...
}
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) { //对粘性事件的处理
...
}
}Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
register方法其实就做了两件事，即获取订阅者的订阅方法、存储订阅者和订阅者方法以及事件类型等等信息。首先通过findSubscriberMethods方法获取订阅者的订阅方法，接着通过遍历，调用subsribe订阅方法，存储订阅信息。
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
...
if (findState.subscriberInfo != null) {
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}由上述代码可知，findSubscriberMethods方法是由findUsingReflectionInSingleClass方法具体实现的，接下来就是重点了。
private void findUsingReflectionInSingleClass(FindState findState) { //利用反射获取订阅者方法
Method[] methods;
try {
methods = findState.clazz.getDeclaredMethods(); //获取本类中的所有方法
} catch (Throwable th) {
...
}
for (Method method : methods) {
int modifiers = method.getModifiers(); //返回此类或接口以整数编码的 Java语言修饰符(PUBLIC、PRIVATE...)
//是否属于PUBLIC方法且不属于抽象、静态等方法
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
Class<?> eventType = parameterTypes[0];
if (findState.checkAdd(method, eventType)) {
ThreadMode threadMode = subscribeAnnotation.threadMode();
//存储订阅方法信息
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
//处于严格方法验证模式以及注解无误，但是缺少方法参数时
} 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");
}
}
}public class SubscriberMethod {
final Method method;
final ThreadMode threadMode;
final Class<?> eventType;
final int priority;
final boolean sticky;
String methodString;
public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky)
{
this.method = method;
this.threadMode = threadMode;
this.eventType = eventType;
this.priority = priority;
this.sticky = sticky;
}
...
}由上述代码可知，通过findSubscriberMethods方法，利用反射机制，将订阅者方法、传递的事件类型、优先级、是否为粘性事件等信息都存储好了。小结一下，其实register方法就是用来获取订阅者的订阅方法、存储订阅信息的。
三、Unregister方法
/** 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) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}unregister方法就是用来取消订阅的，就不再赘述细节了。
四、Post方法
EventBus是通过Post方法进行事件传递（发布）的，在此对post方法源码进行简单分析。
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
...
while (!eventQueue.isEmpty()) { postSingleEvent(eventQueue.remove(0), postingState); } //核心方法
...
}private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) { //是否发布有继承关系类型的事件
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); //核心方法
}
...
}//传递单独事件的方法
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass); //获取订阅对象集合
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
postToSubscription(subscription, event, postingState.isMainThread); //核心方法
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}从Post方法的内部逻辑分析，post方法主要是对eventQueue进行遍历，然后调用postSingleEvent方法进行处理；接着postSingleEvent方法通过遍历事件类型eventTypes，再调用postSingleEventForEventType方法，通过遍历订阅对象集合subscriptions, 将subscription订阅对象, event事件, postingState.isMainThread是否为主线程等参数传递给postToSubscription方法。而postToSubscription方法根据不同的线程模式threadMode对subsription和event进行不同存储操作，最终调用invokeSubscriber方法，即利用反射机制调用处理订阅方法。
//将事件传递给订阅者
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) { //根据订阅者设置的线程模式threadMode进行处理
case POSTING:
invokeSubscriber(subscription, event); //通过反射机制调用处理订阅方法
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);
}
}
void invokeSubscriber(Subscription subscription, Object event) { //利用反射机制调用处理订阅方法
try {
subscription.subscriberMethod.method.invoke(subscription.subscriber, event); //利用反射机制调用处理订阅方法
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}事实上，invokeSubscriber方法就是处理订阅方法被调用的关键代码，是利用java反射机制调用类方法的一个实例。至此，EventBus的普通事件发布处理流程简单地被梳理了一遍。至于粘性等事件的发布流程，本章并未涉及。
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