一：使用步骤
1、eventbus3.1.1版本依赖：
implementation 'org.greenrobot:eventbus:3.1.1'
2、在 Activity 或 Fragment 中进行注册：
EventBus.getDefault().register(this);
3、在注册的 Activity 或 Fragment 中进行接收，需要写一个接收方法：
3.1 、注解上使用默认参数接收方法
@Subscribe
public void receive(String data) {
// TODO: Create by Ysw 2019/7/18 进行相关 data 的数据处理
}
3.2、注解上进行自定义参数值接收方法
@Subscribe(threadMode = ThreadMode.MAIN, sticky = true, priority = 1)
public void receive(Object data) {
// TODO: Create by Ysw 2019/7/18 进行相关 data 的数据处理
}
4、在注册 EventBus 的 Activity 或 Fragment 的 onDestroy 生命周期方法中进行注销
@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}
5、在需要发出通知的地方进行调用，可以是 Fragment Activity 或者别的工具类中，EventBus 目前不支持跨进程通讯，需保持在同一个进程中进行一个完整的操作。
private void sendMessage(){
EventBus.getDefault().post("这个是我发的测试消息");
EventBus.getDefault().post(new Friend("Ysw",18));
}
二：我们从注册开始着手解析。
我们进入 EventBus 的 register() 方法中，看看其做了什么事情
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
register 方法中做了三件事，第一通过反射拿到 class 对象，二，通过这个 class 对象去遍历这个类里面的方法并返回一个呗 @Subscribe 所注释了的方法集合。三，对这个方法集合进行遍历并做相应的操作。
我们来看他是怎么进行类里面方法遍历并返回相关方法集合的，我们进入到 subscriberMethodFinder 中的subscriberMethodFinder() 方法:
// 这是一个 Map 容器，key 值是一个 class 类，value 是一个方法集合
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
// 第一次进来，容器肯定为空，找不到相关的 方法集合
if (subscriberMethods != null) {
return subscriberMethods;
}
// ignoreGeneratedIndex 默认值是 false
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;
}
}
进入 findUsingInfo(Class<?> subscriberClass) 方法：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
// findState.subscriberInfo 没有进行赋值，进入 findUsingReflectionInSingleClass(findState) 方法
findState.subscriberInfo = getSubscriberInfo(findState);
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);
}
findUsingReflectionInSingleClass(findState) 方法
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
// 通过反射 获取这个类中的所有方法集合
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;
}
// 遍历这个集合，获取被 Subscribe 所注释的方法并添加到 subscriberMethods 中
for (Method method : methods) {
int modifiers = method.getModifiers();
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();
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");
}
}
}
getMethodsAndRelease(FindState findState) 方法 直接将 findState.subscriberMethods 方法集合返回
private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
findState.recycle();
synchronized (FIND_STATE_POOL) {
for (int i = 0; i < POOL_SIZE; i++) {
if (FIND_STATE_POOL[i] == null) {
FIND_STATE_POOL[i] = findState;
break;
}
}
}
return subscriberMethods;
}
最后将 subscriberMethods 添加到 METHOD_CACHE map容器中并返回
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
// 代码省略
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
// 代码省略
}
回到 EventBus 的 subscribe(subscriber, subscriberMethod) 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) {
// 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);
}
}
}
在subscriptionsByEventType去查找一个CopyOnWriteArrayList<Subscription> ，如果没有则创建一个新的CopyOnWriteArrayList；
然后将这个CopyOnWriteArrayList放入subscriptionsByEventType中，这里的subscriptionsByEventType是一个Map，key为eventType，value为CopyOnWriteArrayList<Subscription>，这个Map非常重要，后续还会用到它；
接下来，就是添加newSubscription，它属于Subscription类，里面包含着subscriber和subscriberMethod等信息，同时这里有一个优先级的判断，说明它是按照优先级添加的。优先级越高，会插到在当前List靠前面的位置；
typesBySubscriber这个类也是一个Map，key为subscriber，value为subscribedEvents，即所有的eventType列表，这个类我找了一下，发现在EventBus#isRegister()方法中有用到，应该是用来判断这个Subscriber是否已被注册过。然后将当前的eventType添加到subscribedEvents中；
最后，判断是否是sticky。如果是sticky事件的话，到最后会调用checkPostStickyEventToSubscription()方法。
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, isMainThread());
}
}
接着看是如何让发送消息的，post() 方法
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;
}
}
}
进入 postSingleEvent(eventQueue.remove(0), postingState)方法
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) {
logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
进入 postSingleEventForEventType()方法
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;
}
最后调用 postToSubscription() 方法
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);
}
}
最后根据线程类别进行调用
POSTING：执行invokeSubscriber()方法，就是直接反射调用
MAIN：首先去判断当前是否在UI线程，如果是的话则直接反射调用，否则调用mainThreadPoster#enqueue()，即把当前的方法加入到队列之中，然后通过handler去发送一个消息，在handler的handleMessage中去执行方法。具体逻辑在HandlerPoster.java中；
MAIN_ORDERED：与上面逻辑类似，顺序执行我们的方法。
BACKGROUND：判断当前是否在UI线程，如果不是的话直接反射调用，是的话通过。backgroundPoster.enqueue()将方法加入到后台的一个队列，最后通过线程池去执行。
ASYNC：与BACKGROUND的逻辑类似，将任务加入到后台的一个队列，最终由Eventbus中的一个线程池去调用，这里的线程池与BACKGROUND逻辑中的线程池用的是同一个
有参考这位大神的分析并结合自己的理解，给自己做个笔记，感谢大神的分享 EventBus 3.1.1 源码解析
