小知识
观察者模式 ：定义对象间一种一对多的依赖关系，使得每当一个对象改变状态，则所有依赖它的对象都会得到通知并自动更新。三个角色：
1.Subject：就是“被观察”的角色，它将所有观察者对象的引用保存在一个集合中。
2.Observer：是抽象的“观察”角色，它定义了一个更新接口，使得在被观察者状态发生改变时通知自己。
3.ConcreteObserver：具体的观察者。
承接上一篇基本用法的例子来理解这三个角色，其实就是Subject（被观察者）是考试中班里的那个优生，订阅的方法就是提前和人家打好招呼，写完了答案发过来。ConcreteObserve(观察者)就是提前和大神说好的我们这些考试等答案的人。大神做完一条短信发给众多同学，这就是一个发布过程。
这个设计模式也是为了实现上一篇的小知识 解耦。
书归正传。。。
原理图
这是EventBus的官方原理图，抄一下
image.png
这就是个流程图 流程图也可以大概看出 这里使用的是观察者模式。
Publisher就是那个大神，我们就是那些Subscriber。这些Event就是答案。
看下源码
注解 @Subscribe
package org.greenrobot.eventbus;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Subscribe {
ThreadMode threadMode() default ThreadMode.POSTING;
/**
* If true, delivers the most recent sticky event (posted with
* {@link EventBus#postSticky(Object)}) to this subscriber (if event available).
*/
boolean sticky() default false;
/** Subscriber priority to influence the order of event delivery.
* Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before
* others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of
* delivery among subscribers with different {@link ThreadMode}s! */
int priority() default 0;
}
ThreadMode threadMode() default ThreadMode.POSTING;
很直接 设置默认的模式为POSTING，这个上篇的例子讲过，就是接收答案的那些人和大神在一个教室（接收者和发布者在一个线程中）。
boolean sticky() default false;
设置粘性事件开关默认为false，就是我默认还没有注册的情况下不先接收消息。
int priority() default 0;
设置默认优先级为0，这个数越大优先级越高，优先级高的在同一模式下会优先收到消息。
注册 EventBus.getDefault().register(this);
这里先看下EventBus.getDefault()
/** 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;
}
我这少儿英语水平看最上面的英文也猜的出来，这是一个线程安全的单例模式。
其实没什么特别的，跟我们平时写一个单例一样。只是里面的
instance = EventBus.defaultInstance = new EventBus();
需要看下:
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;
}
这个也眼熟，其实就是初始化一些设置参数，这里使用的是构建者模式 。就和大神问你怎么传答案一样。是短信还是小抄，只要选择题还是只要最后几道大题，要能及格还是考高一点。一切满足客户需求。你不设置就是一个默认套餐，你要做一些私人定制 那就自定义:
EventBus.builder()
.eventInheritance(false)
.logSubscriberExceptions(false)
.build()
.register(this);
都商量好了那就定下来了
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
Class<?> subscriberClass = subscriber.getClass();
这个好理解 拿到注册我的那个类。相当于找到一份学生名单
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
这个里面 SubscriberMethod 这个家伙里装的就是那些私人定制(是否黏性事件啦，线程模式啦，优先级啦)，没有私人定制就是默认值。
然后 findSubscriberMethods 方法就是在注册的那个类里寻找
Subscribe注解、有public修饰符、一个参数的方法。相当于在这份学生名单中找到需要答案的那部分学生。
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
最后遍历这些订阅了的方法 完成注册。相当于确认这些人，准备好写完答案发给他们。
我们看下findSubscriberMethods方法 看看具体是怎么找的
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;
}
}
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
这里我们看下 METHOD_CACHE
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
就是一个存储订阅者和其中订阅方法的类,这样做应该是为了重复查找时的效率。这里就相当于将找到班级里对应需要答案的人,记下来.
我们先看
if (ignoreGeneratedIndex) {subscriberMethods =
findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
ignoreGeneratedIndex在这里时一个注解管理器 这个下次说
findUsingInfo
findUsingInfo()方法会在当前要注册的类以及其父类中查找订阅事件的方法，这里出现了一个FindState类，它是SubscriberMethodFinder的内部类，用来辅助查找订阅事件的方法，具体的查找过程在findUsingReflectionInSingleClass()方法里，它主要通过反射来查找订阅事件的方法。
private void findUsingReflectionInSingleClass(FindState findState) {
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;
}
// 循环遍历当前类的方法，筛选出符合条件的
for (Method method : methods) {
// 获得方法的修饰符
int modifiers = method.getModifiers();
// 如果是public类型，但非abstract、static等
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
// 获得当前方法所有参数的类型
Class<?>[] parameterTypes = method.getParameterTypes();
// 如果当前方法只有一个参数
if (parameterTypes.length == 1) {
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
// 如果当前方法使用了Subscribe注解
if (subscribeAnnotation != null) {
// 得到该参数的类型
Class<?> eventType = parameterTypes[0];
// checkAdd()方法用来判断FindState的anyMethodByEventType map是否已经添加过以当前eventType为key的键值对，没添加过则返回true
if (findState.checkAdd(method, eventType)) {
// 得到Subscribe注解的threadMode属性值，即线程模式
ThreadMode threadMode = subscribeAnnotation.threadMode();
// 创建一个SubscriberMethod对象，并添加到subscriberMethods集合
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");
}
}
}
再来看
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;
}
再接着就是判断,没人订阅的时候就抛出一个异常,有人订阅的时候就把这个人从名单中记到自己小本本上(METHOD_CACHE缓存中)
注册方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
// 得到当前订阅了事件的方法的参数类型
Class<?> eventType = subscriberMethod.eventType;
// Subscription类保存了要注册的类对象以及当前的subscriberMethod
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
// subscriptionsByEventType是一个HashMap，保存了以eventType为key,Subscription对象集合为value的键值对
// 先查找subscriptionsByEventType是否存在以当前eventType为key的值
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
// 如果不存在，则创建一个subscriptions，并保存到subscriptionsByEventType
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);
}
}
// 添加上边创建的newSubscription对象到subscriptions中
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;
}
}
// typesBySubscribere也是一个HashMap，保存了以当前要注册类的对象为key，注册类中订阅事件的方法的参数类型的集合为value的键值对
// 查找是否存在对应的参数类型集合
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
// 不存在则创建一个subscribedEvents，并保存到typesBySubscriber
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
// 保存当前订阅了事件的方法的参数类型
subscribedEvents.add(eventType);
// 粘性事件相关的，后边具体分析
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);
}
}
}
subscribe()方法主要是得到了subscriptionsByEventType、typesBySubscriber两个 HashMap。其中，发送事件的时候要用到subscriptionsByEventType来完成事件的处理。当取消 EventBus 注册的时候要用到typesBySubscriber、subscriptionsByEventType，完成相关资源的释放。
网上这样说的居多,其实大白话就是 :
订阅的过程是把之后要做的需要的事准备好,比如发送需要的 按照优先级排列顺序,这样发送的时候按照优先级好发送 把粘性事件的处理下 把取消注册的放到typesBySubscriber中,方便之后取消注册时处理typesBySubscriber
取消注册
EventBus.getDefault().unregister(this);
/** 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());
}
}
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
得到当前注册类对象 对应的 订阅事件方法的参数类型 的集合;
typesBySubscriber这个上面注册方法里刚提到说过,初始化了然后再解除订阅的时候要用,这里用到了
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
遍历参数类型集合，释放之前缓存的当前类中的Subscription
typesBySubscriber.remove(subscriber);
然后删除这个key对应的键值对儿
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
字面意思也看得出来 这个报错是之前没注册无法删除
unsubscribeByEventType(subscriber, eventType);
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
// 得到当前参数类型对应的Subscription集合
List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions != null) {
int size = subscriptions.size();
// 遍历Subscription集合
for (int i = 0; i < size; i++) {
Subscription subscription = subscriptions.get(i);
// 如果当前subscription对象对应的注册类对象 和 要取消注册的注册类对象相同，则删除当前subscription对象
if (subscription.subscriber == subscriber) {
subscription.active = false;
subscriptions.remove(i);
i--;
size--;
}
}
}
}
删除的目的就是
释放typesBySubscriber、subscriptionsByEventType中缓存的资源。
发送事件
EventBus.getDefault().post("Hello AZ!")
post发放发送时间 源码
/** Posts the given event to the event bus. */
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;
}
}
}
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
这里的PostingThreadState类保存了事件队列和线程模式等信息 你是以哪种线程模式处理 并且新建了一个队列 把事件添加了进去
if (!postingState.isPosting) {
// 是否为主线程
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
// 遍历事件队列
while (!eventQueue.isEmpty()) {
// 发送单个事件
// eventQueue.remove(0)，从事件队列移除事件
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
这里就是就是判断线程,然后遍历事件 把事件通过postSingleEvent方法发送出去
其实post发放是把消息添加到了list中 然后遍历之后通过
postSingleEvent方法处理。
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));
}
}
}
其实这里也没处理 而是判断了下这个类的父类有没有接收方法,决定是否向上传递事件 然后把事件交给了postSingleEventForEventType去处理
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;
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;
}
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}
这里是通过事件类型这个key去map里找对应的集合
postingState.event = event;
postingState.subscription = subscription;
保存下来subscription 和event 之后 交给postToSubscription做最终处理
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);
}
}
switch (subscription.subscriberMethod.threadMode)
这里首先判断了一下线程模式是哪种 然后
case POSTING:
invokeSubscriber(subscription, event);
break;
如果是POSTING模式 则发送的在哪个线程 就在那个线程下处理
case MAIN: if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
如果是MAIN模式 如果在主线程就在主线程处理 如果发送者在子线程 就将事件放入消息队列 通过线程切换到主线程处理
case BACKGROUND:
if (isMainThread) {
backgroundPoster.enqueue(subscription, event);
} else {
// 如果在子线程发送事件，则直接在发送事件的线程通过反射处理事件
invokeSubscriber(subscription, event);
}
break;
如果是BACKGROUND模式 如果发送者在主线程 则入消息队列 在子线程处理 如果发送者在子线程 则直接处理
case ASYNC:
asyncPoster.enqueue(subscription, event);
break;
如果是ASYNC模式 则将消息如消息队列 通过线程池处理
这里的postToSubscription()方法就是根据发送者的线程和当线程模式来处理事件,处理方式有两种,一种是invokeSubscriber()方法 ,一种是进线程池处理
进线程池处理就是把事件包装成了BackgroundPoster对象 然后BackgroundPoster类内就是一个线程池实现 然后去处理
final class BackgroundPoster implements Runnable, Poster {
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) {
eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
}
} finally {
executorRunning = false;
}
}
}
他继承了Runnable接口 然后run里实现处理方法
至于invokeSubscriber()方法
void invokeSubscriber(Subscription subscription, Object event) {
try {
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
没啥说的 就是用反射来执行订阅事件的方法，消息就传到了方法中。
粘性事件
EventBus.getDefault().postSticky("Hello AZ!");
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);
}
这个方法将事件类型和对应事件保存到stickyEvents中，方便后续使用；然后执行post(event)继续发送事件，这个post()方法就是之前发送的post()方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
......
......
......
// 如果当前订阅事件的方法的Subscribe注解的sticky属性为true，即该方法可接受粘性事件
if (subscriberMethod.sticky) {
// 默认为true，表示是否向上查找事件的父类
if (eventInheritance) {
// stickyEvents就是发送粘性事件时，保存了事件类型和对应事件
Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
for (Map.Entry<Class<?>, Object> entry : entries) {
Class<?> candidateEventType = entry.getKey();
// 如果candidateEventType是eventType的子类或
if (eventType.isAssignableFrom(candidateEventType)) {
// 获得对应的事件
Object stickyEvent = entry.getValue();
// 处理粘性事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
之前的post()方法中 有执行粘性事件的
可以看到，处理粘性事件就是在 EventBus 注册时，遍历stickyEvents，如果当前要注册的事件订阅方法是粘性的，并且该方法接收的事件类型和stickyEvents中某个事件类型相同或者是其父类，则取出stickyEvents中对应事件类型的具体事件，交给checkPostStickyEventToSubscription()处理
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());
}
}
这个方法中 只是判断了一下粘性事件不为空的话,就调用postToSubscription()方法 postToSubscription()方法就是上面讲过的postToSubscription()方法 最终会判断线程模式将消息发送到接受方法里
这种方式实现了在注册之前 发送了粘性事件
最后 附上原理图
EventBus.png
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