上篇分析了EventBus查找所有的订阅集合List<SubscriberMethod>的过程，这篇继续往下走，分析一下订阅的流程：
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
//通过类名也就是activity的类查找List<SubscriberMethod>集合
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
//开始循环订阅每个方法
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取订阅方法的入参类型，也即是事件类名
Class<?> eventType = subscriberMethod.eventType;
//又搞了个封装类Subscription，封装了事件订阅所在的类名和订阅的方法信息
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//subscriptionsByEventType的格式是Map<Class<?>, CopyOnWriteArrayList<Subscription>>
//缓存用的map，存放的是事件类型和Subscription集合的键值对
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//为空则新建了一个空的subscriptions，并把当前的事件类型和这个subscriptions存起来
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//同一个方法订阅了两次，报错
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//根据优先级排序装入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;
}
}
//又是个缓存map，typesBySubscriber的格式是Map<Object, List<Class<?>>>，存放的是订阅所在类名
//和订阅事件类型集合的键值对，简单来说就是通过这个map可以方便拿到该类下所有的订阅事件类型
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
//放入缓存
subscribedEvents.add(eventType);
//省略干扰代码。。。
}
看下来似乎就是把找到的订阅事件信息整理放在了两个map上：
//存放的是事件类型和Subscription集合的键值对
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//存放的是订阅所在类名和订阅事件类型集合的键值对
private final Map<Object, List<Class<?>>> typesBySubscriber;
猜测一波，后续post发送事件后肯定就是对这两个map进行操作了
接下来看看post(event)方法：
//推送流程1
public void post(Object event) {
//currentPostingThreadState是一个ThreadLocal类型，这是java提供的一个线程内部的存储类,
// 可以在指定线程内存储数据,数据存储以后,只有指定线程可以得到存储数据，这里不详细分析
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
//把事件放在队列里
eventQueue.add(event);
//判断一下状态
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//一个一个顺序推送
while (!eventQueue.isEmpty()) {
//重点代码在这里，接着看这个方法
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
//推送流程2
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//eventInheritance是初始化eventBus的一个值，大意是是否一并通知该订阅事件的父类或实现的接口类的订阅事件，默认是true
//比如 A extends B implements C 发布者post(A),那么找订阅者的时候不仅要找订阅了事件A的订阅者
// 还要找订阅了B和C的订阅者
if (eventInheritance) {
//把订阅事件父类的订阅事件信息也找出来
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
//|=是个或运算，自己和等式右边做或运算
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
//简单点理解可以看这里，上面的分支也最终走到了postSingleEventForEventType()方法
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));
}
}
}
//推送流程3
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//开始用到subscriptionsByEventType这个map了，通过事件类型取出所有订阅了该事件的方法信息集合
subscriptions = subscriptionsByEventType.get(eventClass);
}
//简单判空处理
if (subscriptions != null && !subscriptions.isEmpty()) {
//循环遍历这个list
for (Subscription subscription : subscriptions) {
//在线程存储对象里记录下发送的事件对象和订阅了该事件的信息
postingState.event = event;
postingState.subscription = subscription;
boolean aborted;
try {
//最终调用到这个方法，从方法名也可以看出是推送到订阅方的意思，继续走下去
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;
}
//推送流程4 按照订阅时设置的调用线程分别处理
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);
}
}
最后post事件后会到invokeSubScriber()方法,enqueue()方法只是做个异步调用最后还是会走到invokeSubScriber()，看方法名就知道是用到了反射去执行订阅方法：
void invokeSubscriber(Subscription subscription, Object event) {
try {
//用subscription保存到的方法实例invoke()一下，最终完成了用事件推送来达到订阅方相应的目的
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
至此，从订阅到post事件的流程讲完了，主要核心是缓存的设计，APT和反射的运用。最后，还有一个map在post过程中好像没被用到就是:
//存放的是订阅所在类名和订阅事件类型集合的键值对
private final Map<Object, List<Class<?>>> typesBySubscriber
看他的键值对设计想必应该是用在判断/解除注册里可以方便的查找，看了看源码发现果然是：
//用来判断该类是否注册过
public synchronized boolean isRegistered(Object subscriber) {
return typesBySubscriber.containsKey(subscriber);
}
//解除注册
public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
//相应的在subscriptionsByEventType这个map里也会移除调订阅信息
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
OK，分析完成！看完源码后只是把主干流程梳理了一下，EventBus作为优秀的开源项目，其设计思想还是很值得反复研究的，多看看这种成熟优秀的项目对于自己的代码能力也是有很大的提升！
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