EventBus 源码解析

http://blog.csdn.net/yuanzeyao/article/details/38174537

就从EvnetBus的入口开始看吧：EventBus.register

public void register(Object subscriber) {
register(subscriber, DEFAULT_METHOD_NAME, false, 0);
}

其实调用的就是同名函数register，它的四个参数意义分别是：

subscriber：就是要注册的一个订阅者，

methodName:就是订阅者默认的订阅函数名，其实就是“onEvent”

sticky:表示是否是粘性的，一般默认都是false，除非你调用registerSticky方法了

priority：表示事件的优先级，默认就行，

接下来我们就看看这个函数具体干了什么

private synchronized void register(Object subscriber, String methodName, boolean sticky, int priority) {
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass(),
methodName);
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod, sticky, priority);
}
}

通过一个findSubscriberMethods方法找到了一个订阅者中的所有订阅方法，返回一个 List<SubscriberMethod>，进入到findSubscriberMethods看看如何实现的

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, String eventMethodName) {
//通过订阅者类名+"."+"onEvent"创建一个key
String key = subscriberClass.getName() + '.' + eventMethodName;
List<SubscriberMethod> subscriberMethods;
synchronized (methodCache) {
//判断是否有缓存，有缓存直接返回缓存
subscriberMethods = methodCache.get(key);
}
//第一次进来subscriberMethods肯定是Null
if (subscriberMethods != null) {
return subscriberMethods;
}
subscriberMethods = new ArrayList<SubscriberMethod>();
Class<?> clazz = subscriberClass;
HashSet<String> eventTypesFound = new HashSet<String>();
StringBuilder methodKeyBuilder = new StringBuilder();
while (clazz != null) {
String name = clazz.getName();
//过滤掉系统类
if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
// Skip system classes, this just degrades performance
break;
}

// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
//通过反射，获取到订阅者的所有方法
Method[] methods = clazz.getMethods();
for (Method method : methods) {
String methodName = method.getName();
//只找以onEvent开头的方法
if (methodName.startsWith(eventMethodName)) {
int modifiers = method.getModifiers();
//判断订阅者是否是public的,并且是否有修饰符，看来订阅者只能是public的，并且不能被final，static等修饰
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//获得订阅函数的参数
Class<?>[] parameterTypes = method.getParameterTypes();
//看了参数的个数只能是1个
if (parameterTypes.length == 1) {
//获取onEvent后面的部分
String modifierString = methodName.substring(eventMethodName.length());
ThreadMode threadMode;
if (modifierString.length() == 0) {
//订阅函数为onEvnet
//记录线程模型为PostThread,意义就是发布事件和接收事件在同一个线程执行，详细可以参考我对于四个订阅函数不同点分析
threadMode = ThreadMode.PostThread;
} else if (modifierString.equals("MainThread")) {
//对应onEventMainThread
threadMode = ThreadMode.MainThread;
} else if (modifierString.equals("BackgroundThread")) {
//对应onEventBackgrondThread
threadMode = ThreadMode.BackgroundThread;
} else if (modifierString.equals("Async")) {
//对应onEventAsync
threadMode = ThreadMode.Async;
} else {
if (skipMethodVerificationForClasses.containsKey(clazz)) {
continue;
} else {
throw new EventBusException("Illegal onEvent method, check for typos: " + method);
}
}
//获取参数类型，其实就是接收事件的类型
Class<?> eventType = parameterTypes[0];
methodKeyBuilder.setLength(0);
methodKeyBuilder.append(methodName);
methodKeyBuilder.append('>').append(eventType.getName());
String methodKey = methodKeyBuilder.toString();
if (eventTypesFound.add(methodKey)) {
// Only add if not already found in a sub class
//封装一个订阅方法对象，这个对象包含Method对象，threadMode对象，eventType对象
subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
}
}
} else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
+ methodName);
}
}
}
//看了还会遍历父类的订阅函数
clazz = clazz.getSuperclass();
}
//最后加入缓存，第二次使用直接从缓存拿
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "
+ eventMethodName);
} else {
synchronized (methodCache) {
methodCache.put(key, subscriberMethods);
}
return subscriberMethods;
}
}

对于这个方法的讲解都在注释里面了，这里就不在重复叙述了，到了这里我们就找到了一个订阅者的所有的订阅方法

我们回到register方法：

for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod, sticky, priority);
}

对每一个订阅方法，对其调用subscribe方法，进入该方法看看到底干了什么，找到一个订阅方法的所有订阅者

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
subscribed = true;
//从订阅方法中拿到订阅事件的类型
Class<?> eventType = subscriberMethod.eventType;
//通过订阅事件类型，找到所有的订阅（Subscription）,订阅中包含了订阅者，订阅方法
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
//创建一个新的订阅
Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);
//将新建的订阅加入到这个事件类型对应的所有订阅列表
if (subscriptions == null) {
//如果该事件目前没有订阅列表，那么创建并加入该订阅
subscriptions = new CopyOnWriteArrayList<Subscription>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//如果有订阅列表，检查是否已经加入过
for (Subscription subscription : subscriptions) {
if (subscription.equals(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
}

//根据优先级插入订阅
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//将这个订阅事件加入到订阅者的订阅事件列表中
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<Class<?>>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//这个是对粘性事件的，暂时不讨论
if (sticky) {
Object stickyEvent;
synchronized (stickyEvents) {
stickyEvent = stickyEvents.get(eventType);
}
if (stickyEvent != null) {
postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
}
}
}

好了，到这里差不多register方法分析完了，大致流程就是这样的，我们总结一下：

1、找到被注册者中所有的订阅方法。

2、依次遍历订阅方法，找到EventBus中eventType对应的订阅列表，然后根据当前订阅者和订阅方法创建一个新的订阅加入到订阅列表。事件的订阅列表

3、找到EvnetBus中subscriber订阅的事件列表，将eventType加入到这个事件列表。订阅的事件列表

所以对于任何一个订阅者，我们可以找到它的 订阅事件类型列表，通过这个订阅事件类型，可以找到在订阅者中的订阅函数。

register分析完了就分析一下post吧，这个分析完了，EventBus的原理差不多也完了...

public void post(Object event) {
//这个EventBus中只有一个，差不多是个单例吧，具体不用细究
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
//将事件放入队列
eventQueue.add(event);

if (postingState.isPosting) {
return;
} else {
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
while (!eventQueue.isEmpty()) {
//分发事件
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}

post里面没有什么具体逻辑，它的功能主要是调用postSingleEvent完成的，进入到这个函数看看吧

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<? extends Object> eventClass = event.getClass();
//找到eventClass对应的事件，包含父类对应的事件和接口对应的事件
List<Class<?>> eventTypes = findEventTypes(eventClass);
boolean subscriptionFound = false;
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//找到订阅事件对应的订阅，这个是通过register加入的（还记得吗....）
subscriptions = subscriptionsByEventType.get(clazz);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//对每个订阅调用该方法
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
subscriptionFound = true;
}
}
//如果没有订阅发现，那么会Post一个NoSubscriberEvent事件
if (!subscriptionFound) {
Log.d(TAG, "No subscribers registered for event " + eventClass);
if (eventClass != NoSubscriberEvent.class && eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}

这个方法有个核心方法 postToSubscription方法，进入看看吧

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
//第一个参数就是传入的订阅，第二个参数就是对于的分发事件，第三个参数非常关键：是否在主线程
switch (subscription.subscriberMethod.threadMode) {
//这个threadMode是怎么传入的，仔细想想？是不是根据onEvent,onEventMainThread,onEventBackground,onEventAsync决定的？
case PostThread:
//直接在本线程中调用订阅函数
invokeSubscriber(subscription, event);
break;
case MainThread:
if (isMainThread) {
//如果直接在主线程，那么直接在本现场中调用订阅函数
invokeSubscriber(subscription, event);
} else {
//如果不在主线程，那么通过handler实现在主线程中执行，具体我就不跟踪了
mainThreadPoster.enqueue(subscription, event);
}
break;
case BackgroundThread:
if (isMainThread) {
//如果主线程，创建一个runnable丢入线程池中
backgroundPoster.enqueue(subscription, event);
} else {
//如果子线程，则直接调用
invokeSubscriber(subscription, event);
}
break;
case Async:
//不论什么线程，直接丢入线程池
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}