EventBus底层实现原理

2种思路去读EventBus源码

1.注册 EventBus.getDefault().register(this);

2.发送消息EventBus.getDefault().post();

根据1追到了findSubscriberMethodsWithReflection 根据反射查找订阅者，代码如下里面的内容不解释了，返回的是一个订阅方法的集合。

private List<SubscriberMethod> findSubscriberMethodsWithReflection(Class<?> subscriberClass) {List<SubscriberMethod> subscriberMethods = new ArrayList<SubscriberMethod>();Class<?> clazz = subscriberClass;HashSet<String> eventTypesFound = new HashSet<String>();StringBuilder methodKeyBuilder = new StringBuilder();while (clazz != null) {String name = clazz.getName();if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {// Skip system classes, this just degrades performancebreak;}// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)Method[] methods = clazz.getDeclaredMethods();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) {String methodName = method.getName();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 classThreadMode threadMode = subscribeAnnotation.threadMode();subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,subscribeAnnotation.priority(), subscribeAnnotation.sticky()));}}} else if (strictMethodVerification) {if (method.isAnnotationPresent(Subscribe.class)) {String methodName = name + "." + method.getName();throw new EventBusException("@Subscribe method " + methodName +"must have exactly 1 parameter but has " + parameterTypes.length);}}} else if (strictMethodVerification) {if (method.isAnnotationPresent(Subscribe.class)) {String methodName = name + "." + method.getName();throw new EventBusException(methodName +" is a illegal @Subscribe method: must be public, non-static, and non-abstract");}}}clazz = clazz.getSuperclass();}return subscriberMethods;
}

然后看调用这个方法的地方

public void register(Object subscriber) {Class<?> subscriberClass = subscriber.getClass();// @Subscribe in anonymous classes is invisible to annotation processing, always fall back to reflectionboolean forceReflection = subscriberClass.isAnonymousClass();List<SubscriberMethod> subscriberMethods =subscriberMethodFinder.findSubscriberMethods(subscriberClass, forceReflection);for (SubscriberMethod subscriberMethod : subscriberMethods) {subscribe(subscriber, subscriberMethod);}
}

接下来咱们继续查看 subscribe(subscriber, subscriberMethod);，如下

划线部分核心代码，到此为止EventBus.getDefault().register(this);大概干了一这样一件事情

通过反射得到观察者（订阅者）的方法，事件和所在的线程

然后将这些参数组装后放入Map中，其中key为事件类型

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {Class<?> eventType = subscriberMethod.eventType;CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);Subscription newSubscription = new Subscription(subscriber, subscriberMethod);if (subscriptions == null) {subscriptions = new CopyOnWriteArrayList<Subscription>();subscriptionsByEventType.put(eventType, subscriptions);} else {if (subscriptions.contains(newSubscription)) {throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "+ eventType);}}// Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)// subscriberMethod.method.setAccessible(true);// Got to synchronize to avoid shifted positions when adding/removing concurrentlysynchronized (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;}}}List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);if (subscribedEvents == null) {subscribedEvents = new ArrayList<Class<?>>();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);}}
}

-----------------------------------------------------------------------------------------------------------------------------

接下来看一下发送消息是怎么实现的

/** 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 = Looper.getMainLooper() == Looper.myLooper();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;}}
}

接着看画线部分方法，然后此方法中找到调用的postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass)方法。划线部分是核心代码，头一个划线部分是从map中根据发送事件类型得到的一个类。这个类存储的是订阅的方法，所在的线程，不出意外的话下面应该用的是反射调用订阅的方法。拭目以待

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;
}

接下来咱们看一下第二个划线部分的代码。里面基本全是反射

/**

*subscription 里面存储的是要调用方法的名字等信息

*event 此事件

*isMainThread boolean 当前发送消息线程是不是UI主线程

private void postToSubscription(Subscription subscription, Object event, boolean。isMainThread) {switch (subscription.subscriberMethod.threadMode) {  //当时订阅方法所在的线程case PostThread:   //非主线程invokeSubscriber(subscription, event);break;case MainThread:  //ui主线程if (isMainThread) { //在主线程直接更新invokeSubscriber(subscription, event);} else { //通过handler发送消息，防止在自线程中更新UI的误操作mainThreadPoster.enqueue(subscription, event);}break;case BackgroundThread:if (isMainThread) { //主线程创建runable丢入线程池backgroundPoster.enqueue(subscription, event);} else { //直接调用invokeSubscriber(subscription, event);}break;case Async:  //直接丢入线程池asyncPoster.enqueue(subscription, event);break;default:throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);}
}

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