前言
EventBus是一种发布-订阅事件总线。它有三大要素：Event:事件、Publisher:发布者,可以在任意线程发布事件、Subscrible:订阅者。
下面就让从注册开始慢慢揭开EventBus的神秘面纱。
EventBus.getDefault().register(this);
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
追踪getDefault方法，我们可以清楚的看到它是通过双重判空创建了一个EventBus对象，我们继续往下追
public EventBus() {
this(DEFAULT_BUILDER);
}
EventBus(EventBusBuilder builder) {
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
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;
}
在初始化中创建了部分的参数，我们下面会着重分析下subscriptionsByEventType、typesBySubscriber 、stickyEvents 。简单介绍下它们：
subscriptionsByEventType:以event(即事件类)为key，以订阅列表(Subscription)为value，事件发送之后，在这里寻找订阅者,而Subscription又是一个CopyOnWriteArrayList，这是一个线程安全的容器。你们看会好奇，Subscription到底是什么？Subscription其实是一个封装类，封装了订阅者、订阅方法这两个类。源码中会一一说明。
typesBySubscriber：以订阅者类为key，以event事件类为value，在进行register或unregister操作的时候，会操作这个map。
stickyEvents:保存的是粘性事件。
追到这里我们的EventBus的初始化就结束了，接着我们分析register。
public void register(Object subscriber) {
//获取当前注册类的类名
Class<?> subscriberClass = subscriber.getClass();
//通过类名查找该类下订阅的方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
从代码中不难看出它是先获取了订阅者类的class，接着交给SubscriberMethodFinder的findSubscriberMethods()方法处理，返回结果保存在List中，由此可推测通过上面的方法把订阅方法找出来了，并保存在集合中，那么我们直接看SubscriberMethodFinder的findSubscriberMethods()这个方法。
findSubscriberMethods
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//这里判断条件用到了ignoreGeneratedIndex，从初始化那里我们得知默认值是false
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//重点，默认执行的是findUsingInfo
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;
}
}
public class EventBusBuilder {
private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();
boolean logSubscriberExceptions = true;
boolean logNoSubscriberMessages = true;
boolean sendSubscriberExceptionEvent = true;
boolean sendNoSubscriberEvent = true;
boolean throwSubscriberException;
boolean eventInheritance = true;
boolean ignoreGeneratedIndex;
boolean strictMethodVerification;
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
List<Class<?>> skipMethodVerificationForClasses;
List<SubscriberInfoIndex> subscriberInfoIndexes;
...省略
}
继续往下追，
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
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);
}
用到了FindState这个内部类来保存订阅者类的信息，我们看看它的内部结构：
static class FindState {
final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
final Map<Class, Object> anyMethodByEventType = new HashMap<>();
final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
final StringBuilder methodKeyBuilder = new StringBuilder(128);
Class<?> subscriberClass;
Class<?> clazz;
boolean skipSuperClasses;
SubscriberInfo subscriberInfo;
void initForSubscriber(Class<?> subscriberClass) {
this.subscriberClass = clazz = subscriberClass;
skipSuperClasses = false;
subscriberInfo = null;
}
...省略
}
我们可以看到这里当init的时候会对subscriberInfo置空，因此在findUsingInfo里面，会直接调用findUsingReflectionInSingleClass，这个方法非常重要！！！在这个方法内部，利用反射的方式，对订阅者类进行扫描，找出订阅方法，并用上面的Map进行保存，我们来看下这个方法。
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;
}
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");
}
}
}
虽然该方法比较长，但是逻辑非常清晰，逐一判断订阅者类是否存在订阅方法，如果符合要求，并且调用FindState的checkAdd方法返回true的时候,才会把方法保存在findState的subscriberMethods内。而SubscriberMethod则是用于保存订阅方法的一个类。由上面代码我们就知道了该类下的订阅方法都被添加到了我们findState的subscriberMethods中，接着看findUsingInfo的返回，即getMethodsAndRelease方法。
getMethodsAndRelease
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;
}
它就是返回的findState的subscriberMethods。至此我们EventBus的register中findSubscriberMethods分析完毕，我们获取到了注册类的订阅的方法以及订阅的事件也都保存了下来。
然后就是分析register的subscribe()了：
subscribe
在该方法内，主要是实现订阅方法与事件直接的关联，即放进我们上面提到关键的几个Map中：subscriptionsByEventType、typesBySubscriber、stickyEvents。
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
//将subscriber和subscriberMethod封装成 Subscription
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根据事件类型获取特定的 Subscription
//Arraylist效率高,但线程不安全,在多线程的情况下,使用CopyOnWriteArrayList,避免多线程并发异常
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
//如果为null，说明该subscriber尚未注册该事件
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//如果不为null，并且包含了这个subscription 那么说明该subscriber已经注册了该事件，抛出异常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//根据优先级来设置放进subscriptions的位置，优先级高的会先被通知
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;
}
}
//根据subscriber(订阅者)来获取它的所有订阅事件
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
//把订阅者、事件放进typesBySubscriber这个Map中
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//下面是对粘性事件的处理
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 {
//根据eventType，从stickyEvents列表中获取特定的事件
Object stickyEvent = stickyEvents.get(eventType);
//分发事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
到目前为止，注册流程基本分析完毕，而关于最后的粘性事件的处理，这里暂时不说，下面会详细进行讲述。
注销
与注册相对应的是注销，当订阅者不再需要事件的时候，我们要注销这个订阅者，即调用以下代码：
EventBus.getDefault().unregister(this);
直接上源码分析：
public synchronized void unregister(Object subscriber) {
//根据当前的订阅者来获取它所订阅的所有事件
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
//遍历所有订阅的事件
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
//从typesBySubscriber中移除该订阅者
typesBySubscriber.remove(subscriber);
} else {
Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
追下去unsubscribeByEventType：
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
//根据事件类型从subscriptionsByEventType中获取相应的 subscriptions 集合
List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions != null) {
int size = subscriptions.size();
//遍历所有的subscriptions，逐一移除
for (int i = 0; i < size; i++) {
Subscription subscription = subscriptions.get(i);
if (subscription.subscriber == subscriber) {
subscription.active = false;
subscriptions.remove(i);
i--;
size--;
}
}
}
}
可以清楚的看到，上面两个方法的逻辑就是从typesBySubscriber或subscriptionsByEventType移除相应与订阅者有关的信息。是不是感觉注销流程相对于注册流程简单了很多，其实注册流程更多的分析处理集中于怎样找到订阅方法上。
发送事件
分析完了注册和注销，接着我们继续探究下发送事件。用过的都知道发送有两种一种是post一种是postSticky，我们先从post分析:
/** Posts the given event to the event bus. */
public void post(Object event) {
//获取了PostingThreadState对象,内部封装了消息队列、当前线程的信息、以及订阅者、订阅事件等。（currentPostingThreadState是一个ThreadLocal<PostingThreadState>）
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
//先将事件加入到队列中
eventQueue.add(event);
if (!postingState.isPosting) {
//判断当前线程是否是主线程
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//只要队列不为空，就不断从队列中获取事件进行分发
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<Object>();
boolean isPosting;
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
逻辑非常清晰，首先是获取一个PostingThreadState（内部封装了消息队列、当前线程的信息、以及订阅者、订阅事件等），然后将事件添加到它的eventQueue中，再往下就是一个while循环，这里不断地从队列中取出事件，并且分发出去，调用的是EventBus的postSingleEvent方法。
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//该eventInheritance上面有提到，默认为true，即EventBus会考虑事件的继承树
//如果事件继承自父类，那么父类也会作为事件被发送
if (eventInheritance) {
//向上遍历找到该类以及该类的所有父类（直到Object）
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
//遍历所有事件
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
//如果没找到订阅该事件的订阅者
if (!subscriptionFound) {
if (logNoSubscriberMessages) {
Log.d(TAG, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
从上述源码中我们可以得到：当发送一个事件时，我们默认会将它和它的父类一起发送给订阅者，接着调用了EventBus的postSingleEventForEventType，把事件、postingState、事件的类传递进去，那么我们来看看这个方法。
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//从subscriptionsByEventType获取响应的subscriptions
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//发送事件
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
}
//...
}
return true;
}
return false;
}
接着追一下postToSubscription。
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 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);
}
}
首先获取threadMode，即订阅方法运行的线程，如果是POSTING（发送者是什么线程就在什么线程处理），那么直接调用invokeSubscriber()方法即可，如果是MAIN（不管发送者是什么线程，需要切到主线程处理），则要判断当前线程是否是MAIN线程，如果是也是直接调用invokeSubscriber()方法，否则会交给mainThreadPoster来处理，其他情况相类似。几个Poster的最后也都是调用invokeSubscriber（），而EventBus中invokeSubscriber方法的实现也很简单，主要实现了利用反射的方式来调用订阅方法，这样就实现了事件发送给订阅者，订阅者调用订阅方法这一过程。如下所示：
mainThreadPoster的handleMessage
@Override
public void handleMessage(Message msg) {
boolean rescheduled = false;
try {
long started = SystemClock.uptimeMillis();
while (true) {
PendingPost pendingPost = queue.poll();
if (pendingPost == null) {
synchronized (this) {
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null) {
handlerActive = false;
return;
}
}
}
//重点看这里
eventBus.invokeSubscriber(pendingPost);
long timeInMethod = SystemClock.uptimeMillis() - started;
if (timeInMethod >= maxMillisInsideHandleMessage) {
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
rescheduled = true;
return;
}
}
} finally {
handlerActive = rescheduled;
}
}
BackgroundPoster的run
@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;
}
}
}
//重点看这里
eventBus.invokeSubscriber(pendingPost);
}
} catch (InterruptedException e) {
Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
}
} finally {
executorRunning = false;
}
}
AsyncPoster的run
@Override
public void run() {
PendingPost pendingPost = queue.poll();
if(pendingPost == null) {
throw new IllegalStateException("No pending post available");
}
//重点看这里
eventBus.invokeSubscriber(pendingPost);
}
EventBus的invokeSubscriber
void invokeSubscriber(Subscription subscription, Object event) {
try {
//重点看这里
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
postSticky的分析
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);
}
postSticky也是最终调用的post，只是区别是它在之前会先把事件塞到stickyEvents中。处理还是走的上述的一套流程，唯一的区别是：当sticky事件没有从stickyEvents移除的话，后续注册的Class在注册中会接收处理sticky的事件，代码在subscribe中有分析过，这边就再截取部分带着看一下吧。
if (subscriberMethod.sticky) {
//eventInheritance为默认false,所以走else
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 {
//读取stickyEvents中的事件
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
checkPostStickyEventToSubscription
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, Looper.getMainLooper() == Looper.myLooper());
}
}
又回到了我们熟悉的postToSubscription方法。至此我们EventBus的分析完结！！！由于本文很长，建议读者可以追着源码看着解读，这样能加深理解。最后，为坚持读完本文的你点赞，谢谢你们的阅读！
