EventBus基本使用
发送事件：
EventBus.getDefault().post(new EmptyEvent());
订阅事件：
EventBus.getDefault().register(this);
处理事件：
@Subscribe(threadMode = ThreadMode.MAIN)
public void onRefresh(EmptyEvent event) {
//TODO 接收到通知后需要进行的操作
}
取消订阅：
EventBus.getDefault().unregister(this);
EventBus的操作很简单，引入也很方便，那么它内部是怎么实现的呢？事件的接收方是怎么收到事件的，会不会有遗漏或错乱？它的线程是怎么处理的？好，接下来一步一步看源码～
class EventBus {
//map中的key为Event类，value为订阅该Event的Subscription(Object subscriber, SubscriberMethod subscriberMethod)列表
//Subscription 类只有两个成员变量，订阅类 和 该类下的某个订阅方法
//核心参数，post一个事件的时候能快速获取到接收该事件的方法并调用
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//map中key为订阅类，value为订阅类订阅的Event列表
//该参数主要用于判断一个类是否已经register
private final Map<Object, List<Class<?>>> typesBySubscriber;
/**
* Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
* are no longer interested in receiving events.
* <p/>
* Subscribers have event handling methods that must be annotated by {@link Subscribe}.
* The {@link Subscribe} annotation also allows configuration like {@link
* ThreadMode} and priority.
*/
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
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);
}
}
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<>();
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);
}
}
}
/** 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());
}
}
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions != null) {
int size = subscriptions.size();
for (int i = 0; i < size; i++) {
Subscription subscription = subscriptions.get(i);
if (subscription.subscriber == subscriber) {
subscription.active = false;
subscriptions.remove(i);
i--;
size--;
}
}
}
}
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;
}
}
}
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));
}
}
}
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;
}
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);
}
}
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);
}
}
}
总结：
A类订阅事件 EventBus.getDefault().register(this); 做的事情：
1> 获取订阅类A下的所有SubscriberMethod（含@Subscribe注解的，public的，非static的，非abstract的，只有一个参数的方法），得到 List<SubscriberMethod> methods；在此过程中，会遍历父类（跳过系统类），添加父类的不同的（方法名，事件类型不完全相同）SubscriberMethod到methods
2> 加锁synchronized，接下来的操作都在锁内进行
3> 遍历订阅类A下的所有SubscriberMethod，每个SubscriberMethod都进行如下操作
4> 根据优先级，在 subscriptionsByEventType 中为该Event加入new Subscription(subscriber, subscriberMethod)
5> 在typesBySubscriber 中为该订阅类加入新的EventType
6> subscriberMethod.sticky相关操作，待阅读A类取消订阅事件 EventBus.getDefault().unregister(this); 做的事情：
1> typesBySubscriber 获取订阅类A订阅的所有EventType
2> 针对每个EventType，将subscriptionsByEventType 移除EventType下的所有Subscription.subscriber 为该类的Subscription
3> typesBySubscriber 移除该类B类发送一个事件 EventBus.getDefault().post(new EmptyEvent()); 做的事情：
0> eventInheritance为true的时候（默认为true），获取到该Event的父类Event/继承的接口类Event列表，遍历该列表，执行以下操作
1> 从subscriptionsByEventType 获取到接收该Event的Subscription
2> 遍历Subscription列表，依次调用Subscription.subscriberMethod
3> 这个过程中，确认subscriberMethod的执行线程，通过AsyncPoster，BackgroundPoster，HandlerPoster处理线程切换问题
class AsyncPoster implements Runnable, Poster {
private final PendingPostQueue queue;
private final EventBus eventBus;
AsyncPoster(EventBus eventBus) {
this.eventBus = eventBus;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
queue.enqueue(pendingPost);
eventBus.getExecutorService().execute(this);
}
@Override
public void run() {
PendingPost pendingPost = queue.poll();
if(pendingPost == null) {
throw new IllegalStateException("No pending post available");
}
eventBus.invokeSubscriber(pendingPost);
}
}
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);
//区别于SyncPoster的地方
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;
}
}
}
public class HandlerPoster extends Handler implements Poster {
private final PendingPostQueue queue;
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
private boolean handlerActive;
protected HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
super(looper);
this.eventBus = eventBus;
this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
queue.enqueue(pendingPost);
if (!handlerActive) {
handlerActive = true;
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
}
}
}
@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;
}
}
}
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;
}
}
}
EventBus提供的几种线程模式：
/**
* Each subscriber method has a thread mode, which determines in which thread the method is to be called by EventBus.
* EventBus takes care of threading independently from the posting thread.
*
* @see EventBus#register(Object)
* @author Markus
*/
public enum ThreadMode {
/**
* Subscriber will be called directly in the same thread, which is posting the event. This is the default. Event delivery
* implies the least overhead because it avoids thread switching completely. Thus this is the recommended mode for
* simple tasks that are known to complete in a very short time without requiring the main thread. Event handlers
* using this mode must return quickly to avoid blocking the posting thread, which may be the main thread.
*/
/**
* 订阅服务器将直接在发布事件的同一线程中调用。这是默认设置。
* 事件传递意味着开销最小，因为它完全避免了线程切换。
* 因此，对于已知在非常短的时间内完成而不需要主线程的简单任务，建议使用这种模式。
* 使用此模式的事件处理程序必须快速返回，以避免阻塞可能是主线程的发布线程。
*/
POSTING,
/**
* On Android, subscriber will be called in Android's main thread (UI thread). If the posting thread is
* the main thread, subscriber methods will be called directly, blocking the posting thread. Otherwise(否则，另外) the event
* is queued for delivery (non-blocking). Subscribers using this mode must return quickly to avoid blocking the main thread.
* If not on Android, behaves the same as {@link #POSTING}.
*/
MAIN,
/**
* On Android, subscriber will be called in Android's main thread (UI thread). Different from {@link #MAIN},
* the event will always be queued for delivery. This ensures that the post call is non-blocking.
*/
MAIN_ORDERED,
/**
* On Android, subscriber will be called in a background thread. If posting thread is not the main thread, subscriber methods
* will be called directly in the posting thread. If the posting thread is the main thread, EventBus uses a single
* background thread, that will deliver all its events sequentially. Subscribers using this mode should try to
* return quickly to avoid blocking the background thread. If not on Android, always uses a background thread.
*/
BACKGROUND,
/**
* Subscriber will be called in a separate thread. This is always independent from the posting thread and the
* main thread. Posting events never wait for subscriber methods using this mode. Subscriber methods should
* use this mode if their execution might take some time, e.g. for network access. Avoid triggering a large number
* of long running asynchronous subscriber methods at the same time to limit the number of concurrent threads. EventBus
* uses a thread pool to efficiently reuse threads from completed asynchronous subscriber notifications.
*/
ASYNC
}
POSTING：默认线程，和发布事件的线程一致；
MAIN：主线程，如果发布事件的线程是主线程，则直接调用订阅者的方法，会阻塞发布线程，否则事件排队等待传递，非阻塞；
MAIN_ORDERED：主线程，事件始终排队等待传递，确保发布事件的调用是非阻塞的；
BACKGROUND：子线程，如果发布事件的线程不是主线程，则和发布事件的线程一致，否则使用单个后台线程，按顺序传递所有事件，共用一个子线程；
ASYNC：子线程，一个单独的线程，区别于发布事件的线程以及主线程，每次都从线程池中execute一个新的线程，不共用。
使用EventBus需要注意的地方：
- 一个类注册监听的时候，必须有含@Subscribe注解的公有方法
- 一个类只能注册一次监听，否则subscriptionsByEventType添加Subscription的时候会抛出异常
new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event " + eventType) - 父类和子类有相同的订阅方法（方法名，事件类型相同，线程无所谓）的时候，编译通过，只会执行子类的订阅方法，因为register的时候会遍历父类，添加父类的不同的（方法名，事件类型不完全相同）SubscriberMethod，相同的则不添加
- 默认情况下，发送一个子类Event的时候，接收子类Event和父类Event的方法都会调用，如果不想接收父类Event的方法被调用，设置eventInheritance为false即可；发送父类Event，只有接收父类Event的方法被调用
- 一定要记得unregister，否则subscriptionsByEventType中一直含有该订阅方法，发送事件的时候仍然会去调用，造成异常
