前言
相信大家已经非常熟练的使用EventBus了，简单的说EventBus是一个Android事件发布/订阅框架，通过解耦发布者和订阅者简化 Android 事件传递，是个典型的观察者模式，那么是什么是观察者模式，有个很形象的比喻：西游记中各路神仙一挥手，自己的坐骑就现出原形了，那么这些妖怪就是观察者，当他们观察到神仙挥手时就会现出原形。本文源码基于EventBus3.0。
一、获取EventBus实例
EventBus.getDefault().register(this);
使用前我们要进行注册，EventBus.getDefault().register(this)；首先我们来看下getDefault方法
/** Convenience singleton for apps using a process-wide EventBus instance. */
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
我们可以看出这是一个典型的双重锁校验的单例模式，如果不存在则新建，接着我们来看EventBus的构造方法
/**
* Creates a new EventBus instance; each instance is a separate scope in which events are delivered. To use a
* central bus, consider {@link #getDefault()}.
*/
public EventBus() {
this(DEFAULT_BUILDER);
}
看到这里有没有疑惑的地方？既然是单例模式为什么构造方法不是private而是public，这是因为EventBus可能有多条总线，订阅者注册到不同线上的 EventBus，通过不同的实例来发送数据，不同的 EventBus 是相互隔离开的，订阅者都只会收到注册到该线上事件。
构造方法中只有一个DEFAULT_BUILDER，接着我们来看DEFAULT_BUILDER
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
我们可以看出EventBus是通过一个EventBusBuilder的实例来构建的
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;
}
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
前三行是三个hashMap但有所不同:
subscriptionsByEventType 对应的hashMap是：
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
它的key是订阅事件，value是所有订阅者集合。当我们发送Event事件的时候都是从这个集合中去寻找，
typesBySubscriber 对应的hashMap是：
private final Map<Object, List<Class<?>>> typesBySubscriber;
它的key是订阅者对象，value是这个订阅者订阅的所有事件集合。当我们注册或者反注册的时候都是操作这个集合然后操作subscriptionsByEventType。
stickyEvents 对应的hashMap是：
private final Map<Class<?>, Object> stickyEvents;
它的key是粘性事件的class对象，value是粘性事件对象。接着我们看后面三行代码：
mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10); backgroundPoster = new BackgroundPoster(this);asyncPoster = new AsyncPoster(this);
mainThreadPoster
我们知道，在此之前我们线程间通信或者传值的话 都是使用Hanlder，而mainThreadPoster 的本质就是一个Hanlder，我们点击mainThreadPoster 进去看
既然是hanlder所以我们主要来看它的hanleMessage方法
@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;
}
}
从队列中不断的去处pendingPost，使用 eventBus.invokeSubscriber(pendingPost)进行事件分发，而pendingPost是一个可复用对象的复用池，通过obtainPendingPost方法进行复用，releasePendingPost方法进行回收，这个HandlerPoster是运行在主线程中的，因为初始化的时候有个 Looper.getMainLooper()。
pendingPost主要有三个对象：事件、订阅和一个节点
backgroundPoster
backgroundPoster本质是一个Runnable，主要在后台处理事件，所以我们主要看它的run方法
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) {
Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
}
} finally {
executorRunning = false;
}
}
和mainThreadPoster是一样的，从复用池中取出，如果队列为空就间隔1秒再取，然后调用invokeSubscriber方法进行分发
asyncPoster
asyncPoster的本质也是一个Runnable
@Override
public void run() {
PendingPost pendingPost = queue.poll();
if(pendingPost == null) {
throw new IllegalStateException("No pending post available");
}
eventBus.invokeSubscriber(pendingPost);
}
但是每次只取一个pendingPost。不论发布线程是否为主线程，都使用一个空闲线程来处理。Async类的所有线程是相互独立的，因此不会出现卡线程的问题。
说到这里这三个poster都是负责线程调度的，最后都调用invokeSubscriber进行事件分发，那么我们有必要来来看下这个invokeSubscriber方法。
invokeSubscriber
/**
* Invokes the subscriber if the subscriptions is still active. Skipping subscriptions prevents race conditions
* between {@link #unregister(Object)} and event delivery. Otherwise the event might be delivered after the
* subscriber unregistered. This is particularly important for main thread delivery and registrations bound to the
* live cycle of an Activity or Fragment.
*/
void invokeSubscriber(PendingPost pendingPost) {
Object event = pendingPost.event;
Subscription subscription = pendingPost.subscription;
PendingPost.releasePendingPost(pendingPost);
if (subscription.active) {
invokeSubscriber(subscription, event);
}
}
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);
}
}
这样就比较一目了然了，从peningPost中得到事件和订阅，通过反射调用了订阅者的订阅函数并把event对象作为参数传入。
二、注册事件
/**
* 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);
}
}
}
我们可以看到这个注册方法
首先获取订阅者的class对象，通过subscriberMethodFinder寻找这个class对象的所有订阅方法集合SubscriberMethod，SubscriberMethod，而SubscriberMethod中包含了相应的线程、Method对象、事件类型、优先级记忆是否是粘性事件等。
然后通过 subscribe(subscriber, subscriberMethod);订阅事件，而subscriberMethod对象是通过subscriberMethodFinder获取的，所以我们先来看下subscriberMethodFinder的实现
subscriberMethodFinder
subscriberMethodFinder类是用来查找和缓存订阅者响应函数的信息的类，那么我们如何获取订阅者响应函数的信息，这里就要提到APT运行时注解了，当然我们这里不对APT进行介绍，EventBus中是通过@Subscribe()注解来获取的，我们来看下@Subscribe()
我们可以看到默认粘性事件为false，retention生命是运行时注解，相比反射而言效率更高，如果你想了解APT注解是什么，这里推荐一篇不错的博文。http://www.jcodecraeer.com/a/anzhuokaifa/androidkaifa/2018/0423/9629.html，如果这篇博文作者看到可以联系我，如有侵权可删除。
言归正传我们来看subscriberMethodFinder方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
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;
}
}
从METHOD_CACHE取看是否有缓存,key是保存订阅类的类名,value是保存类中订阅的方法数据,如果忽略注解器生成的MyEventBusIndex类就调用反射来获取订阅类中的订阅方法信息，如果没有忽略就从注解生成的是否忽略注解器生成的MyEventBusIndex类中来获取订阅类中的订阅方法信息。
findUsingReflection
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//通过反射来获得订阅方法信息
findUsingReflectionInSingleClass(findState);
//查找父类的订阅方法
findState.moveToSuperclass();
}
//返回订阅方法集合
return getMethodsAndRelease(findState);
}
findUsingReflectionInSingleClass
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
//反射得到方法数组
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;
}
//遍历Method
for (Method method : methods) {
int modifiers = method.getModifiers();
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?>[] parameterTypes = method.getParameterTypes();
//保证必须只有一个事件参数
if (parameterTypes.length == 1) {
//得到注解
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
Class<?> eventType = parameterTypes[0];
//校验是否添加该方法
if (findState.checkAdd(method, eventType)) {
ThreadMode threadMode = subscribeAnnotation.threadMode();
//实例化SubscriberMethod对象并添加
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");
}
}
}
findUsingInfo
findUsingInfo是通过运行时注解生成的MyEventBusIndex类来获取订阅方法信息
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//得到订阅者信息
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
//获取方法数组
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
//检验是否添加
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
findUsingReflectionInSingleClass(findState);
}
//到父类中查找
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
getSubscriberInfo
private SubscriberInfo getSubscriberInfo(FindState findState) {
//判断FindState对象中是否有缓存的订阅方法
if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
if (findState.clazz == superclassInfo.getSubscriberClass()) {
return superclassInfo;
}
}
//从注解器生成的MyEventBusIndex类中获得订阅类的订阅方法信息
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}
这样，订阅类的所有SubscriberMethod都已经被保存了，最后再通过getMethodsAndRelease()返回List<SubscriberMethod>。
然后我们再回到注册中的subscribe(subscriber, subscriberMethod);方法，方法代码如下：
//必须在同步代码块里调用
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取订阅的事件类型
Class<?> eventType = subscriberMethod.eventType;
//创建Subscription对象
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//从subscriptionsByEventType里检查是否已经添加过该Subscription,如果添加过就抛出异常
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);
}
}
//根据优先级priority来添加Subscription对象
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;
}
}
//将订阅者对象以及订阅的事件保存到typesBySubscriber里.
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//如果接收sticky事件,立即分发sticky事件
if (subscriberMethod.sticky) {
//eventInheritance 表示是否分发订阅了响应事件类父类事件的方法
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);
}
}
}
三、发送事件
EventBus是通过post()方法进行事件发送的，接下来我们来看post方法
/** 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;
}
}
}
首先通过 PostingThreadState postingState = currentPostingThreadState.get();来获取当前线程的状态
currentPostingThreadState代码如下：
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
ThreadLocal概念引用:是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据，而这段数据是不会与其他线程共享的。其内部原理是通过生成一个它包裹的泛型对象的数组，在不同的线程会有不同的数组索引值，通过这样就可以做到每个线程通过get() 方法获取的时候，取到的只能是自己线程所对应的数据。 所以这里取到的就是每个线程的PostingThreadState状态，然后得到当前事件的队列，最后调用postSingleEvent方法来发送事件
postSingleEvent
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) {
Log.d(TAG, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
首先得到事件类型对应的class类，查找eventClass类所有的父类以及接口如果没找到则会打印日志，存在最终是调用postSingleEventForEventType方法进行分发。
postSingleEventForEventType
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;
}
首先从subscriptionsByEventType里获得所有订阅了这个事件的Subscription列表,然后在通过postToSubscription()方法来分发
事件,在postToSubscription()通过不同的threadMode在不同的线程里invoke()订阅者的方法。
ThreadMode：
public enum ThreadMode {
/**
* Subscriber will be called 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 is 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.
*/
POSTING,
/**
* Subscriber will be called in Android's main thread (sometimes referred to as UI thread). If the posting thread is
* the main thread, event handler methods will be called directly. Event handlers using this mode must return
* quickly to avoid blocking the main thread.
*/
MAIN,
/**
* Subscriber will be called in a background thread. If posting thread is not the main thread, event handler 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. Event handlers using this mode should try to
* return quickly to avoid blocking the background thread.
*/
BACKGROUND,
/**
* Event handler methods are called in a separate thread. This is always independent from the posting thread and the
* main thread. Posting events never wait for event handler methods using this mode. Event handler 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 handler methods at the same time to limit the number of concurrent threads. EventBus
* uses a thread pool to efficiently reuse threads from completed asynchronous event handler notifications.
*/
ASYNC
}
ThreadMode一共有四类
PostThread：默认的 ThreadMode，表示在执行 Post 操作的线程直接调用订阅者的事件响应方法，不论该线程是否为主线程（UI 线程）。当该线程为主线程时，响应方法中不能有耗时操作，否则有卡主线程的风险。适用场景：对于是否在主线程执行无要求，但若 Post 线程为主线程，不能耗时的操作；
MainThread：在主线程中执行响应方法。如果发布线程就是主线程，则直接调用订阅者的事件响应方法，否则通过主线程的 Handler 发送消息在主线程中处理——调用订阅者的事件响应函数。显然，MainThread类的方法也不能有耗时操作，以避免卡主线程。适用场景：必须在主线程执行的操作；
BackgroundThread：在后台线程中执行响应方法。如果发布线程不是主线程，则直接调用订阅者的事件响应函数，否则启动唯一的后台线程去处理。由于后台线程是唯一的，当事件超过一个的时候，它们会被放在队列中依次执行，因此该类响应方法虽然没有PostThread类和MainThread类方法对性能敏感，但最好不要有重度耗时的操作或太频繁的轻度耗时操作，以造成其他操作等待。适用场景：操作轻微耗时且不会过于频繁，即一般的耗时操作都可以放在这里；
Async：不论发布线程是否为主线程，都使用一个空闲线程来处理。和BackgroundThread不同的是，Async类的所有线程是相互独立的，因此不会出现卡线程的问题。适用场景：长耗时操作，例如网络访问。
