介绍
EventBus是一种用于Android的发布/订阅事件总线。在我们开发中经常将其应用于Activity之间，Fragment之间的通讯传值等。它能达到简化组件间的通信，以及解耦事件的发送者和接受者的作用。
EventBus使用十分简单，在需要发送数据的地方调用post方法，并将数据对象传入
EventBus.getDefault(this).post("test");
在我们期望接受到数据的地方，注册EventBus，并写一个带有@Subscribe注解的方法，该方法只有一个参数，并且其数据类型与post方法发送的数据类型一样。该方法就会接收到post发送的数据。
public class TestActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
EventBus.getDefault().register(this);
}
@Subscribe()
public void receiveValue(String msg){
}
@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}
}
源码分析
基于EventBus3.1.1
如上，EventBus有三个关键节点，register注册、post发送和unregister取消注册。这里就根据这三个节点来简单分析下源码。
register注册流程
首先通过getDefault方法获取EventBus单例对象
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
下面是register方法，该方法是注册给定的用来接收事件的subscriber订阅者。
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
首先我们看到，调用了subscriber.getClass()方法得到了订阅者的class对象，如上面的例子我们得到的就是Activity的class对象。这里又将这个class对象传到findSubscriberMethods中，findSubscriberMethods的作用是找到订阅者中所有@Subscribe注解标记的方法，将其封装成SubscriberMethod对象并将其添加到集合中返回。
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;
}
}
首先会去METHOD_CACHE这个Map中取订阅者的方法集合，如果存在就将其直接返回。如果没有缓存，再去查找。ignoreGeneratedIndex属性值默认为false，这个变量在初始化SubscriberMethodFinder对象时传入
subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
builder.strictMethodVerification, builder.ignoreGeneratedIndex);
所以，此时执行findUsingInfo方法。
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);
}
这里先初始化FindState对象，并调用其initForSubscriber方法为FindState的clazz属性赋值为订阅者的class对象，如下
void initForSubscriber(Class<?> subscriberClass) {
this.subscriberClass = clazz = subscriberClass;
skipSuperClasses = false;
subscriberInfo = null;
}
那么此时while (findState.clazz != null) 判断就成立了，再执行getSubscriberInfo方法获取subscriberInfo
private SubscriberInfo getSubscriberInfo(FindState findState) {
if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
if (findState.clazz == superclassInfo.getSubscriberClass()) {
return superclassInfo;
}
}
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}
上面在initForSubscriber方法中为subscriberInfo赋值为空，而subscriberInfoIndexes属性在实例化SubscriberMethodFinder对象时通过EventBusBuilder对象传入，其值也为空。所以这里的if (findState.subscriberInfo != null)条件不成立，继续执行else分支的findUsingReflectionInSingleClass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");
}
}
}
首先获取所有声明在订阅者类中的方法数组methods，然后遍历methods数组，获取方法的修饰符modifiers，判断语句(modifiers & Modifier.PUBLIC) != 0通过修饰符和PUBLIC对应的值进行按位与运算来判断方法的修饰符是否为PUBLIC,不等于0即为PUBLIC;判断语句(modifiers & MODIFIERS_IGNORE) == 0，其中MODIFIERS_IGNORE = Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;这个MODIFIERS_IGNORE的值与modifiers按位与即判断修饰符是否是MODIFIERS_IGNORE中的任意一个，结果为0即没有这些修饰符。所以经过这个判断，即保证了@Subscribe注解的方法必须是public，非静态以及非抽象的方法。
进入到if 分支，通过getParameterTypes获取到方法的参数类型数组parameterTypes，判断数组长度为1时继续执行，否则抛出异常，也就是这里限制了@Subscribe方法只能有一个参数。然后获取方法上的@Subscribe注解，通过parameterTypes[0]获取到了方法参数的类型eventType。if (findState.checkAdd(method, eventType))这个判断的checkAdd方法做了两层检查，即判断该方法是否已经添加到集合中了，返回true即没有添加过，继而获取到注解上声明的线程threadMode，并将method方法名、eventType参数类型、threadMode线程、priority优先级、sticky粘性封装成SubscriberMethod对象添加到findState.subscriberMethods集合中。到这里，subscribe方法中findSubscriberMethods查找订阅方法的流程执行结束，并得到了List<SubscriberMethod> 格式的集合。遍历前面得到的集合并调用Subscribe方法
// 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);
}
}
}
首先将subscriber订阅者对象和SubscriberMethod订阅方法对象封装成Subscription对象。在该方法中建立了两个Map关联关系，第一个：以事件类型为key，CopyOnWriteArrayList<Subscription>为value存到subscriptionsByEventType这个Map中，这样就建立了 eventType事件类型和CopyOnWriteArrayList<Subscription>（（订阅者对象以及订阅者类中的订阅方法）的对象的集合）。第二个：以订阅者为key，以订阅者类中所有@Subscribe方法的参数类型（即订阅的事件类型）的集合为value存到typesBySubscriber这个Map中。
至此register流程就结束了，这里主要的操作就是找到订阅类中注解为@Subscribe的并且修饰符为public非静态非抽象且只有一个参数的方法，将其封装为SubscriberMethod添加到集合中保存起来。
post发送流程
post方法将给定的event事件发送到Event bus
/** 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 = 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;
}
}
}
首先获取PostingThreadState对象，这个对象中封装了发送相关的状态，如eventQueue为事件集合，这里将event事件添加到这个集合中。判断isPosting为false表示当前未处于发送状态，再将当前是否处于主线程赋值给isMainThread属性，判断当事件集合不为空时，调用postSingleEvent方法发送集合中的第一个事件。
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));
}
}
}
eventInheritance属性代表EventBus是否会考虑event事件的继承结构，当该值为true时发送一个事件，注册了这个事件父类的方法也会收到通知。不管该值为true或false都会调用到postSingleEventForEventType方法
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;
}
通过发送的事件类型从subscriptionsByEventType中获取对应的subscriptions集合。遍历subscriptions集合，调用postToSubscription将事件传到订阅者中的订阅方法中
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);
}
}
判断订阅方法指定的执行线程，回调到指定线程中执行订阅方法，即调用invokeSubscriber方法
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);
}
}
最后通过反射调用subscriber对象中的method方法，并将event事件传递到方法中，至此post发送流程就结束了。通过Post发送事件的eventType类型从Register流程中构造的subscriptionsByEventType这个Map对象中获取到对应的订阅者以及其中订阅方法的集合，遍历集合再通过反射来执行订阅者中的订阅方法。
unregister取消注册流程
将订阅者从所有事件类中取消注册。
/** 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());
}
}
遍历事件类型集合subscribedTypes，调用unsubscribeByEventType方法，并从subscribedTypes移除订阅者subscriber。
/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
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--;
}
}
}
}
根据事件类型获取到subscriptions集合，遍历集合，并将当前订阅者subscriber对应的Subscription对象从集合中移除。取消注册流程比较简单，从typesBySubscriber中移除subscriber，从subscriptions集合中将subscriber对应的Subscription对象移除。
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