EventBus源码解析
注册
EventBus-> register
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
方法流程：
- 先根据传入的subscriber拿到它的class对象
- 然后调用findSubscriberMethods拿到subscriber中有subscriber注解的method
- 然后遍历收集的方法集调subscribe方法
SubscriberMethodFinder-> findSubscriberMethods
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
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;
}
}
方法流程：
- 首先从METHOD_CACHE缓存中拿到对应class的subscriberMethod集合，然后存在就直接返回
- 然后根据ignoreGeneratedIndex参数判断转调哪个方法根据subcriberClass拿到subscriber方法集合，它的默认值为false
SubscriberMethodFinder-> findUsingInfo
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);
}
方法流程：
- 调用prepareFindState方法从对象池中拿到一个FindState对象并初始化
- 调用getSubscriberInfo拿到SubcriberInfo，而SubscriberInfo其实就是Class和Subsriber方法信息之间关系的封装
- 然后做一下校验工作之后就把拿到的方法集合添加到findState中
- 接下来就是再往上找父类
SubscriberMethodFinder-> getSubscriberInfo
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;
}
方法流程：
- 遍历从EventBusBuilder中传进来的subscriberInfoIndexes，然后通过index根绝subscriberClass拿到SubscriberInfo
SubscriberInfoIndex-> getSubscriberInfo
public interface SubscriberInfoIndex {
SubscriberInfo getSubscriberInfo(Class<?> subscriberClass);
}
- SubscriberInfoIndex是一个接口，因为实现类是通过apt生成的
/** This class is generated by EventBus, do not edit. */
public class MyEventBusIndex implements SubscriberInfoIndex {
private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;
static {
SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();
putIndex(new SimpleSubscriberInfo(MainActivity.class, true, new SubscriberMethodInfo[] {
new SubscriberMethodInfo("onEventA", PayEvent.class, ThreadMode.ASYNC),
}));
putIndex(new SimpleSubscriberInfo(MyIntentService.class, true, new SubscriberMethodInfo[] {
new SubscriberMethodInfo("onEventBB", PayEvent.class),
}));
}
private static void putIndex(SubscriberInfo info) {
SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
}
@Override
public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
if (info != null) {
return info;
} else {
return null;
}
}
设计思路：
- 通过apt实现在编译器生成SubscriberInfoIndex的实现类，然后这个实现类维护了一个Map<Class，SubscriberInfo>，然后在static块就把SubscriberClass和SubscriberInfo存进这个Map中
- 而SubscriberInfo其实就是Class和Subsriber方法信息之间关系抽象的数据结构
- 所以用这种apt的方式做Class和SubscriberInfo的映射就不需要反射找方法了，这也是这个类命名叫index的原因吧
SimpleSubscriberInfo-> getSubscriberMethods
@Override
public synchronized SubscriberMethod[] getSubscriberMethods() {
int length = methodInfos.length;
SubscriberMethod[] methods = new SubscriberMethod[length];
for (int i = 0; i < length; i++) {
SubscriberMethodInfo info = methodInfos[i];
methods[i] = createSubscriberMethod(info.methodName, info.eventType, info.threadMode,
info.priority, info.sticky);
}
return methods;
}
- 其实就是把SubscriberInfo储存方法信息转存并返回
SubscriberMethodFinder-> 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;
}
- 把FindState中的methods信息取出来，然后回收到对象池中
小结
到这里其实针对apt的Subscriber方法查找的流程就走完了，其实原理就是
- 通过apt生成SubscriberInfoIndex的子类，然后在这个类中维护了一个Map储存了Class和Subscriber方法的映射的关系
- 然后在注册阶段就可以以Class为索引找到对应SubscriberInfo对象，SubsriberInfo对象储存了对应类中所有的Subsriber方法以及Event类型的信息
- 通过这种优化策略就可以避免在注册过程中使用反射找到这些Subsriber方法
SubscriberMethodFinder-> findUsingReflection
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
- 这个方法的执行流程和findUsingInfo方法很类似，就是少了那部分根据index查找info再找方法的逻辑
- 而是直接采用反射进行Subsriber方法的搜索
SubscriberMethodFinder-> findUsingReflectionInSingleClass
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");
}
}
}
方法流程：
- 通过反射拿到这个类中所有的方法
- 然后遍历这些方法，第一步先先判断是否是public，是否有需要忽略修饰符，如果既不是public，又不需要忽略修饰符就抛出异常
- 如果是public的修饰符：
- 方法参数只有一个：这就说明EventBus只允许Subscriber方法绑定的Event只能是一个，然后再判断是不是被Subscribe注解修饰，如果是，就从注解中拿到线程信息，然后进行一些校验工作之后就把方法添加到FindState中
- 如果是strict校验模式而且Event不止一个，就抛出异常
- 如果是strict校验模式而且Subscribe方法不是public，非静态，非抽象就抛出异常
EventBus-> subscribe
// 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);
}
}
}
方法流程：
- 从subscriberMethod中拿到绑定的Event的类型，然后创建一个Subscription对象，其实就是对subscriberMethod和subscriber的一层封装
- 然后在EventBus中维护一个subscriptionsByEventType，也就是Map<Event类型，Subscription集合>，然后就是根绝Event类型拿到这个Subscription集合，根据优先级把Subscription插入到这个集合中
- 然后在EventBus还维护一个typesBySubscriber，也就是Map<接收者Object，subscribed的所有Event类型列表>，然后把这个Event的类型添加到对应的subscriber的类型列表中
- 然后就是对sticky的处理，这里很有意思，通过eventInheritance这个标记位做区分处理：
- eventInheritance为true：会判断当前Event有没有父类，没有就直接做一次方法调用，如果有父类就往上找它的父类
- eventInheritance为false：直接做一次方法调用
小结
总的来说注册的过程分为2步：
搜索方法：
- apt在编译器生成一个SubscriberIndex类，维护一个Map储存Class和SubscriberInfo的关系，SubscriberInfo则保存所有Subscribe方法名称以及Event的类型，用这种方式避免了在注册过程中反射搜索方法
- 通过反射拿到Class的所有方法，然后遍历参数只有1个的方法找它是否被注解修饰
注册：
- 注册这一步操作说明白了就是把信息缓存在EventBus中
- 注册部分中涉及的EventBus缓存包含两个：
- Map<Event类型，Subscription集合> subscriptionsByEventType，用于记录Event类型和接收地的关系，Subscription不止包含Subsriber，还包含了接收方法
- Map<接收者Object，subscribed的所有Event类型列表>，用于记录接收者所关注的所有事件类型
发送Event
EventBus-> post
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
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;
}
}
}
方法流程：
- 拿到当前线程的PostingThreadState对象，这个对象其实就是线程信息的封装，在PostingThreadState中还维护了一个event队列，然后post会把event添加到这个队列中
- isPosting这个标记其实就是用来控制并发，在post过程中，当前线程的其他post操作无法生效
- 然后给postingState赋值是否为主线程，isPosting置为true
- 转调postSingleEvent函数，遍历eventQueue进行Event发送
PostingThreadState
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<>();
boolean isPosting;
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
EventBus-> 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 (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
方法流程：
- 这里依然有对eventInheritance标记位的兼容，与注册时同理是为了对继承关系进行处理
- 而实际的subscription搜索和Event发送都是由postSingleEventForEventType方法执行的
EventBus-> 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;
}
方法流程：
- subscriptions集合的获取就是根据注册时候填入的Map，根据Event的class拿到所有subscriptions的集合
- 然后给postingState，也就是当前线程发送状态赋值信息
- 转调postToSubscription方法发送消息
EventBus-> 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 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);
}
}
- 根据方法搜索时储存的threadMode这个字段，根据不同情况进行处理
- 主要的思想就是判断当前是否处于指定的线程，是的话就直接invokeSubscriber，不是的话就加入队列中
- 但是MAIN_ORDERED这种情况下，不管是否处于指定线程都会入队，这样就保证了顺序
- 而ASYNC就是相反了，不管是否处于指定线程都直接入队，从而异步
- 而非主线程的Poster本身也是一个Runnable，会不停从队列中取出事件去发送，而本质其实也是调用invokeSubscriber方法，而EventBus会维护一个线程池执行这些线程
- 而主线程的Poster的线程切换的秘密其实也还是通过Handler去实现的
EventBus->invokeSubscriber
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);
}
}
- 可见实际在找到对应关系之后就是通过invoke调用subscribe方法即可
EventBus-> postSticky
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);
}
- 把sticky的event用一个Map保存起来，key依旧是event的类型
- 转调post方法，而当接收方还没有被register的情况下postSingleEventForEventType就会因为subscriptions为null而退出了
总结
EventBus这个框架的设计思想其实就是通过apt或者反射找到了被注解描述的方法缓存起来，然后当调post的时候从早就做好的这个缓存中找到目标，反射调用一下就好了
而对粘性事件，其实也是活用了缓存，postSticky的时候缓存起来，注册的时候检查缓存中是否存在粘性事件，存在就调用目标方法就好了
