我们在使用EventBus的时候，首先要注册EventBus：
EventBus.getDefault().register(this);
那么我们就从这段代码作为入口，一步一步探索EventBus的秘密！
我们先看看getDefault()这个方法：
static volatile EventBus defaultInstance;
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
很明显，采用了单例的设计模式。
进入register()方法查看：
public void register(Object subscriber) {
//通过反射，拿到这个类
Class<?> subscriberClass = subscriber.getClass();
//通过这个类，拿到所有subcriber注解的方法，并返回到一个集合里
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder
.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
我们在进findSubscriberMethods方法里面看看：
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//如果有缓存，从缓存里面拿
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//默认false
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;
}
}
接着看findUsingInfo这个方法：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findState.subscriberInfo = getSubscriberInfo(findState);
//findState.subscriberInfo == null
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 {
//走else
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
我们并不知道FindState为何物，为了方便理解，我们先来看看什么是FindState。
static class FindState {
//持有subscriberMethods这个list
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;
//一开始为null
subscriberInfo = null;
}
//释放对象，内存优化
void recycle() {
subscriberMethods.clear();
anyMethodByEventType.clear();
subscriberClassByMethodKey.clear();
methodKeyBuilder.setLength(0);
subscriberClass = null;
clazz = null;
skipSuperClasses = false;
subscriberInfo = null;
}
boolean checkAdd(Method method, Class<?> eventType) {
// 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
// Usually a subscriber doesn't have methods listening to the same event type.
Object existing = anyMethodByEventType.put(eventType, method);
if (existing == null) {
return true;
} else {
if (existing instanceof Method) {
if (!checkAddWithMethodSignature((Method) existing, eventType)) {
// Paranoia check
throw new IllegalStateException();
}
// Put any non-Method object to "consume" the existing Method
anyMethodByEventType.put(eventType, this);
}
return checkAddWithMethodSignature(method, eventType);
}
}
private boolean checkAddWithMethodSignature(Method method, Class<?> eventType) {
methodKeyBuilder.setLength(0);
methodKeyBuilder.append(method.getName());
methodKeyBuilder.append('>').append(eventType.getName());
String methodKey = methodKeyBuilder.toString();
Class<?> methodClass = method.getDeclaringClass();
Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass);
if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
// Only add if not already found in a sub class
return true;
} else {
// Revert the put, old class is further down the class hierarchy
subscriberClassByMethodKey.put(methodKey, methodClassOld);
return false;
}
}
void moveToSuperclass() {
if (skipSuperClasses) {
clazz = null;
} else {
clazz = clazz.getSuperclass();
String clazzName = clazz.getName();
/** Skip system classes, this just degrades performance. */
if (clazzName.startsWith("java.") || clazzName.startsWith("javax.") || clazzName.startsWith("android.")) {
clazz = null;
}
}
}
}
可见，FindState为SubscriberMethodFinder的一个静态内部类。将持有注解的Class赋值给了findState.clazz。
初始化的时候findState.subscriberInfo == null，所以我们再看看findUsingReflectionInSingleClass(findState)：
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
//拿到注册类所有的方法
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;
}
//遍历方法
for (Method method : methods) {
//通过反射获取方法的修饰符
int modifiers = method.getModifiers();
//如果是public，进入if
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//获取方法参数类型
Class<?>[] parameterTypes = method.getParameterTypes();
//判断参数长度是不是1
if (parameterTypes.length == 1) {
//获取注解Subscriber
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
//如果注解不为空
if (subscribeAnnotation != null) {
Class<?> eventType = parameterTypes[0];
//判断有没有已加入
if (findState.checkAdd(method, eventType)) {
//拿到线程
ThreadMode threadMode = subscribeAnnotation
.threadMode();
//将方法，参数类型，线程，优先级等加入SubscriberMethods类
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");
}
}
}
最后再看看getMethodsAndRelease方法：
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;
}
先将findState持有的集合赋值，然后释放findState。
我们来总结一下EventBus.getDefault().register(this)这段代码到底干了什么，其实很简单，找到所有注册了bus的类，然后找到该类所有Subcriber注解的方法，并将该方法保存到一个List里面去。
我们再继续往下看：
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
上面这段代码的意思是循环我们保存得到的List，将订阅者（Activity，Fragment）和对应方法一起作为参数执行下面的方法。
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//方法参数类型 MessageEvent
Class<?> eventType = subscriberMethod.eventType;
//封装的一个对象 封装了订阅者和Subscriber注解的方法
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//这个集合到底存了什么？见分析一
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);
}
}
//list数量 其实为0
int size = subscriptions.size();
//按照优先级加入list，这段算法设计很巧妙
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//订阅者作为key，注解方法参数类型作为value
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
//添加参数类型进list
subscribedEvents.add(eventType);
//暂时不用理解
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);
}
}
}
分析一：subscriptionsByEventType到底是一个什么东西？
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
首先，这是一个Map，key是注解方法参数的类型类，value是一个存有Subscription的List。而Subscription又封装了subscriber和注解方法。
我们再来看一看时间的发布源码：
EventBus.getDefault().post(new MessageEvent(hospitalList
.get(currentPos).getCompanyName(),hospitalList.get(currentPos)
.getCompanyID()+""));
进post源码：
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
if (!postingState.isPosting) {
//是不是主线程
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
//post方法参数类型
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));
}
}
}
进入post核心方法：
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState,
Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//从Map里面取集合
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//核心方法
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;
}
我们看一下下面的方法：
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);
}
}
我们拿上面的代码其中一段分析，其他同理：
case MAIN:
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
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);
}
}
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
最终，利用invoke反射执行方法！
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