深入EventBus 事件总线的框架源码学习
在看源码之前，需要掌握的主要知识点：集合、反射、注解。
框架基本上是用这三方面的知识点写的，没掌握的最好去掌握下，不然看的时候会晕头转向。
一、注册源码解析
注册的一系列流程，其流程图(来自网络)如下：
我们在使用EventBus的时候，首先做的第一件事就是给事件注册对象了，通俗来讲就是说要接收消息，需要登记信息，方便有消息可以通知到你。
那么EventBus是如何注册信息的呢？又是如何接收事件？
我们带着疑问看代码，效果会更好。
注册订阅者代码：
//将当前对象进行注册，表示这个对象要接收事件
EventBus.getDefault().register(this);
注册源码：
public void register(Object subscriber) {
//获取订阅者的Class类型
Class<?> subscriberClass = subscriber.getClass();
//根据订阅者的Class类型，获取订阅者的订阅方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
List< SubscriberMethod > subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
这句代码表示的是：通过订阅者的Class类型获取订阅者的所有的订阅方法。那它是如何获取所有的订阅方法的呢？
我们再看一下findSubscriberMethods方法的源码
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//先从缓存中查找是否存在订阅方法
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//是否忽略注解器生成的MyEventBusIndex类
if (ignoreGeneratedIndex) {
//利用反射来获取订阅类中的订阅方法信息
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//从注解器生成的MyEventBusIndex类中获得订阅类的订阅方法信息
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;
}
}
上面的源码在查找订阅方法信息是用了两种方式，如下所示：
if (ignoreGeneratedIndex) {
//利用反射来获取订阅类中的订阅方法信息
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//从注解器生成的MyEventBusIndex类中获得订阅类的订阅方法信息
subscriberMethods = findUsingInfo(subscriberClass);
}
ignoreGeneratedIndex的判断是看你的build配置文件是否配置相关注解处理器信息如下：
这个方法的大致逻辑是这样：
先从缓存中查找订阅方法信息，如果没有就利用反射或注解来获取订阅方法信息。
1、使用注解处理器
findUsingInfo(Class< ? > subscriberClass)
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);
}
EventBus提供了一个EventBusAnnotationProcessor注解处理器来在编译期通过读取@Subscribe()注解并解析,
处理其中所包含的信息,然后生成java类来保存所有订阅者关于订阅的信息,这样就比在运行时使用反射来获得这些订阅者的
信息速度要快。
EventBus 3.0使用的是编译时注解，而不是运行时注解。通过索引的方式去生成回调方法表，通过表可以看出，极大的提高了性能。
2、通过反射获取的订阅方法信息
findUsingReflection(subscriberClass)源码
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//通过发射获得订阅方法信息
findUsingReflectionInSingleClass(findState);
//查找父类的订阅方法信息
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
FindState是一个实体类，用来保存订阅者和订阅方法的信息，以及校验订阅方法。写了那么多，其实最终是调用findUsingReflectionInSingleClass(findState)来获得订阅方法的相关信息。
看代码：
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
//获取订阅者的所有订阅方法
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
for (Method method : methods) {
//获取订阅方法的修饰权限
int modifiers = method.getModifiers();
//如果是public修饰的方法且不是静态的和抽象的，否则抛出异常
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//获取方法的参数类型数组
Class<?>[] parameterTypes = method.getParameterTypes();
//方法至少要有一个参数，否则抛出异常
if (parameterTypes.length == 1) {
//获取注解对象
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
//获取事件对象的参数类型信息
Class<?> eventType = parameterTypes[0];
//检查是否添加了订阅方法
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");
}
}
}
开始订阅事件的源码：
订阅者有了，订阅方法也有了。接下来是当做参数进行订阅，
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取事件类型
Class<?> eventType = subscriberMethod.eventType;
//实例一个订阅者信息
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根据事件类型获取订阅者对象
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();
//根据优先级来添加订阅者对象
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//根据订阅者对象来获取订阅者事件
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
//将订阅者对象和订阅者事件保存
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
if (subscriberMethod.sticky) {
if (eventInheritance) {
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);
}
}
}
注册只有一句代码，但是深入里面去，却是做了许许多多的工作。
其实，做了那么多的工作，无非就是存储订阅者的相关信息（订阅者、订阅方法、事件对象等），为后面的事件分发做的铺垫。
订阅逻辑：
1. 首先调用register（）注册订阅对象。
2. 根据订阅对象获取该订阅对象的所有订阅方法。
3. subscriptionsByEventType.put(eventType, subscriptions)，根据该订阅者的所有订阅的事件类型，将订阅者存入到每个以 事件类型为key， 以订阅者信息（subscriptions）为values的map集合中。
4. typesBySubscriber.put(subscriber, subscribedEvents)，然后将订阅事件添加到以订阅者为key ，以订阅者所有订阅事件为values的map集合中.
二、事件分发解析
事件分发的流程图（来源网络）
注册订阅者是为了后面的事件分发，以便知道该将事件发送给谁，而事件的接收就是订阅者的订阅方法。
事件分发代码:
EventBus.getDefault().post(new Event());
看post方法的源码：
参数即要发送的事件对象
public void post(Object event) {
//获取当前线程的状态
PostingThreadState postingState = currentPostingThreadState.get();
//获取当前线程的事件队列
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
//判断当前线程是否处于发送状态
if (!postingState.isPosting) {
//设置为主线程
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
//不断从队列中循环取出事件
try {
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
PostingThreadState 是EventBust的静态内部类，它保存当前线程的事件队列和线程状态。
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<Object>();//当前线程的事件队列
boolean isPosting;//是否有事件正在分发
boolean isMainThread;//post的线程是否是主线程
Subscription subscription;//订阅者
Object event;//订阅事件
boolean canceled;//是否取消
}
参数一：事件对象
参数二：发送状态
postSingleEvent(eventQueue.remove(0), postingState)执行源码
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
//获取事件对象的Class类型
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//判断事件对象是否有父类对象
if (eventInheritance) {
//查找事件对象所有的事件类型以及它的父类类型
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));
}
}
}
postSingleEventForEventType(event, postingState, eventClass)
看了那么多的源码才发现这个方法才是罪魁祸首。最终发射事件的方法。
那好接着看看他的源码：
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//通过事件对象类型获取所有的订阅者
subscriptions = subscriptionsByEventType.get(eventClass);
}
//
if (subscriptions != null && !subscriptions.isEmpty()) {
//遍历所有的订阅者
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;
}
postToSubscription(subscription, event, postingState.isMainThread);
开始处理订阅者的信息
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);
}
}
invokeSubscriber(subscription, event)
参数一：订阅者
参数二：事件对象
最后是通过反射调用实现的。
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);
}
}
事件分发逻辑：
1. 获取事件队列。
2. 循环取出队列中的事件。
3. 获取订阅者集合，然后遍历，最后通过反射调用订阅者的订阅方法。
三、取消注册解析
public synchronized void unregister(Object subscriber) {
//根据订阅者获取订阅者的所有订阅的事件类型
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
//根据事件类型获取事件类型的所有订阅者
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--;
}
}
}
}
取消注册逻辑：
1、首先根据订阅者获取所有订阅事件。
2、遍历订阅事件集合。
3、根据订阅事件获取订阅该事件的订阅者集合。
4、遍历订阅者集合，然后根据传入的订阅者做比较，判断是否是同一订阅者。如果是则移除，反之。
四、post和postSticky的区别
/** 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;
}
}
}
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);
}
从上面的比较来看postSticky利用 Map<Class<?>, Object> 数据结构来存储sticky事件，再调用post方法。接下去就是等待时机触发
了，这个时机就是等待注册的那个时候，然后标有sticky标记的方法就好被触发执行。
在EventBus类中有一个subscribe方法里面有如下的一段代码，这段主要是判断是否是黏性事件，如果是的话就执行，不是的话直接跳过。
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);
}
}
其实post和postSticky的区别用通俗的话来讲就是post要先注册，然后立刻开始处理事件对象。而postSticky就是不管你有没有注册，先发送事件对象再说，然后存储起来，直到有对象来注册这个sticky事件。
五、其它类分析
订阅方法类：主要用来存储订阅方法的相关信息。
//有方法、线程模型、事件类型、优先级、粘性等属性。
public class SubscriberMethod {
final Method method;
final ThreadMode threadMode;
final Class<?> eventType;
final int priority;
final boolean sticky;
/** Used for efficient comparison */
String methodString;
}
订阅者信息类，用来存储订阅者和订阅方法。
final class Subscription {
final Object subscriber;
final SubscriberMethod subscriberMethod;
/**
* Becomes false as soon as {@link EventBus#unregister(Object)} is called, which is checked by queued event delivery
* {@link EventBus#invokeSubscriber(PendingPost)} to prevent race conditions.
*/
volatile boolean active;
}
EventBus的事件处理函数通常需要指定处理线程，EventBus提供了四种线程模型POSTING（默认）、MAIN、BACKGROUND、ASYNC
public enum ThreadMode {
//POSTING表示如果使用事件处理函数指定了线程模型为PostThread，那么该事件在哪个线程发布出来的，事件处理函数就会在这个线程中运行，也就是说发布事件和接收事件在同一个线程。在线程模型为PostThread的事件处理函数中尽量避免执行耗时操作，因为它会阻塞事件的传递，甚至有可能会引起ANR。
POSTING,
//如果使用事件处理函数指定了线程模型为MainThread，那么不论事件是在哪个线程中发布出来的，该事件处理函数都会在UI线程中执行。该方法可以用来更新UI，但是不能处理耗时操作。
MAIN,
//如果使用事件处理函数指定了线程模型为BackgroundThread，那么如果事件是在UI线程中发布出来的，那么该事件处理函数就会在新的线程中运行，如果事件本来就是子线程中发布出来的，那么该事件处理函数直接在发布事件的线程中执行。在此事件处理函数中禁止进行UI更新操作
BACKGROUND,
//如果使用事件处理函数指定了线程模型为Async，那么无论事件在哪个线程发布，该事件处理函数都会在新建的子线程中执行。同样，此事件处理函数中禁止进行UI更新操作
ASYNC
}
