前言
在项目中遇到了有关EventBus数据方面的问题，于是就参考源码梳理一下整个数据流程，最后整理输出。
简介
EventBus是一个基于观察者模式的发布/订阅事件总线，解决了以往广播在组件间通信的不安全、耗时问题，替代Intent,Handler,BroadCast在Fragment，Activity，Service，线程之间传递消息。
EventBus
优缺点
优点->简化组件之间的通信，实现解耦使业务简洁，高效，可以自己设置事件处理的线程以及优先级。
缺点->需要维护很多事件类。
使用
-
定义事件:
public static class MessageEvent { /* Additional fields if needed */ } -
准备订阅者: 申明并注解你的订阅方法, 指定线程模式(可选)
@Subscribe(threadMode = ThreadMode.MAIN) public void onMessageEvent(MessageEvent event) {/* Do something */};在使用的类中，注册和解注册订阅者，如Activity/fragment中:
@Override public void onStart() { super.onStart(); EventBus.getDefault().register(this); } @Override public void onStop() { super.onStop(); EventBus.getDefault().unregister(this); } -
发送事件:
EventBus.getDefault().post(new MessageEvent());
内部实现
1.EventBus.getDefault().register(this);
注册给定的订阅用户以接收事件。订阅者对接收事件不再感兴趣后，必须调用{@link #unregister（Object）}。
public void register(Object subscriber) {
//通过反射获取订阅者Class对象
Class<?> subscriberClass = subscriber.getClass();
//1.5
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
//遍历订阅
for (SubscriberMethod subscriberMethod : subscriberMethods) {
//2
subscribe(subscriber, subscriberMethod);
}
}
}
通过Class对象获取它的订阅事件的集合
1.SubscriberMethodFinder#List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass)
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
//这个Map key为注册事件的类(比如某个Activity)，value为承载着事件类(MessageEvent)和处理事件的方法(onMessageEvent(MessageEvent event))
//先从此处查找 不为空返回 为空 就添加到map中
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//1.1 获取所有订阅者的方法列表
subscriberMethods = findUsingInfo(subscriberClass);
}
//不为空 就添加到map中
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;
}
}
1.1 SubscriberMethodFinder#List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass)
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
//从FIND_STATE_POOL (private static final FindState[] FIND_STATE_POOL = new FindState[POOL_SIZE]; POOL_SIZE->4)
//中获取FindState对象
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findState.subscriberInfo = getSubscriberInfo(findState);
//第一次为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 {
//通过反射获取订阅者类中的所有方法
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
//从findState中获取订阅者所有方法的集合
return getMethodsAndRelease(findState);
}
2.EventBus#subscribe(Object subscriber, SubscriberMethod subscriberMethod)
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取事件类型的Class对象
Class<?> eventType = subscriberMethod.eventType;
//将信息封装到对象中
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根据事件类型查找对应的CopyOnWriteArrayList
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//只能注册一次
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//根据优先级将新构建的subscription(subscriber, subscriberMethod)添加到CopyOnWriteArrayList中
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);
//如果是sticky事件 调用checkPostStickyEventToSubscription方法发送粘性事件
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);
}
}
}
注册部分主要通过通过订阅者的Class对象获取其中的订阅方法，将方法和订阅者绑定，之后又处理了粘性事件。
2. EventBus.getDefault().post(new MessageEvent());
public void post(Object event) {
//currentPostingThreadState(是ThreadLocal)获取PostingThreadState
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 {
//如果事件队列中还有事件 就一直发送
while (!eventQueue.isEmpty()) {
//3
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<>(); // 线程的事件队列
boolean isPosting; //是否正在发送
boolean isMainThread; //是否在主线程中发送
Subscription subscription; //关于事件的封装
Object event; //事件对象
boolean canceled; //是否被取消发送
}
3.EventBus#postSingleEvent(Object event, PostingThreadState postingState)
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) {
//查找所有Class对象，包括超类和接口
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
//调用postSingleEventForEventType方法发送事件
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));
}
}
}
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//获取所有订阅者
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//发送事件
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;
}
//粘性事件会直接调用这个
//根据注解上的线程模式在不用的线程调用
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case POSTING://直接反射调用方法 不切换线程
invokeSubscriber(subscription, event);
break;
case MAIN://在主线程直接反射调用
if (isMainThread) {
invokeSubscriber(subscription, event);
} else { //否则加入到列队中到主线程调用
mainThreadPoster.enqueue(subscription, event);
}
break;
case MAIN_ORDERED://与MAIN类似
if (mainThreadPoster != null) {
mainThreadPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
case BACKGROUND:
if (isMainThread) {//在主线程就加入后台队列执行
backgroundPoster.enqueue(subscription, event);
} else {//直接反射调用
invokeSubscriber(subscription, event);
}
break;
case ASYNC://直接在子线程调用
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}
post方法主要将事件加入集合并且集合还有数据就一直发送，然后获取所有订阅该事件类的订阅者，判断他们需要运行的线程来决定在哪个线程运行。
3.EventBus.getDefault().postSticky(new MessageEvent());
//加入到粘性事件列表中供之后判断
public void postSticky(Object event) {
synchronized (stickyEvents) {
stickyEvents.put(event.getClass(), event);
}
//发送事件
post(event);
}
将数据加入到粘性事件列表在registe方法中判断是否post事件
4.EventBus.getDefault().unregister(this);
public synchronized void unregister(Object subscriber) {
//获取订阅者的所有订阅事件
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
//将订阅者的订阅事件移除
unsubscribeByEventType(subscriber, eventType);
}
//将订阅者移除
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
总结
到这里EventBus源码流程就结束了，它的源码相较于其他来说还是比较简单的，如果文中有不足之处望指出。
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