EventBus源码再分析
一、概述
EventBus是一个开源的用于Android和Java上的一个：订阅--->发布事件总线。
优点：
1.只要是在一个JVM内，就可以实现通信
2.小巧灵活、不占内存
3.解耦，切换线程灵活
4.库小，不占内存
缺点：
1.注册和反注册时一对，如果忘记了就会出现内存泄漏
2.拿到注解方法组是通过反射来进行的，效率稍低
3.在整个项目中需要定义不少的Event组件，事件如果很多就会看起来比较臃肿
ps：就目前来说，如果项目中使用kotlin，那么事件总线可以使用SharedFlow或者StateFlow来代替。
二、原理分析
从使用上来说EventBus就三板斧。注册：EventBus.getDefault().register(obj)、取消注册:EventBus.getDefault().unregister(obj)、发送事件:EventBus.getDefault().post(event)、接收事件@Subscribe(threadMode=ThreadMode.MAIN)接收事件
一、先说注册
public void register(Object subscriber) { //获取注册对象的class字节码对象 Class<?> subscriberClass = subscriber.getClass();
//这一步比较关键，作用是用来获取subscriber对象中包含@Subscribe注解的方法集合 List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); synchronized (this) { for (SubscriberMethod subscriberMethod : subscriberMethods) {
//循环对方法集合进行注册，其实就是往集合中去添加找到的方法名。 subscribe(subscriber, subscriberMethod); } } }
接下来看看subscriberMethodFinder.findSubscriberMethods(subscriberClass)方法中都干了啥
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//先判断之前装载了没有，如果装载了就直接拿来使用，不用继续往下执行，如果没有装载则往下执行
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);//拿到注解方法集合
} 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;
}
}
接下来看：subscriberMethods = findUsingReflection(subscriberClass);//拿到注解方法集合
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// 获取注册对象的所有方法
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
....
}
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) {
//获取方法上的@Subscribe注解
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {//如果Subscribe对象不为空，说明这个方法上有@Subscribe注解
Class<?> eventType = parameterTypes[0];//拿到第一个参数，用于确定事件类型
if (findState.checkAdd(method, eventType)) {
//获取与这个注解相关联的线程模型
ThreadMode threadMode = subscribeAnnotation.threadMode();
//将这个注解的信息存入集合，包括：方法对象、事件类型、线程模型等
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
....
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
....
}
}
}
到这里：subscriberMethodFinder.findSubscriberMethods(subscriberClass);获取注解方法列表的方法已经返回了方法列表，算是走完了。
下面看下循环注册subscribe(subscriber, subscriberMethod);方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
//将注册对象和注册对象中的@Subscribe注解信息封装成为一个Subscription，方便后面使用
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//先看看以前有没有存过，如果没有就new一个
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
//把事件类型和时间类型对应的对象及对象注解信息存入map，这个后面unRegister的时候也会用的到
subscriptionsByEventType.put(eventType, subscriptions);
} else {
....
}
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
//把封装好的注册对象和对象包含的注解方法信息存入集合
subscriptions.add(i, newSubscription);
break;
}
}
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);//把事件类型存起来，放入typesBySubscriberd对象中，后面会用到
.....
}
到这里注册就分析完了，其实整体上看起来是比较简单的，流程无非是：拿到订阅对象，通过订阅对象拿到对应的class。在运行时拿到class的所有方法，过滤出带有@subscribe注解的方法，并发放发信息存入对象，对象存入集合，然后返回。通过拿到返回的对象集合，循环的去进行注册，其实就是把对象方法，和事件类型封装到map中，供后面使用。
二、下面说说EventBus.getDefault.Post(event)方法
public void post(Object event) {
//事件入队列
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();//判断post所在的线程是主线程还是子线程，并记录下来
postingState.isPosting = true;
....
try {
while (!eventQueue.isEmpty()) {
//分发事件
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
下面看看postSingleEvent都干了什么事情
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();//依然是通过时间对象获取其字节码对象
boolean subscriptionFound = false;
......
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);//接着对事件分发
.....
}
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);//这句话很重要，表示通过事件类型在subscriptionsByEventType的map中拿，@Subscribe对象中此注解的方法信息集合
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {//@Subscribe信息集合拿到后开始遍历
postingState.event = event;
postingState.subscription = subscription;
boolean aborted;
try {
postToSubscription(subscription, event, postingState.isMainThread);//分发并执行方法
aborted = postingState.canceled;
} finally {
.....
}
return true;
}
return false;
}
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case POSTING://表示默认线程，post在哪里调用，次方发就在哪里执行
invokeSubscriber(subscription, event);//这个方法表示通过java的反射机制，通过method.invoke来执行方法，其实就是主动调用
break;
case MAIN://如果是主线程则不切换线程，直接执行，如果不是主线程则切换到主线程中执行
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) {//如果是主线程则切换到工作线程执行
backgroundPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);//如果不是主线程，则直接在当前线程中执行
}
break;
case ASYNC:
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}
我们看下invokeSubscriber方法中时什么东西
void invokeSubscriber(Subscription subscription, Object event) {
try {
//从对象中拿到注解方法对象，然后调用方法对象的invoke，把参数传递进去主动执行
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
}
....
}
到这里post方法算是执行完事了。其实Post方法说是发送事件倒不如说是，主动执行事件。我们可以看下它所做的操作：1.根据事件类型，拿到注解方法集合、2.遍历集合、3.遍历后拿到@Subscribe注解标注的方法对象，然后执行method.invoke方法。这样看起来就是就是主动执行。
三、最后看看EventBus.getDefault().unregister(obj)干了啥
public synchronized void unregister(Object subscriber) {
//根据对象拿到类型集合，这是register的时候存好的
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
//循环遍历取消注册
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);//取消注册的时候一定要移除
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
//注册的时候才存好的结合，根据事件类型拿到Subscription集合 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) {//遍历出自己，然后移除就行了 subscription.active = false; subscriptions.remove(i); i--; size--; } } } }
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