EventBus源码学习
看了别人的源码文章，一开始有点懵，还要复习下反射和注解器的知识。而后本身也作了下笔记，加深印象。设计模式
参考文章：缓存
www.jianshu.com/p/c4d106419….bash
描述
EventBus是Android和Java的发布/订阅事件总线。数据结构
优缺点：
- 简化了组件之间的通讯，将事件发布者和订阅者分离
- 代码简单
- 包体积小缺点：利用反射，性能可能会差一点。可使用EventBus annotation processor（EventBus注解处理器），在编译期间建立了订阅者索引。（提高EventBus性能：greenrobot.org/eventbus/do…
三要素：
- Event：自定义事件类型
- Subscriber：事件订阅者。在EventBus3.0以前咱们必须定义以onEvent开头的那几个方法，分别是onEvent、onEventMainThread、onEventBackgroundThread和onEventAsync，而在3.0以后事件处理的方法名能够随意取，不过须要加上注解@subscribe()，而且指定线程模型，默认是POSTING。
- Publisher：事件发布者。能够在任意线程里发布事件，通常状况下，使用EventBus.getDefault()就能够获得一个EventBus对象，而后再调用post(Object)方法便可。
4个线程：
- POSTING:在发布线程的事件执行
- MAIN：在主线程执行。
- BACKGROUND：若是发布事件的线程为主线程则新建一个线程执行，不然在发布事件的线程执行。
- ASYNC：在新的线程执行
利用Handler切换到主线程。BACKGROUND线程，使用到了线程池ExecutorService。async
说说EventBus里面用到的知识点
- 注解处理器：
使用EventBus annotation processor（EventBus注解处理器），在编译期间建立了订阅者索引。（提高EventBus性能：greenrobot.org/eventbus/do…）。能够看到，会在build包下面生成一个EventBus的索引类。SUBSCRIBER_INDEX是一个Map类型数据，存放着每个类里面的注解信息ide
/** 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(com.example.camera_learning.eventbus.EventBusActivity.class, true,
new SubscriberMethodInfo[] {
new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent.class),
new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent2.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;
}
}
}
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- 反射
若是索引类中有缓存相关的注解信息的话，那就会直接用索引类里面的注解信息。若是没有的话，就须要利用反射去获取类中的全部方法，而后拿到有@Subscribe的方法。post
post方法最后须要利用invoke来调用订阅者对象的订阅方法（具体代码能够看下面）性能
- 设计模式：单例模式、建造者模式、观察者模式
- ThreadLocal：EventBus会经过ThreadLocal为每一个线程维护一个PostingThreadState对象。
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
final static class PostingThreadState {
//分发队列
final List<Object> eventQueue = new ArrayList<>();
//是否正在分发
boolean isPosting;
//是不是主线程
boolean isMainThread;
//订阅关系
Subscription subscription;
//当前正在分发的事件
Object event;
//是否取消
boolean canceled;
}
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EventBus里面的一些数据结构
比较重要的就是下面两个map集合。ui
//key为事件类型，value值为Subscription的集合。这样能够根据事件的类型，直接获取订阅该事件的全部类中的全部方法
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//key为订阅对象，value为该类中的全部订阅事件类型的集合。
private final Map<Object, List<Class<?>>> typesBySubscriber;
//粘性事件
private final Map<Class<?>, Object> stickyEvents;
//Subscription类，表示一个订阅关系，包含订阅对象和订阅方法
final class Subscription {
final Object subscriber;
final SubscriberMethod subscriberMethod;
}
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阅读源码
getDefault()方法
- 利用双重校验锁实现单例模式，保证线程安全
- 使用Builder建造者模式，将一个复杂对象的构建和它的表示分离。建造者模式通常用来建立复杂对象，用户能够不用关心其建造过程和细节。在EventBus中，指的是EventBusBuilder类，可使用自定义参数建立EventBus实例。
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
public EventBus() {
this(DEFAULT_BUILDER);
}
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
backgroundPoster = new BackgroundPoster(this);
asyncPoster = new AsyncPoster(this);
indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
builder.strictMethodVerification, builder.ignoreGeneratedIndex);
logSubscriberExceptions = builder.logSubscriberExceptions;
logNoSubscriberMessages = builder.logNoSubscriberMessages;
sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
throwSubscriberException = builder.throwSubscriberException;
eventInheritance = builder.eventInheritance;
executorService = builder.executorService;
}
public class EventBusBuilder {
private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();
boolean logSubscriberExceptions = true;
boolean logNoSubscriberMessages = true;
boolean sendSubscriberExceptionEvent = true;
boolean sendNoSubscriberEvent = true;
boolean throwSubscriberException;
boolean eventInheritance = true;
boolean ignoreGeneratedIndex;
boolean strictMethodVerification;
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
List<Class<?>> skipMethodVerificationForClasses;
List<SubscriberInfoIndex> subscriberInfoIndexes;
Logger logger;
MainThreadSupport mainThreadSupport;
EventBusBuilder() {
}
/** Builds an EventBus based on the current configuration. */
public EventBus build() {
return new EventBus(this);
}
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register方法
register的最终目的是，将注解的信息保存到对应的map中。而获取的过程，优先经过注解器生成的注解信息获取，若是没有的话，再利用反射获取注解方法。
public void register(Object subscriber) {
//拿到class对象
Class<?> subscriberClass = subscriber.getClass();
//利用SubscriberMethodFinder类来找出这个类中的全部订阅方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
//拿到类中全部的订阅方法后，调用subscribe方法
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
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);
}
}
}
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下面看一下SubscriberMethodFinder类。
//这个map对象，key值为订阅者类，value为该类中的全部订阅方法的list集合。已经register过的话，相应的数据已经在map中存在了，这个时候就能够直接从map中读取就好了，不用再次利用反射进行多余的遍历类中的方法。有点空间换时间的思想。
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
//经过这个方法，遍历出某个类中的全部订阅方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//根据缓存值，判断是否注册过EventBus
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//默认为false，因此通常是先走findUsingInfo方法，经过自动生成的索引类好比MyEventBusIndex来获取到被注解的办法。这种办法比经过findUsingReflection（）利用反射去获取方法的性能好一点。若是没有获取到索引中存储的订阅者Subscriber信息的话，才去走利用反射的办法。
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
//若是订阅者类和父类中，没有@Subscribe利用监听事件的方法，就会抛这个错误。
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//获取到订阅者类中的全部注解方法后，保存到cache中
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
//利用注解处理器生成的索引类，来获取类中的订阅方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//在生成的索引类中，获取保存的订阅信息
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
//获取拿到这个订阅类中的全部的订阅方法
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
//遍历，检查后，添加到list集合
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
//若是在索引类中找不到订阅信息，那就经过反射的办法来获取
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
//在生成的索引类中，获取保存的订阅信息
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;
}
}
//若是subscriberInfoIndexes为空，也就是没有利用生成索引的办法来提升EventBus性能，直接返回null
//不为空的话，遍历，拿到这个订阅类中的全部注解方法
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}
//使用subscribe方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//订阅方法的事件类型
Class<?> eventType = subscriberMethod.eventType;
//建立一个Subscription，包含订阅类对象和订阅方法
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//获取到这种事件类型的list集合
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//保存到subscriptionsByEventType里面，key为事件类型，value为Subscription
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//处理重复注册的状况
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//这部分的做用，是根据事件优先级进行排序
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;
}
}
//根据订阅对象，获取订阅的全部的事件类型
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//处理粘性事件
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);
}
}
}
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post方法
post方法最后会走到postSingleEventForEventType方法,拿到注册这个事件的订阅者信息，利用反射调用订阅方法的执行。
public void post(Object event) {
//获取当前线程的PostingThreadState对象
PostingThreadState postingState = currentPostingThreadState.get();
//获取当前的分发队列
List<Object> eventQueue = postingState.eventQueue;
//将事件添加到队列中
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 {
//当事件队列不为空的时候，一个一个进行分发
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
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private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//去map集合中，根据事件类型，获取到全部的订阅方法
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
//遍历订阅该事件的list集合
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;
}
//根据线程进行分发事件
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);
}
}
//最后是利用反射，调用订阅者对象的订阅方法
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);
}
}
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unregister方法
unregister方法就是要把这个类中的注解信息都给移除掉。
public synchronized void unregister(Object subscriber) {
//获取这个订阅者种的全部订阅事件类型list
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
//遍历事件类型list，移除subscriptionsByEventType中包含这个订阅者的全部订阅关系
unsubscribeByEventType(subscriber, eventType);
}
//移除这个订阅对象
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
//移除subscriptionsByEventType中包含这个订阅者的全部订阅关系
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
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--;
}
}
}
}
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