EventBus 开源库学习（二）
整体流程阅读
EventBus在使用的时候基本分为以下几步：
1、注册订阅者
EventBus.getDefault().register(this);
2、订阅者解注册，否者会导致内存泄漏
EventBus.getDefault().unregister(this);
3、在订阅者中编写注解为Subscribe的事件处理函数
@Subscribe(threadMode = ThreadMode.MAIN, sticky = true, priority = 1)
public void onMsgEventReceived(MsgEvent event) {
Toast.makeText(this, event.getMsg(), Toast.LENGTH_LONG).show();
}
4、事件发送
EventBus.getDefault().post("msg1 - coming！！！");
我们先按使用的流程大体看下源码逻辑，源码版本3.3.1：
注册源码逻辑
public static EventBus getDefault() {
EventBus instance = defaultInstance;
if (instance == null) {
synchronized (EventBus.class) {
instance = EventBus.defaultInstance;
if (instance == null) {
instance = EventBus.defaultInstance = new EventBus();
}
}
}
return instance;
}
EventBus 使用了双重校验锁的单例设计模式，保证用到的对象是唯一的，首次使用对象为空的时候通过下面构造创建一个。
public EventBus() {
this(DEFAULT_BUILDER);
}
DEFAULT_BUILDER是一个final常量，在加载的时候就进行初始化，赋一个EventBusBuilder对象如下面代码所示。
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
EventBusBuilder是EventBus的建造类，里面参数在加载的时候进行了初始化。
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;
...
}
如果有需要的话，我们也可以通过配置EventBusBuilder来更改EventBus的属性，在EventBus中有一个静态方法直接返回一直新的EventBusBuilder对象，设置完参数后调用build()来以新的配置来新建一个EventBus对象。
#EventBus
public static EventBusBuilder builder() {
return new EventBusBuilder();
}
#EventBusBuilder
/** Builds an EventBus based on the current configuration. */
public EventBus build() {
return new EventBus(this);
}
然后通过下面的调用来设置：
EventBus.builder()
.eventInheritance(false)
.logSubscriberExceptions(false)
.build()
.register(this);
拿到EventBus对象以后，我们可以调用其register方法进行订阅者注册了。
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
首先获取订阅者类subscriber，然后通过findSubscriberMethods方法获取该类中以@Subscribe注解的函数，由于一个类中可能监听多个事件，因此获取的方法可能是多个，所有的方法赋值到一个List列表中，然后遍历这个列表进行注册。
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);
....
}
上面就是注册最主要的代码，步骤解析如下：
-
根据注解方法获取监听事件的类型
eventType，并对订阅者subscriber和订阅函数subscriberMethod建立一个订阅关系对象newSubscription； -
根据
eventType在subscriptionsByEventType（HashMap）中获取所有该事件类型的订阅关系列表subscriptions； -
如果订阅关系列表
subscriptions为空就新建一个，然后以key为eventType，value为newSubscription添加进去； -
如果订阅关系列表
subscriptions不为空，判断是否存在newSubscription，如果存在，说明之前已经注册过，抛出异常； -
如果订阅关系列表
subscriptions不为空，列表页没有订阅关系newSubscription，我们遍历添加进去，这里通过订阅函数的priority来决定存放在列表中的位置，从这里也能看出priority越大，存放位置越靠前，和上一篇中分析的：值越大，优先级越高，越优先接收到事件。我们可以猜出是通过遍历这个表来进行事件发送的，在表里的位置越靠前，越先收到事件。 -
然后通过订阅者
subscriber在另一个HashMap- typesBySubscriber中获取该订阅者订阅的所有事件，因为一个订阅者可以订阅多个不同的事件，因此获取的是个List列表subscribedEvents； -
首先判断
subscribedEvents列表是不是空的，如果是空说明以前没有订阅过任何事件，新建一个List，然后以key为subscriber，value为subscribedEvents添加到typesBySubscriber； -
然后在新建的
subscribedEvents中添加我们订阅的事件eventType。
这里出现了两个HashMap：
subscriptionsByEventType与typesBySubscriber，通过上面的解析可以知道：
subscriptionsByEventType：一个事件可能有多个订阅者，key是事件，value是所有订阅该事件的所有的订阅者；
typesBySubscriber：一个订阅者可能订阅多个事件，key是订阅者，value是订阅者订阅的所有事件；
看命名就知道By后面的是key值，前面的是value值，一个好的命名就是这样吧。
解注册源码逻辑
在使用EventBus时，注册完后我习惯接着去写解注册的代码，怕后面会忘，因此按照写代码习惯在讲下解注册的源码实现。
/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
如果是我们自己写这部分代码，根据前面注册的过程，我们肯定是要把添加到两个HashMap中的值移除掉。好了，看上面源码，首先在typesBySubscriber中获取该订阅者订阅的事件列表subscribedTypes；
如果为空说明该订阅者没有订阅任何事件，无任何操作；如果不为空，遍历所有事件调用unsubscribeByEventType方法进行解注册，然后在typesBySubscriber中移除这个订阅者subscriber。
/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
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--;
}
}
}
}
在上一个方法中，已经把typesBySubscriber这个订阅者移除了，那么unsubscribeByEventType函数就是遍历事件所有的订阅者，然后把解注册的订阅者在subscriptionsByEventType中给移除掉。上面的逻辑就是干这个事情。不过一边遍历一边移除是有风险的，这个大家要注意，index需要也跟着进行减少。
总结下：注册和解注册就是往两个HashMap添加和移除数据的过程。
事件发送post源码逻辑
/** 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);
····
}
上面是部分post代码，第一行先解释这个变量currentPostingThreadState及内部类PostingThreadState。
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
/** For ThreadLocal, much faster to set (and get multiple values). */
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<>();
boolean isPosting;
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
currentPostingThreadState是一个ThreadLocal，相当于是线程的私有财产，里面维护的变量只属于当前线程，线程间不会共享。维护的变量是一个自定义类PostingThreadState，用来保存发送线程的发送状态信息：当前线程是否为主线程，是否在发送事件，发送的事件列表、接收事件的订阅者等。
/** 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 = isMainThread();
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;
}
}
}
现在继续看post逻辑。首先，获取到当前线程的发送状态postingState，然后拿到事件列表，并把需要post的事件加入到列表中。判断是否启动了事件发送流程，如果已经启动了，不在做处理，加入列表中的事件会轮到处理。如果没有启动就启动处理流程，并将isPosting赋值为true。循环从事件列表中获取事件，通过postSingleEvent进行处理。
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
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) {
logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
在postSingleEvent中，有一个变量的判断eventInheritance，这个变量表示是否查找发送事件的父类或接口的订阅者，默认是true。例如，发送事件MsgEvent这个对象，MsgEvent继承至Event这个抽象类，发送后，不光所有订阅MsgEvent的订阅者可以收到事件，所有订阅Event的这个事件的订阅者也会收到事件。前面有分析这个字段的值是可以重新配置的，代码如下。
EventBus.builder()
.eventInheritance(false) //发送的时候不考虑事件父类
.logSubscriberExceptions(false)
.build()
.register(this);
eventInheritance为true，通过lookupAllEventTypes，向上找到所有父类事件类，然后遍历找到所有事件的订阅者，并发送事件，如果为false，直接将当前事件发送给订阅者。
postSingleEvent函数主要是找到需要post的所有相关事件，然后进一步调用postSingleEventForEventType发送给订阅者，因此函数后面的逻辑是判断postSingleEventForEventType的返回值subscriptionFound，即判断当前事件有没有订阅者进行处理，如果没有处理的，会发送一个NoSubscriberEvent。例如：如果我在上一节的例子中post一个新的对象student，但是没有订阅者和接收函数，如果监听了NoSubscriberEvent，会收到一个NoSubscriberEvent的事件，告知调用者你post的对象没有订阅者。
@Subscribe()
public void onMsgEventReceived(NoSubscriberEvent event) {
Log.i(TAG, "NoSubscriberEvent : " + event);
}
Student student = new Student(1,"jane");
EventBus.getDefault().post(student);
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted;
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;
}
从前面分析可知，最后都走到了postSingleEventForEventType这个函数里面，首先在subscriptionsByEventType中找到所有订阅该事件的订阅者subscriptions，subscriptions为空或者个数是0则返回false。有订阅者的话，遍历订阅者，然后通过postToSubscription进行发送，并返回true。
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);
}
}
这段代码比较简单就是根据订阅者中注解@Subscribe中threadMode值来分别进行处理，上一节也讲过，POSTING表示在哪个线程发送就在哪个线程接收处理，因此直接调用invokeSubscriber通过反射来调用订阅者中的接收事件的方法。
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);
}
}
如果threadMode是MAIN，而当前线程是子线程，通过 mainThreadPoster.enqueue(subscription, event)将事件加入主线程队列。
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
public interface MainThreadSupport {
boolean isMainThread();
Poster createPoster(EventBus eventBus);
}
public class DefaultAndroidMainThreadSupport implements MainThreadSupport {
public boolean isMainThread() {
return Looper.getMainLooper() == Looper.myLooper();
}
@Override
public Poster createPoster(EventBus eventBus) {
return new HandlerPoster(eventBus, Looper.getMainLooper(), 10);
}
}
mainThreadPoster是由mainThreadSupport 创建的，mainThreadSupport是MainThreadSupport实例，而MainThreadSupport是一个接口，实现类为DefaultAndroidMainThreadSupport，因此，最终是调用到的DefaultAndroidMainThreadSupport中的createPoster，新建了一个HandlerPoster。
public class HandlerPoster extends Handler implements Poster {
private final PendingPostQueue queue;
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
private boolean handlerActive;
public HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
super(looper);
this.eventBus = eventBus;
this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
queue = new PendingPostQueue();
}
public void enqueue(Subscription subscription, Object event) {
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
queue.enqueue(pendingPost);
if (!handlerActive) {
handlerActive = true;
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
}
}
}
@Override
public void handleMessage(Message msg) {
...
}
}
从上面的源码可以看到，HandlerPoster就是一个Handler，当执行mainThreadPoster.enqueue(subscription, event)时，会将订阅者和事件封装成一个PendingPost，然后加入到PendingPostQueue这个队列中，如果handlerActive为true表示当前Handler正常处理事件，将入队列的事件等着被处理即可。如果为false则启动处理，调用sendMessage发送消息。
public class HandlerPoster extends Handler implements Poster {
private final PendingPostQueue queue;
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
private boolean handlerActive;
public HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
super(looper);
...
}
public void enqueue(Subscription subscription, Object event) {
...
}
@Override
public void handleMessage(Message msg) {
boolean rescheduled = false;
try {
long started = SystemClock.uptimeMillis();
while (true) {
PendingPost pendingPost = queue.poll();
if (pendingPost == null) {
synchronized (this) {
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null) {
handlerActive = false;
return;
}
}
}
eventBus.invokeSubscriber(pendingPost);
long timeInMethod = SystemClock.uptimeMillis() - started;
if (timeInMethod >= maxMillisInsideHandleMessage) {
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
rescheduled = true;
return;
}
}
} finally {
handlerActive = rescheduled;
}
}
}
在handleMessage()方法将中将PendingPost对象循环出队列，交给invokeSubscriber()方法进一步处理。这样就把线程通过Handler切回了主线程。
backgroundPoster.enqueue()和asyncPoster.enqueue也类似，内部都是先将事件入队列，然后再出队列，但是会通过线程池去进一步处理事件。
粘性事件发送postSticky源码逻辑
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);
}
从上面代码可以看到，先把事件放在了stickyEvents列中中，然后调用了post，也就是上面我们解析过的流程。让我们再来回顾下注册的代码：
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
...
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).
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);
}
}
}
注册源码分析的时候，就分析到subscriberMethod.sticky这个句上面，现在让我们看看下面的逻辑，同样先判断eventInheritance的值，然后将之前放在stickyEvents中的事件拿出来，执行checkPostStickyEventToSubscription。
private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
if (stickyEvent != null) {
// If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
// --> Strange corner case, which we don't take care of here.
postToSubscription(newSubscription, stickyEvent, isMainThread());
}
}
checkPostStickyEventToSubscription中的代码比较水，又调用了postToSubscription，这个方法上面贴出来了，这里方面看再贴一遍，熟悉的味道：
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);
}
}
所以，在注册的时候，如果你订阅者的接收方法加了sticky注解，那么在注册的时候就会看下订阅的事件之前有没有通过postSticky发送过，如果有就会立马收到这个事件。
以上是我们平时使用过程的源码解析，码字不易，喜欢就点赞收藏啊。
参考文章：
EventBus 原理解析