前言
EventBus是Android的发布/订阅事件总线，简化了Android的事件传递。提高了代码的简洁性。
官方图.png
1、EventBus的简单使用
public class MainActivity extends BaseActivity {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//注册
EventBus.getDefault().register(this);
}
@Subscribe(threadMode = ThreadMode.MAIN)
public void receiveEventBusMessage(EventBusMessage message){
Log.e("receive", "receiveEventBusMessage: "+message.getText());
}
@Override
protected void onDestroy() {
super.onDestroy();
//解注册
EventBus.getDefault().unregister(this);
}
}
//可以在其它组件、fragment或工作线程中发送事件
private void sendMessage(){
EventBus.getDefault().post(new EventBusMessage("消息："+ new Random().nextInt(50)));
}
从上面代码可以发现，EventBus实现事件传递有多简洁，无任何花哨。首先，通过register()进行注册，这是使用EventBus的前提；然后声明一个订阅方法用于事件的接收(利用@Subscribe注解)，threadMode指定线程模式(在这里指定于主线程)，其中EventBusMessage是自己定义一个事件类；再然后通过在其它组件、Fragment或工作线程中使用post()发送事件，这样就完成了事件的传递。当组件销毁或者EventBus不再使用时千万别忘记了unregister()解注册。这很重要！！！
2、经过上面的一个简单分析，初步的了解了EventBus的使用。那么，看似简单的几个步骤，里面究竟蕴含了什么内功心法或者招式呢？（故事说起来就长了）前排准备瓜子顺带点小酒。接下来从EventBus.getDefault()开始：
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
//EventBusBuilder以建造者模式初始化一以下属性
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>(); //订阅的事件类型
typesBySubscriber = new HashMap<>(); //订阅类型
stickyEvents = new ConcurrentHashMap<>(); //粘性事件
mainThreadSupport = builder.getMainThreadSupport(); //主线程接口类
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
backgroundPoster = new BackgroundPoster(this); //在后台提交的事件
asyncPoster = new AsyncPoster(this);
indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
//查找订阅方法的类
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;
//获取一个线程池
executorService = builder.executorService;
}
很清晰，EventBus.getDefault()利用了单例模式中的双重校验方式获取一个EventBus实例。在构造函数最后一步通过Executors.newCachedThreadPool()创建一个线程池（主要在后台任务或异步任务时使用），对于线程池的创建可以了解这篇文章【探索Java 线程池】
3、EventBus的注册过程(register)。
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
//获取一个订阅方法List，包含了所订阅的类声明的订阅方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
在上面的代码中可以知道，做了两件事，
1、查找订阅的方法，SubscriberMethodFinder # findSubscriberMethods()
2、进行订阅。subscribe()
3.1、是如何查找已声明的订阅方法？进入SubscriberMethodFinder类的indSubscriberMethods()方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//1
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) { //2
subscriberMethods = findUsingReflection(subscriberClass);
} 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 { //3
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
在上面代码1处，首先会从缓存中查找，subscriberClass正是调用register()注册的订阅者，METHOD_CACHE是一个ConcurrentHashMap的实例，从这里可以知道，subscriberClass作为ConcurrentHashMap的key，而ConcurrentHashMap的value（List<SubscriberMethod>）存放的是当前订阅者中声明的订阅方法。注释2处表示是否忽略索引的位置，默认为false，因此会在通过findUsingInfo()查找，注释3处把找到的方法保存到METHOD_CACHE中。一起研究findUsingInfo()方法：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
//从对象池中寻找FindState实例（数组的形式实现对象池）
FindState findState = prepareFindState();
//初始化需要寻找的方法的所属类
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//找到后，获取订阅者的信息
findState.subscriberInfo = getSubscriberInfo(findState);
//开始保存获取的订阅方法
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 {
//如果上面没有找到，则通过反射、注解的方式寻找
findUsingReflectionInSingleClass(findState);
}
//移动到父类
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState); //找到方法后释放findState
}
经过上面代码注释，对寻找订阅方法的步骤已经很清晰了，下面贴出getSubscriberInfo()和findUsingReflectionInSingleClass()两个方法：
private SubscriberInfo getSubscriberInfo(FindState findState) {
if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
//判断是否是需要寻找的具体类
if (findState.clazz == superclassInfo.getSubscriberClass()) {
return superclassInfo;
}
}
//通过订阅信息索引进行查找
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
//遍历订阅者中的所有方法
for (Method method : methods) {
int modifiers = method.getModifiers();
//对订阅方法的作用域进行判断
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
Class<?> eventType = parameterTypes[0];
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");
}
}
}
3.2、是如何进行订阅的？subscribe()
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
//通过订阅者和订阅方法创建一个订阅事件
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根据eventType从缓存中获取subscriptions
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
//若缓存中没有，则添加到缓存
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);
}
}
//遍历订阅事件，根据优先级重新排序
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;
}
}
//这里根据订阅者查找EventType缓存
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//对粘性进行处理
if (subscriberMethod.sticky) {
if (eventInheritance) { //同时考虑子类
// 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);
}
}
}
上面的代码，就是整个的订阅过程了，比较关键的两个东西subscriptionsByEventType和typesBySubscriber，首先把订阅者和订阅方法封装进subscriptionsByEventType，当post事件时根据eventType查找具体的订阅者，然后进行处理。而typesBySubscriber存放的订阅者，在解注册时根据订阅者查找对应的eventType。在最后，会判断是否是粘性事件，如果是，则立刻进行处理。
private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
if (stickyEvent != null) {
//post事件是最终也调用这个方法，所以在下一步进行分析
postToSubscription(newSubscription, stickyEvent, isMainThread());
}
}
4、发送事件 post()
public void post(Object event) {
//每个线程都有一个提交事件的状态
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event); //把提交的事件对象添加到eventQueue
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
// 2
try {
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
从上面代码可以发现，首先根据PostingThreadState确定当前线程的提交状态，从PostingThreadState获取eventQueue，用于存放提交的事件；然后会在注释2处，对事件进行遍历，直到eventQueue内部元素为空。
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//考虑子类
if (eventInheritance) {
//查找所有的EventType
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
//遍历，对eventTypes内部逐一处理
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));
}
}
}
在上面的代码中，先查找出所有的EventType，然后通过postSingleEventForEventType()逐个逐个的处理，postSingleEventForEventType()内部又调用了postToSubscription()。
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);
}
}
分析到这里，可以知道，会根据在最初声明的订阅方法中标识的threadMode进行对应处理，这里就选择文章第一步中的MAIN标识进行分析。
1、isMainThread为true，表示在主线程中执行，直接调用invokeSubscriber()。
2、若不在主线程执行，调用mainThreadPoster.enqueue()，mainThreadPoster实际是HandlerPoster(继承了Handler)类对象的引用。
//HandlerPoster # enqueue()
public void enqueue(Subscription subscription, Object event) {
//把订阅者和提交的事件封装在pendingPost
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
//内部利用了单向链表的方式进行摆放
queue.enqueue(pendingPost);
if (!handlerActive) {
handlerActive = true;
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
}
}
}
//HandlerPoster # handleMessage()
@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;
}
}
根据上面HandlerPoster中的两段代码发现，最终是以Handler消息机制的方式进行处理事件，其中HandlerPoster的Looper是在初始化EventBus的时候获取了主线程中的Looper对象，在MainThreadSupport的实现类AndroidHandlerMainThreadSupport中。最后在handleMessage()中轮询queue，queue是PendingPostQueue的一个实例。调用poll()获取pendingPost，再调用eventBus.invokeSubscriber()进行处理。
synchronized PendingPost poll() {
PendingPost pendingPost = head;
//从链表头部head开始，取完后就移除，下一个链节点作为head，如此循环
if (head != null) {
head = head.next;
if (head == null) {
tail = null;
}
}
return pendingPost;
}
//从链表中获取消息后的处理
void invokeSubscriber(PendingPost pendingPost) {
Object event = pendingPost.event;
Subscription subscription = pendingPost.subscription;
PendingPost.releasePendingPost(pendingPost); //释放PendingPost
if (subscription.active) {
invokeSubscriber(subscription, event);
}
}
在（threadMode == MAIN）时，经过上面的分析可以知道，不管是在主线程调用，还是工作线程中(通过Handler机制处理消息)，都是调用了EventBus # invokeSubscriber(Subscription subscription, Object event) 这个方法。
void invokeSubscriber(Subscription subscription, Object event) {
try {
//通过反射进行处理
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
5、解注册unregister()
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());
}
}
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--;
}
}
}
}
解注册的过程就比较简单了，还记得提交事件时讲到的typesBySubscriber吗？里面以键值对的形式存储subscriber和eventType的List。因此，在这根据订阅者subscriber找到eventType的List，然后遍历，进行释放。
完篇
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