最近去維護另外一個項目，各種庫使用的都比較老，eventbus使用的是2.x版本，於是來個升級，順便讀下eventbus的源碼,在此做個筆記:
EventBus2.x升級3.x
2.x與3.x之間的對應關係：
onEvent--註解ThreadMode.POSTING;
onEventMainThread--註解ThreadMode.MAIN
onEventAsync--註解ThreadMode.BACKGROUND
onEventBackgroundThread--註解ThreadMode.ASYNC
1.包名替換
Eventbus升級之後包名變了，可以ctrl+shift+r全局替換，也可以給studio設置自動導入包名將
import de.greenrobot.event.EventBus;替換成 import org.greenrobot.eventbus.EventBus; 全部替換.
2.方法需要使用註解方式
ctrl+shift+f 全局搜索, 依次添加上面提到的4個註解方法.
例如對onEvent方法,搜索"public void onEvent (" 添加"@Subscribe(threadMode = ThreadMode.POSTING)"
3.EventBus 3.0版本去掉了registerSticky, 換成到每個方法的註解中配置sticky = true. 搜索".registerSticky(", registerSticky替換回register, 再在該類下的方法註解上添加sticky.
EventBus源碼解讀：
1.EventBus的構造方法
/** Convenience singleton for apps using a process-wide EventBus instance. */
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
這裏的單利模式用了雙重檢查模式，接下來看下EventBus的構造方法都做了什麼：
...
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
...
public EventBus() {
this(DEFAULT_BUILDER);
}
this調用了eventbus的另一個構造方法：
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;
}
這裏採用建造者模式，通過構造一個EventbusBuilder來對Eventbus進行配置
2.訂閱註冊
獲取到Eventbus後，就可以將訂閱者註冊到Eventbus中，註冊方法如下：
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
//查找訂閱者（傳進來的subscriber）的所有訂閱方法的集合
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
//遍歷訂閱者的方法，完成註冊
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
1）查找訂閱者的訂閱方法
可以看到，註冊方法做了兩件事：①查找訂閱者中所有訂閱方法，②訂閱者的註冊。SubscriberMethod類主要用來保存訂閱方法Method對象、線程模式（threadMode）、事件類型（eventType）、優先級（priority）、是否粘性事件（sticky）等。findSubscriberMethods如下：
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//檢查緩存中是否有
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
```
//判斷是否忽略註解器生成的MyEventBusIndex（參考http://greenrobot.org/eventbus/documentation/subscriber-index/）
//默認false，可以通過EventbusBuilder來設置。因此我們使用通常使用的是findUsingInfo方法來獲取訂閱方法集合
```
if (ignoreGeneratedIndex) {
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 {
//將訂閱方法集合放入緩存，下次直接取用，避免重複查找
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
接下來看下findUsingInfo方法：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//獲取訂閱者信息
findState.subscriberInfo = getSubscriberInfo(findState);
//如果我們通過EventBuilder配置了MyEventBusIndex,便會獲取到subscriberInfo
if (findState.subscriberInfo != null) {
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
//沒有通過EventBuilder配置MyEventBusIndex，調用此方法將訂閱信息保存到findState，默認沒有配置，走此方法
} else {
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
//對findState做回收處理，並返回訂閱方法的集合
return getMethodsAndRelease(findState);
}
more沒有配置MyEventBusIndex，下面看下findUsingReflectionInSingleClass方法：
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
//通過反射獲取訂閱者中的所有方法，這個方法比getMethods()快，特別是代碼比較多的類如Activity
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
for (Method method : methods) {
//獲取方法的修飾符，eg. public
int modifiers = method.getModifiers();
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();
//將訂閱方法相關信息保存到findstate中
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");
}
}
}
至此，查找訂閱者的訂閱方法結束。
2）訂閱者的註冊過程
查找完訂閱者的訂閱方法之後，就開始對再所有訂閱者方法進行註冊，回到register方法中，看那裏調用subscribe方法：
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//訂閱者訂閱的方法的類型
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根據eventType獲取訂閱對象集合，如果爲空則重新創建
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);
}
}
}
subscribe主要做了兩件事，①將Subscription根據evntType封裝到subscriptionsByEventType中，將subscribedEvents根據subscriber封裝到typeBySubscriber中；②對粘性事件的處理。
3.事件發送
在獲取到Eventbus對象之後，通過post方法，完成對事件的發送：
/** Posts the given event to the event bus. */
public void post(Object event) {
//PostingThreadState 保存着事件隊列（eventQueue）和線程狀態信息（isPosting，isMainThread，subscription，event，canceled）
PostingThreadState postingState = currentPostingThreadState.get();
//獲取事件隊列，並將當前事件插入事件隊列
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 {
//處理隊列中的事件
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
看看postSingleEvent做了什麼：
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//是否向上查找父類，默認true，可以通過EventBusBuilder配置
if (eventInheritance) {
//查找所有註冊事件
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));
}
}
}
通過postSingleEventForEventType方法對事件進行處理：
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//取出事件對應的訂閱對象集合
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
//將事件event和對應的Subscription（訂閱對象）傳遞給postingState，通過調用postToSubscription方法對事件進行處理
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;
}
看下postToSubscription做了什麼：
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);
}
}
根據訂閱方法的線程模式，分別處理，如果是Main，如果提交的線程是主線程，則通過反射直接運行訂閱的方法，若不是主線程，需要mainThreadPoster將我們的訂閱事件添加到主線程隊列中。mainThreadPoster繼承自Handler，通過Handler將訂閱方法切換到主線程。
至此，事件發送結束。
4.訂閱者取消註冊
取消註冊是獲取Eventbus對象之後，調用unregieter方法。
/** 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());
}
}
/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object
typesBySubscriber是一個map集合，在訂閱的時候使用過。先通過subscriber找到subscribedTypes集合，然後將subscriber對應的eventTpye從typesBySubscriber中移除。遍歷subscribedTypes並調用unsubscribeByEventType方法：
/** 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--;
}
}
}
}
從subscriptions移除Subscriptions中移除，完成取消訂閱。
參考資料：
1.https://www.jianshu.com/p/e28e1692d0c7
2.https://github.com/greenrobot/EventBus
3.《Android進階之光》（劉望舒）第七章