Android EventBus事件釋出流程淺析
如上圖所示,簡而言之,EventBus是一個用於Android/Java的釋出/訂閱事件匯流排,可以簡化元件間的資料傳輸。它的優勢在於解耦事件傳送者和接受者,在Activities、Fragments以及後臺執行緒和主執行緒間的資料傳輸方面表現不錯,避免了複雜和容易出錯的依賴關係和生命週期問題。
從原始碼分析的角度來看,EventBus的資料傳輸機制,主要是依賴於Java反射機制和自定義註解處理器。對於後者,本章暫且按下不表,集中精力分析EventBus是如何通過反射機制傳輸資料的。
一、EventBus的例項獲取
/** Convenience singleton for apps using a process-wide EventBus instance. */ 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; }
public EventBus() {
this(DEFAULT_BUILDER);
}
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
從上述程式碼可知,EventBus的getDefault()方法就是一個採用了單例模式的同步方法,同時分析EventBus的構造器方法可知,EventBus內部的一些引數屬性是由EventBusBuilder這個類物件來負責定義的。
public class EventBusBuilder { private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool(); ... MainThreadSupport mainThreadSupport; ... MainThreadSupport getMainThreadSupport() { if (mainThreadSupport != null) { return mainThreadSupport; } else if (AndroidLogger.isAndroidLogAvailable()) { Object looperOrNull = getAndroidMainLooperOrNull(); return looperOrNull == null ? null : new MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull); } else { return null; } } //獲取主執行緒Looper Object getAndroidMainLooperOrNull() { try { return Looper.getMainLooper(); } catch (RuntimeException e) { // Not really a functional Android (e.g. "Stub!" maven dependencies) return null; } } public EventBus installDefaultEventBus() { synchronized (EventBus.class) { if (EventBus.defaultInstance != null) { throw new EventBusException("Default instance already exists." + " It may be only set once before it's used the first time to ensure consistent behavior."); } EventBus.defaultInstance = build(); return EventBus.defaultInstance; } } public EventBus build() { return new EventBus(this); } }
針對EventBusBuilder類,可以簡單過一遍,主要是設定了一些屬性和生成一些工具,暫時知道它是配合EventBus的構造器方法就好。接著,咱們將注意力轉向register方法。
二、EventBus的register方法流程
public void register(Object subscriber) { Class<?> subscriberClass = subscriber.getClass(); List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); //獲取訂閱者方法 synchronized (this) { for (SubscriberMethod subscriberMethod : subscriberMethods) { subscribe(subscriber, subscriberMethod); } } }
// 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);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions); //儲存事件型別和訂閱物件們
} else {
...
}
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) { //對粘性事件的處理
...
}
}
Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
register方法其實就做了兩件事,即獲取訂閱者的訂閱方法、儲存訂閱者和訂閱者方法以及事件型別等等資訊。首先通過findSubscriberMethods方法獲取訂閱者的訂閱方法,接著通過遍歷,呼叫subsribe訂閱方法,儲存訂閱資訊。
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
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 {
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
...
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 {
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
由上述程式碼可知,findSubscriberMethods方法是由findUsingReflectionInSingleClass方法具體實現的,接下來就是重點了。
private void findUsingReflectionInSingleClass(FindState findState) { //利用反射獲取訂閱者方法
Method[] methods;
try {
methods = findState.clazz.getDeclaredMethods(); //獲取本類中的所有方法
} catch (Throwable th) {
...
}
for (Method method : methods) {
int modifiers = method.getModifiers(); //返回此類或介面以整數編碼的 Java語言修飾符(PUBLIC、PRIVATE...)
//是否屬於PUBLIC方法且不屬於抽象、靜態等方法
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();
//儲存訂閱方法資訊
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
//處於嚴格方法驗證模式以及註解無誤,但是缺少方法引數時
} 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");
}
}
}
public class SubscriberMethod {
final Method method;
final ThreadMode threadMode;
final Class<?> eventType;
final int priority;
final boolean sticky;
String methodString;
public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky)
{
this.method = method;
this.threadMode = threadMode;
this.eventType = eventType;
this.priority = priority;
this.sticky = sticky;
}
...
}
由上述程式碼可知,通過findSubscriberMethods方法,利用反射機制,將訂閱者方法、傳遞的事件型別、優先順序、是否為粘性事件等資訊都儲存好了。小結一下,其實register方法就是用來獲取訂閱者的訂閱方法、儲存訂閱資訊的。
三、Unregister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());
}
}
unregister方法就是用來取消訂閱的,就不再贅述細節了。
四、Post方法
EventBus是通過Post方法進行事件傳遞(釋出)的,在此對post方法原始碼進行簡單分析。
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);
...
while (!eventQueue.isEmpty()) { postSingleEvent(eventQueue.remove(0), postingState); } //核心方法
...
}
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) { //是否釋出有繼承關係型別的事件
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); //核心方法
}
...
}
//傳遞單獨事件的方法
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass); //獲取訂閱物件集合
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
postToSubscription(subscription, event, postingState.isMainThread); //核心方法
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}
從Post方法的內部邏輯分析,post方法主要是對eventQueue進行遍歷,然後呼叫postSingleEvent方法進行處理;接著postSingleEvent方法通過遍歷事件型別eventTypes,再呼叫postSingleEventForEventType方法,通過遍歷訂閱物件集合subscriptions, 將subscription訂閱物件, event事件, postingState.isMainThread是否為主執行緒等引數傳遞給postToSubscription方法。而postToSubscription方法根據不同的執行緒模式threadMode對subsription和event進行不同儲存操作,最終呼叫invokeSubscriber方法,即利用反射機制呼叫處理訂閱方法。
//將事件傳遞給訂閱者
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) { //根據訂閱者設定的執行緒模式threadMode進行處理
case POSTING:
invokeSubscriber(subscription, event); //通過反射機制呼叫處理訂閱方法
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);
}
}
void invokeSubscriber(Subscription subscription, Object event) { //利用反射機制呼叫處理訂閱方法
try {
subscription.subscriberMethod.method.invoke(subscription.subscriber, event); //利用反射機制呼叫處理訂閱方法
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
事實上,invokeSubscriber方法就是處理訂閱方法被呼叫的關鍵程式碼,是利用java反射機制呼叫類方法的一個例項。至此,EventBus的普通事件釋出處理流程簡單地被梳理了一遍。至於粘性等事件的釋出流程,本章並未涉及。