EventBus是目前比較常用的數據通訊方案,如何使用有很多文章介紹,官網也介紹的很詳細,今天就對其框架實現原理做個簡單梳理。
以下爲官方GitHub地址:
https://github.com/greenrobot/EventBus
1.使用前首先要註冊
EventBus.getDefault().register(this);
先進入EventBus的register方法查看(此方法重點邏輯是把將註冊類含有subscriber註解的方法和註冊類以鍵值對的方式加入subscriptionsByEventType中):
public void register(Object subscriber) { Class<?> subscriberClass = subscriber.getClass(); //找到此類含有subscriber註解的所有方法 List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); synchronized (this) { for (SubscriberMethod subscriberMethod : subscriberMethods) { //將含有subscriber註解的方法和對應類添加至 subscriptionsByEventType subscribe(subscriber, subscriberMethod); } } }
subscriber參數一般是我們傳入的activity,重點看subscriberMethodFinder.findSubscriberMethods()方法
/** * 找到activity中添加註解的方法 ,並添加至METHOD_CACHE * @param subscriberClass 註冊的類 * @return 找到activity中添加註解的方法 */ 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); //返回activity中添加註解的方法 return subscriberMethods; } }
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) { FindState findState = prepareFindState(); findState.initForSubscriber(subscriberClass); while (findState.clazz != null) { //將有註解的方法添加至 findState.subscriberMethods findUsingReflectionInSingleClass(findState); findState.moveToSuperclass(); } //返回List<SubscriberMethod> subscriberMethods return getMethodsAndRelease(findState); }
private List<SubscriberMethod> getMethodsAndRelease(FindState findState) { List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods); findState.recycle(); synchronized (FIND_STATE_POOL) { for (int i = 0; i < POOL_SIZE; i++) { if (FIND_STATE_POOL[i] == null) { FIND_STATE_POOL[i] = findState; break; } } } return subscriberMethods; }
第一步主要是將註冊類與其對應註解的方法放置緩存SubscriberMethodFinder類的
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
變量中,這裏有Subscribe註解(用於接收數據方法的標識)和SubscriberMethod對象(用於存儲添加了註解的方法),以下爲對應源碼:
@Documented @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.METHOD}) public @interface Subscribe { ThreadMode threadMode() default ThreadMode.POSTING; /** * If true, delivers the most recent sticky event (posted with * {@link EventBus#postSticky(Object)}) to this subscriber (if event available). */ boolean sticky() default false; /** Subscriber priority to influence the order of event delivery. * Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before * others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of * delivery among subscribers with different {@link ThreadMode}s! */ int priority() default 0; }
public class SubscriberMethod { final Method method; final ThreadMode threadMode; final Class<?> eventType; final int priority; final boolean sticky; /** Used for efficient comparison */ 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; } @Override public boolean equals(Object other) { if (other == this) { return true; } else if (other instanceof SubscriberMethod) { checkMethodString(); SubscriberMethod otherSubscriberMethod = (SubscriberMethod)other; otherSubscriberMethod.checkMethodString(); // Don't use method.equals because of http://code.google.com/p/android/issues/detail?id=7811#c6 return methodString.equals(otherSubscriberMethod.methodString); } else { return false; } } private synchronized void checkMethodString() { if (methodString == null) { // Method.toString has more overhead, just take relevant parts of the method StringBuilder builder = new StringBuilder(64); builder.append(method.getDeclaringClass().getName()); builder.append('#').append(method.getName()); builder.append('(').append(eventType.getName()); methodString = builder.toString(); } } @Override public int hashCode() { return method.hashCode(); } }
2.發送數據
EventBus.getDefault().post(new MessageEvent());
/** 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; } } }
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)); } } }
post方法最終調用了以下方法(主要邏輯時通過註冊存儲的subscriptionsByEventType獲取到含有註解的方法,調用invoke方法傳的所傳的event)
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 = 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; } 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); } }
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); } }
3.接收參數
@Subscribe(threadMode = ThreadMode.MAIN) public void onMessageEvent(MessageEvent event) {/* Do something */};此處主要是添加註解,用於rigister()註冊時查找到此方法並保存在內存中subscriptionsByEventType,在post()發送數據後,通過保存在內存的subscriptionsByEventType中拿到此方法並將數據傳遞,達到效果。