由淺入深瞭解EventBus:(五)

事件分發

   EventBus3.0的事件的分發時經過EventBus類中的post(粘性事件爲postSticky)方法，post與postSticky的惟一區別就是，在postSticky內部首先會向EventBus類中的stickyEvents集合中添加事件類實例，而後在調用post方法；post方法的參數就是咱們本身定義的事件類的實例；

post

 public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);

        if (!postingState.isPosting) {
            postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
            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;
            }
        }
    }

    在事件分發以前，首先經過currentPostingThreadState內獲取PostingThreadState;currentPostingThreadState是一個ThreadLocal<PostingThreadState>,咱們知道ThreadLocal在java是保存每一個線程中獨立的數據,通常狀況下，經過ThreadLocal.set() 到線程中的對象是該線程本身使用的對象，其餘線程是不須要訪問的，也訪問不到的;而ThreadLocal存儲的PostingThreadState類,就是個信息類;

    final static class PostingThreadState {
        final List<Object> eventQueue = new ArrayList<Object>();//事件類隊列
        boolean isPosting;
        boolean isMainThread;//是不是主線程
        Subscription subscription;
        Object event;//事件類實例
        boolean canceled;
    }

 

    獲取到當前線程的PostingThreadState，並將須要處理的事件類實例添加到PostingThreadState的eventQuenue隊列中;若是當前線程的Looper與主線程的Looper一致，這PostingThreadState的isMainThread爲true;而PostingThreadState類中的isPosting來判斷是否正在進行分發;最後經過postSingleEvent方法循環進行數據的分發處理;

  

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {//是否觸發訂閱了該事件的基類以及接口類的相應方法
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);//查找event類全部的基類以及接口
            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) {
                Log.d(TAG, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

     postSingleEvent方法首先經過lookupAllEventTypes方法查找出全部的event類中全部的基類以及接口;而後對這些循環進行分發;

    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;
    }

在postSingleEventForEventType方法中咱們能夠看到咱們從EventBus類中subscriptionsByEventType中取出全部匹配的Subscription;並將Subscription實例中的event,subscription賦值給postingState;

    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 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);
        }
    }

 在postToSubscription方法中，subscription.subscriberMethod.threadMode 4種線程模型,當線程模型爲POSTING 時，直接執行invokeSubscriber方法，效率也最快;而invokeSubscriber方法也很簡單，就是反射中的invoke方法;

   subscription.subscriberMethod.method.invoke(subscription.subscriber, event);