由浅入深了解EventBus:(五)

事件分发

EventBus3.0的事件的分发时通过EventBus类中的post(粘性事件为postSticky)方法，post与postSticky的唯一区别就是，在postSticky内部首先会向EventBus类中的stickyEvents集合中添加事件类实例，然后在调用post方法；post方法的参数就是我们自己定义的事件类的实例；

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;
            }
        }
    }

在事件分发之前，首先通过currentPostingThreadState内获取PostingThreadState;currentPostingThreadState是一个ThreadLocal<PostingThreadState>,我们知道ThreadLocal在java是保存每个线程中独立的数据,一般情况下，通过ThreadLocal.set() 到线程中的对象是该线程自己使用的对象，其他线程是不需要访问的，也访问不到的;而ThreadLocal存储的PostingThreadState类,就是个信息类;

    final static class PostingThreadState {
        final List<Object> eventQueue = new ArrayList<Object>();//事件类队列
        boolean isPosting;
        boolean isMainThread;//是否是主线程
        Subscription subscription;
        Object event;//事件类实例
        boolean canceled;
    }

获取到当前线程的PostingThreadState，并将需要处理的事件类实例添加到PostingThreadState的eventQuenue队列中;如果当前线程的Looper与主线程的Looper一致，这PostingThreadState的isMainThread为true;而PostingThreadState类中的isPosting来判断是否正在进行分发;最后通过postSingleEvent方法循环进行数据的分发处理;

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {//是否触发订阅了该事件的基类以及接口类的相应方法
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);//查找event类所有的基类以及接口
            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方法首先通过lookupAllEventTypes方法查找出所有的event类中所有的基类以及接口;然后对这些循环进行分发;

    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方法中我们可以看到我们从EventBus类中subscriptionsByEventType中取出所有匹配的Subscription;并将Subscription实例中的event,subscription赋值给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种线程模型,当线程模型为POSTING 时，直接执行invokeSubscriber方法，效率也最快;而invokeSubscriber方法也很简单，就是反射中的invoke方法;

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