EventBus框架原理解析(结合源码)(下)

上一篇文章EventBus框架原理解析(结合源码)(上),给大家讲述了EventBus中实体类的封装和register()的具体代码。

接下来我们看另外一个重要方法post(),这个方法显然是要根据传入的参数类型,从subscriptionsByEventType取出对应的subscription

 /** Posts the given event to the event bus. */
    public void post(Object event) {//event就是参数
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);//将参数加入队列

        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;
            }
        }
    }

整个post方法的执行主体是postSingleEvent(),我们看到,这里传入了event

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();//参数类型
        boolean subscriptionFound = false;
        if (eventInheritance) {
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);//查找父类
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {//遍历
                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));
            }
        }
    }
首先调用了lookupAllEventTypes()方法

/**
     * Looks up all Class objects including super classes and interfaces. Should also work for interfaces.<br>
     * 根据参数类型,查找其父类类型和接口类型
     */
    private List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
        synchronized (eventTypesCache) {
            List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
            if (eventTypes == null) {
                eventTypes = new ArrayList<Class<?>>();
                Class<?> clazz = eventClass;
                while (clazz != null) {
                    eventTypes.add(clazz);
                    addInterfaces(eventTypes, clazz.getInterfaces());
                    clazz = clazz.getSuperclass();
                }
                eventTypesCache.put(eventClass, eventTypes);
            }
            return eventTypes;
        }
    }

这个方法可以根据参数类型,找到其父类类型和接口类型

为什么要找到父类类型呢?这也是为了继承需要,也就是说如果一个类本身没有订阅方法,而它的父类有,那么它也应该有这个订阅方法了,我们要把它找出来。

可以看到,这里做了一个缓存机制,就是eventTypesCache

 /**
     * key为参数类型,value为参数类型及其父类型,接口类型列表
     */
    private static final Map<Class<?>, List<Class<?>>> eventTypesCache = new HashMap<Class<?>, List<Class<?>>>();


找出来以后,遍历所有类型,调用了postSingleEventForEventType()方法

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(eventClass);//找到该类型下所有订阅
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {/遍历订阅
            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;
    }
如上,使用subscriptionsByEventType找到参数类型(及其父类)的所有subscription,调用postSubscription()方法,如下:

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case PostThread:
                invokeSubscriber(subscription, event);
                break;
            case MainThread:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            case BackgroundThread:
                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);
        }
    }
在这个方法里面,我们可以看到,我们根据threadMode,将subscription和event交给不同Poster去处理

这时我们知道,有了subscription和event,我们就有了触发反射的所有条件,不同的Poster只是为了在不同线程中调用反射而已。


这里我只举其中一个poster来做说明,看HandlerPoster,这个是为了在主线程中调用反射

final class HandlerPoster extends Handler {

    private final PendingPostQueue queue;
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;

    HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
        super(looper);
        this.eventBus = eventBus;
        this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
        queue = new PendingPostQueue();
    }

    void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
    }

    @Override
    public void handleMessage(Message msg) {
        boolean rescheduled = false;
        try {
            long started = SystemClock.uptimeMillis();
            while (true) {
                PendingPost pendingPost = queue.poll();
                if (pendingPost == null) {
                    synchronized (this) {
                        // Check again, this time in synchronized
                        pendingPost = queue.poll();
                        if (pendingPost == null) {
                            handlerActive = false;
                            return;
                        }
                    }
                }
                eventBus.invokeSubscriber(pendingPost);
                long timeInMethod = SystemClock.uptimeMillis() - started;
                if (timeInMethod >= maxMillisInsideHandleMessage) {
                    if (!sendMessage(obtainMessage())) {
                        throw new EventBusException("Could not send handler message");
                    }
                    rescheduled = true;
                    return;
                }
            }
        } finally {
            handlerActive = rescheduled;
        }
    }
}

看到enqueue()方法,其实就是将subscription和event包装成PendingPost

final class PendingPost {
    private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>();

    Object event;
    Subscription subscription;
    PendingPost next;

    private PendingPost(Object event, Subscription subscription) {
        this.event = event;
        this.subscription = subscription;
    }

    static PendingPost obtainPendingPost(Subscription subscription, Object event) {
        synchronized (pendingPostPool) {
            int size = pendingPostPool.size();
            if (size > 0) {
                PendingPost pendingPost = pendingPostPool.remove(size - 1);
                pendingPost.event = event;
                pendingPost.subscription = subscription;
                pendingPost.next = null;
                return pendingPost;
            }
        }
        return new PendingPost(event, subscription);
    }

    static void releasePendingPost(PendingPost pendingPost) {
        pendingPost.event = null;
        pendingPost.subscription = null;
        pendingPost.next = null;
        synchronized (pendingPostPool) {
            // Don't let the pool grow indefinitely
            if (pendingPostPool.size() < 10000) {
                pendingPostPool.add(pendingPost);
            }
        }
    }

}
其中pendingPostQueue是起到缓存池的作用。

重要的是handlerMessage(),这个方法从队列中取出一个pendingPost,然后调用EventBus的invokeSubscriber()方法来执行反射

void   invokeSubscriber(PendingPost pendingPost) {
        Object event = pendingPost.event;
        Subscription subscription = pendingPost.subscription;
        PendingPost.releasePendingPost(pendingPost);//释放缓存
        if (subscription.active) {
            invokeSubscriber(subscription, event);
        }
    }

    void invokeSubscriber(Subscription subscription, Object event) {
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);//触发反射!
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }
这个方法很简单,就是触发了反射,在注释中我们可以看到触发反射的位置


有人会问为什么需要队列,因为同一个post()方法,可能有很多订阅者订阅了这个信息,而这些订阅是有优先级的,handler对于消息的执行,也是在一个队列里面,逐个取出的,所以不可能一次性执行所有的操作。


文章的最后,我说明一下sticky这个属性的作用,EventBus里面,还有一个stickyPost()方法

public void postSticky(Object event) {
        synchronized (stickyEvents) {
            stickyEvents.put(event.getClass(), event);
        }
        // Should be posted after it is putted, in case the subscriber wants to remove immediately
        post(event);
    }
可以看到,里面还是调用了post()方法,但是在这之前,将参数event的类型,和其本身都放入stickyEvents这个map里面保存了起来

/**
     * 可以看做缓存,key参数类型,value是参数
     */
    private final Map<Class<?>, Object> stickyEvents;
其实这就是一个缓存的作用,为什么要保存呢?


还记得EventBus有一个subscribe()方法里面,有关于sticky的判断吗?

if (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);
            }
        }
如果是stickyregister(),那么就会进入这个判断里面

里面stickyEvents会根据参数类型,获得了之前postSticky()传递的参数,然后调用checkPostStickyEventToSubscription()方法

private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
        if (stickyEvent != null) {
            // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
            // --> Strange corner case, which we don't take care of here.
            postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
        }
    }

可以看到,调用了postToSubscription(),也就是说,马上post了。

在register()以后,如果是sticky,马上就转到post(),而这次的参数,是上次postSticky()保留下来的。

通过这种方式,我们可以获得最近一次post()传递的消息。

sticky的作用就是这样。

OK,整个框架解析得差不多了,有问题可以留言。

转载请注明出处。

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