Analysis of eventbus source code

summary

This article is suitable for EventBus have interest in , Or have been right EventBus Readers with certain use experience .
If the reader has not used it before EventBus, It is recommended to read the author's previous article ：

android Why use EventBus

register

Key logic ：

• Traverse the currently registered class , Get which used eventBus Method of annotation .

• Register these methods to two HashMap in , Namely subscriptionsByEventType and typesBySubscriber. adopt synchronized Lock , To ensure thread safety .

• subscriptionsByEventType：key yes eventType,value yes List
  typesBySubscriber:key Is a registered object ,value yes List

• Method execution , Will judge whether there is stcky event , If any, it will trigger directly .

    public void register(Object subscriber) {
    
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        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);
        } else {
    
            if (subscriptions.contains(newSubscription)) {
    
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        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) {
    
            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);
            }
        }
    }

post

Key logic ：

• post The source code does two things , The first thing is to change the current event Add to eventQueue in . The second thing is to change the current thread to posting state .
• posting The status will be processed circularly eventQueue Medium event, Put it in the corresponding subscription In the implementation of .
  – lookup subscription The process is ： First find and event Related to the class , Then traverse these classes and their related subscription.

    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;
            }
        }
    }

    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) {
    
                Log.d(TAG, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
    
                post(new NoSubscriberEvent(this, 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 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;
    }

postSticky

Key logic ：

• And post comparison ,postSticky Will be will be event Add to stickyEvents This Map in .
• in front register The logic in has been mentioned , When a class is registered , Will judge whether there is stickyEvent, If any, it will trigger directly .

    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);
    }

Multithreaded logic

Key logic ：

• Finally trigger subscription when , Will be in postToSubscription Select the thread to execute .
• POSTING: Trigger directly on the current thread
• MAIN： If the current thread is the main thread , Then trigger directly . If you are not currently in the main thread , Then it will pass handler Throw it to the main thread to execute .
• BACKGROUND: If it is currently a child thread , Then trigger directly . If you are not currently in a child thread , Then it will be thrown to eventBus In the thread pool .
• ASYNC： Throw to eventBus In the thread pool .

    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);
        }
    }

final class BackgroundPoster implements Runnable {
    

    private final PendingPostQueue queue;
    private final EventBus eventBus;

    private volatile boolean executorRunning;

    BackgroundPoster(EventBus eventBus) {
    
        this.eventBus = eventBus;
        queue = new PendingPostQueue();
    }

    public void enqueue(Subscription subscription, Object event) {
    
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
    
            queue.enqueue(pendingPost);
            if (!executorRunning) {
    
                executorRunning = true;
                eventBus.getExecutorService().execute(this);
            }
        }
    }

    @Override
    public void run() {
    
        try {
    
            try {
    
                while (true) {
    
                    PendingPost pendingPost = queue.poll(1000);
                    if (pendingPost == null) {
    
                        synchronized (this) {
    
                            // Check again, this time in synchronized
                            pendingPost = queue.poll();
                            if (pendingPost == null) {
    
                                executorRunning = false;
                                return;
                            }
                        }
                    }
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
    
                Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
    
            executorRunning = false;
        }
    }

}