Reading and understanding of eventbus source code

EventBus

Android Mainstream event delivery Library in , Simplify callbacks and thread operations in our code

EventBus introduce

implementation 'org.greenrobot:eventbus:3.2.0'

register

EventBus.getDefault().register(this);

Next, let's see how to register here

    public static EventBus getDefault() {
    
        EventBus instance = defaultInstance;
        if (instance == null) {
    
            synchronized (EventBus.class) {
    
                instance = EventBus.defaultInstance;
                if (instance == null) {
    
                    instance = EventBus.defaultInstance = new EventBus();
                }
            }
        }
        return instance;
    }

The first is a single example of a standard double lock hungry man , Let's look at regsiter()

   public void register(Object subscriber) {
    
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
    
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
    
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

First of all to see List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

  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 {
    
        	// Use in this class Subscribe Annotated functions are added to the cache 
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

If there is no cache, it will execute subscriberMethods = findUsingInfo(subscriberClass); Let's move on

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        // there findState.clazz It was passed in when we registered this
        while (findState.clazz != null) {
    
            findState.subscriberInfo = getSubscriberInfo(findState);
            // Here for null Executive else Branch 
            if (findState.subscriberInfo != null) {
    
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
    
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
    
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
    
            	// Continue to look at the source code here 
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

    private void findUsingReflectionInSingleClass(FindState findState) {
    
        Method[] methods;
        try {
    
            methods = findState.clazz.getDeclaredMethods();
        } catch (Throwable th) {
    
            methods = findState.clazz.getMethods();
            findState.skipSuperClasses = true;
        }
        // Get the function in this class in the signature 
        for (Method method : methods) {
    
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
    
                Class<?>[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
    
                	// Check whether this function has Subscribe annotation 
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
    
                    	// Get the parameters of this function class It's the event we deliver class
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
    
                        	// Get what we set threadMode
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            // Use this class Subscribe Annotated functions are added subscriberMethods in 
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                }
            } 
        }
    }

It's over findSubscriberMethods() Let's keep looking subscribe() What have you done

  private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    
        Class<?> eventType = subscriberMethod.eventType;
        // This object holds our subscription class and subscriber Annotated functions 
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
    
            subscriptions = new CopyOnWriteArrayList<>();
            // Put this type of Event Add to this Map
            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) {
    
            	// This List In the above function, it has been stored in subscriptionsByEventType It's in 
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
    
            subscribedEvents = new ArrayList<>();
            // Which types of events this class focuses on are stored in typesBySubscriber
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
		// Each registration will trigger a sticky Event Sending of 
        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);
            }
        }
    }

Come here register I have finished reading , This is actually getting it for use Subscribe Annotated functions and related information ( Thread mode , priority , Whether it is a sticky event )

Event sending

Let's continue to look at the sending of events and how they are called , Take a look first EventBus.getDefaule().post()

    public void post(Object event) {
    
    	// First, get the thread status , Because the default threadMode yes ThreadMode.POSTING
        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()) {
    
                	// Send this event , Add this to the front list
                    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;
        // Creating EventBus Whether this of the object is true Of 
        if (eventInheritance) {
    
        	// Here, the event class class  as well as Object.class Save in eventTypes
            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));
            }
        }
    }

	
    private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
    
        synchronized (eventTypesCache) {
    
        	// Get the... In the cache first 
            List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
            if (eventTypes == null) {
    
                eventTypes = new ArrayList<>();
                Class<?> clazz = eventClass;
                while (clazz != null) {
    
                	// Our event class
                    eventTypes.add(clazz);
                    // This function will not execute internally without inheriting the interface 
                    addInterfaces(eventTypes, clazz.getInterfaces());
                    // The second loop will add the parent of the event class to this list
                    clazz = clazz.getSuperclass();
                }
                // Add to cache , Go down and get it directly from the cache , It's better 
                eventTypesCache.put(eventClass, eventTypes);
            }
            // Now there are events class,Object.class, Because all classes implicitly inherit Object
            return eventTypes;
        }
    }

   private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
    
        	// stay register Called subscribe The function takes eventClass Events of type are stored in subscriptionsByEventType
        	// All this here if you register this type Event,subscriptions Not for null
        	// For details of this data storage, please see subscribe()
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
    
            for (Subscription subscription : subscriptions) {
    
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted;
                try {
    
                	// Here is the sending of events , Continue to look at the source code of this function 
                    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) {
    
        	//threadMode The default is POSTING
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
            	//Android Main thread execution 
                if (isMainThread) {
    
                    invokeSubscriber(subscription, event);
                } else {
    
              		//mainThreadPoster When it was created, it was passed into Looper.getMainLooper()
					// And use this Looper Created Handler object , So here is the use of Handler The mechanism is executed in the main thread 
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            case MAIN_ORDERED:
            	// Execute in the main thread first ,mainThreadPoster==null Execute in the sending thread 
                if (mainThreadPoster != null) {
    
                    mainThreadPoster.enqueue(subscription, event);
                } else {
    
                    // temporary: technically not correct as poster not decoupled from subscriber
                    invokeSubscriber(subscription, event);
                }
                break;
            case BACKGROUND:
            	// Execute... In the process pool 
                if (isMainThread) {
    
                    backgroundPoster.enqueue(subscription, event);
                } else {
    
                    invokeSubscriber(subscription, event);
                }
                break;
            case ASYNC:
            	// Asynchronous execution , Like and BACKGROUND There's no difference 
                asyncPoster.enqueue(subscription, event);
                break;
            default:
                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
        }
    }

	// Here is the function to be executed , Use reflection to execute 
  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);
        }
    }

clazz = clazz.getSuperclass(); It's worth noting , That is to say, we write a parameter of type Object Function of , And use Subscribe Annotation, then it can receive all our events

summary

• stay EventBus Instantiation time , Use the standard double lock mechanism to realize the singleton
• In execution register() Function EventBus You will find that the modifier under this class is public, The number of parameters is 1, And use Subscribe Annotated functions , stay map For storage
• register() The function also sends sticky events , This is why we send sticky events first , Later, you can also receive the reason for the event by subscribing
• stay post() We will first find the event type in Map The corresponding function in , And use reflection to execute this function
• EventBus Use in Handler Thread scheduling