EventBus相關(guān)解析

1.注冊

使用EventBus的第一步就是需要給當(dāng)前的Activiy注冊，注冊方式如下

EventBus.getDefault().register(this);

直接看register里面的方法

    private synchronized void register(Object subscriber, boolean sticky, int priority) {
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass());
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod, sticky, priority);
        }
    }

這里首先調(diào)用了findSubscriberMethods方法 ，我們看下這個方法里面究竟干了什么

  List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        String key = subscriberClass.getName();
        List<SubscriberMethod> subscriberMethods;
        synchronized (methodCache) {
            subscriberMethods = methodCache.get(key);
        }
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        ...
        while (clazz != null) { 
            ...
            // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
            Method[] methods = clazz.getDeclaredMethods();
            for (Method method : methods) {
                String methodName = method.getName();
                if (methodName.startsWith(ON_EVENT_METHOD_NAME)) {
                    int modifiers = method.getModifiers();
                    if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                        Class<?>[] parameterTypes = method.getParameterTypes();
                        if (parameterTypes.length == 1) {
                            ...
                            Class<?> eventType = parameterTypes[0];
                            methodKeyBuilder.setLength(0);
                            methodKeyBuilder.append(methodName);
                            methodKeyBuilder.append('>').append(eventType.getName());
                            String methodKey = methodKeyBuilder.toString();
                            if (eventTypesFound.add(methodKey)) {
                                // Only add if not already found in a sub class
                                subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
                            }
                        }
                    } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
                        Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
                                + methodName);
                    }
                }
            }
            clazz = clazz.getSuperclass();
        }
        ...
    }

這里我僅僅保留了與我們解析相關(guān)的代碼，可以看到這里主要是獲取我們注冊的Class里面onEvent方法，并將相關(guān)方法信息放入SubscriberMethod里面，同時存儲在一個List里面并返回，通過這個方法就可以獲取到注冊Class里面所有的onEvent方法。

接下來就是對每個獲取到的方法執(zhí)行subscribe方法，方法如下

// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
        Class<?> eventType = subscriberMethod.eventType;
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<Subscription>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
        // subscriberMethod.method.setAccessible(true);

        int size = subscriptions.size();
        for (int i = 0; i <= size; i++) {
            if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<Class<?>>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        if (sticky) {
            Object stickyEvent;
            synchronized (stickyEvents) {
                stickyEvent = stickyEvents.get(eventType);
            }
            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());
            }
        }
    }

這里做了幾件事情

  • 這里通過className，method以及priority（優(yōu)先級）構(gòu)建Subscription

    然后通過priority來將Subscription放到CopyOnWriteArrayList的安全線程list中，這個list保存在一個Map中，Map根據(jù)subscriberMethod.eventType來存儲list

    而這個eventType是Class<?>，它是個通配泛型，?可以代表任何類型，我測試的類型如下

    class com.xinyuanhxy.libreadprogejct.TestEvent

    當(dāng)然不同的類的類型也不同

    所以當(dāng)不同類的onEvent是同一個Event類，則會加載在同一個List中存儲相關(guān)數(shù)據(jù)

  • 通過subscriber即當(dāng)前類的名稱獲取list列表，并存儲上面的eventType，即建立class與Event類集合的map信息

  • 如果該事件處理方法為粘性事件，即設(shè)置了“sticky = true”，則需要調(diào)用checkPostStickyEventToSubscription
    判斷是否有粘性事件需要處理，如果需要處理則觸發(fā)一次事件處理函數(shù)

    這個和BroadCastReceiver中的stickyBroadCastReceiver類似

那么這里就將我們注冊相關(guān)的信息解析并存儲下來了，接下來就是發(fā)送消息相關(guān)了。

總結(jié)一下整個注冊過程，大致分為3步：

獲取注冊類的onEvent方法，存儲相關(guān)信息
建立Event類與相關(guān)onEvent方法類的map信息，與建立class與Event類集合的map信息
處理粘性事件

2.發(fā)送消息與接收消息相關(guān)

發(fā)送消息如下

EventBus.getDefault().post(new TestEvent());

直接看post里面的方法

    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;
            }
        }
    }

currentPostingThreadState是一個ThreadLocal類型，里面存儲了PostingThreadState，PostingThreadState包含了一個事件隊列eventQueue和一些標(biāo)志信息。eventQueue存放所有待post的事件對象。

接下來會調(diào)用postSingleEvent方法post該事件

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));
            }
        }
    }

如果允許事件繼承，則會調(diào)用lookupAllEventTypes查找所有的父類和接口類。并將其保存到eventTypesCache中，方便下次使用。

接下來調(diào)用postSingleEventForEventType方法，如下

    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;
    }

這里就是根據(jù)eventClass為key從subscriptionsByEventType對象中獲取Subscription列表。在上面講register的時候我們已經(jīng)看到EventBus在register的時候會將Subscription列表存儲在subscriptionsByEventType中。接下來會遍歷subscriptions列表然后調(diào)用postToSubscription方法進(jìn)行下一步處理。

最后調(diào)用到invokeSubscriber方法，如下

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);
        }
    }

這里通過反射調(diào)用相應(yīng)的onEvent()方法，接下來就會執(zhí)行我們的處理邏輯了

到這里關(guān)于發(fā)送與接收就分析完了，

總結(jié)一下整個post過程，大致分為3步：

將事件對象添加到事件隊列eventQueue中等待處理
遍歷eventQueue隊列中的事件對象并調(diào)用postSingleEvent處理每個事件
找出訂閱過該事件的所有事件處理函數(shù)，并在相應(yīng)的線程中執(zhí)行該事件處理函數(shù)

3.取消事件注冊
public synchronized void unregister(Object subscriber) {
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {
            unubscribeByEventType(subscriber, eventType);
        }
        typesBySubscriber.remove(subscriber);
    } else {
        Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

將register過程注冊到EventBus的事件處理函數(shù)移除掉。

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