Eventbus基礎功能源碼解析

BroadcastReceiver

? 本質上是一個監聽器，分為接收者和發送者。用于監聽（接收）應用發出的廣播消息，并做出響應。可用于進程間通信、線程間通信和監聽系統的特定時間（如：電話呼入，網絡狀態變更等）。

  • 使用觀察者模式：基于消息的發布/訂閱事件模型。

  • 常用做與android 系統事件的監聽和交互。

  • 消耗時間、空間較多，在onRecieve()方法內可以獲得Context和Intent參數，通過這兩個方法，可以調用AndroidSDK中的很多方法，屬于重量級組件。

  • 可跨進程通信。

Eventbus

? 針對android跨進程、線程通信的優化方案，一定程度上可以代替廣播、handler、Intent。

優點

  • 調度靈活，不依賴于context。
  • 可繼承。
  • 有事件優先級。
  • 粘性事件機制。
  • 量級較輕。

1. 實現原理

  • getDefault()：通過懶漢單例模式獲取到對象

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

    在new對象時，使用builder模式：

    EventBus(EventBusBuilder builder) {
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        throwSubscriberException = builder.throwSubscriberException;
        eventInheritance = builder.eventInheritance;
        executorService = builder.executorService;
    }

  • register()實現如下：

    public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);//通過反射的方式獲取到所有通過注解注冊的回調方法
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);//這里是具體的注冊方法
            }
        }
    }

    Subscribe()：

    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;//獲取觸發本次消費的事件類型
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);//創建訂閱對象（把注冊者和事件消費方法綁定）
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {//若該事件（注冊者和消費方法）已注冊，則拋異常
                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) {//輪詢的目的是為了按照優先級插入到指定的位置(事件消費隊列)
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);//獲取該訂閱者下的所有訂閱事件
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);//將本事件添加進觸發事件列表中

        if (subscriberMethod.sticky) {//粘性事件處理
            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);
            }
        }
    }

    以上就是EventBus注冊時的邏輯，接下來我們看看發布事件的時候的邏輯：

  • post()：

    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;//獲取當前線程的消息隊列
        eventQueue.add(event);//將本事件添加進消息隊列中

        if (!postingState.isPosting) {//若當前
            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()) {//循環遍歷事件隊列（在這兒可以類比handler機制不過不是死循環，也不存在阻塞，每post一次就進行判斷，是否處于posting）
                    postSingleEvent(eventQueue.remove(0), postingState);//主要的消費邏輯
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

    這個currentPostingThreadState是做什么的呢，我們可以看一下它的聲明和實現：

        private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
        @Override
        protected PostingThreadState initialValue() {
            return new PostingThreadState();
        }
    };

    這里使用了ThreadLocal，說明在每個線程里都會有這個對象的副本。我們接著往下看PostingThreadState是維護什么的類：

    final static class PostingThreadState {
        final List<Object> eventQueue = new ArrayList<Object>();//事件隊列，用一個list維護
        boolean isPosting;//是否在發布事件（就是之后的循環遍歷是否在執行）
        boolean isMainThread;//是否為主線程，通過判斷當前looper是否為MainLooper
        Subscription subscription;//當前的訂閱類，上面有說明
        Object event;//當前正在處理的事件
        boolean canceled;//當前事件是否被取消
    }

    我們繼續看postSingleEvent()方法，這里是事件消費的主要邏輯：

    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));
            }
        }
    }

    這里可以看到最終都會回調postSingleEventForEventType()這個方法，我們繼續看這個方法：

    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;//當前正在處理的事件
                postingState.subscription = subscription;//當前正在輪詢的訂閱事件
                boolean aborted = false;
                try {
                    postToSubscription(subscription, event, postingState.isMainThread);//事件處理方法
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            return true;
        }
        return false;
    }

    我們接著看：

    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {//這里就是處理線程模式的不同策略
            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);
        }
    }

    我們從默認的POSTING模式往下看：

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

    我們接著看：

    void invokeSubscriber(Subscription subscription, Object event) {
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);//最終是通過反射的方法調用事件的處理
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }

    這樣我們可以知道，最后調用方法是通過反射來調用與之對應的消費事件的方法。

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