EventBus for Android 源碼分析

EventBus for Android 源碼分析

上文執行個體講解EventBus for Android介紹了EventBus的基本用法，本文將介紹EventBus的實現原理。EventBus的實現主要圍繞兩個函數register和post,下面分別介紹之。

1 register（Object subscriber)

功能
註冊subscriber中以onEvent開頭的方法
代碼：

private synchronized void register(Object subscriber, boolean sticky, int priority) {    //構造出方法列表    List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass());    for (SubscriberMethod subscriberMethod : subscriberMethods) {        //註冊每個方法        subscribe(subscriber, subscriberMethod, sticky, priority);    }}

重要資料結構：
SubscriberMethod:儲存方法、執行緒模式和Event事件處理的參數類型
subscriber函數用兩個Map:subscriptionByEventType和typesBySubscriber來維護事件訂閱者和事件類型的關係，如代碼所示

// Must be called in synchronized blockprivate void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {    Class eventType = subscriberMethod.eventType;    CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);    Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);    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);        }    }    // 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++) {        //以優先順序順序來插入隊列中        if (i == size || newSubscription.priority > subscriptions.get(i).priority) {            subscriptions.add(i, newSubscription);            break;        }    }    List> subscribedEvents = typesBySubscriber.get(subscriber);    if (subscribedEvents == null) {        subscribedEvents = new ArrayList>();        typesBySubscriber.put(subscriber, subscribedEvents);    }    subscribedEvents.add(eventType);    ...}

這個函數中用到了CopyOnWriteArrayList,下次研究一下。

2 post(Object event)

先看源碼：

public void post(Object event) {    //postingState是一個ThreadLocal型變數， 表示發送事件的進程狀態    PostingThreadState postingState = currentPostingThreadState.get();    //將事件加如到隊列中    List eventQueue = postingState.eventQueue;    eventQueue.add(event);    //發送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 {            //執行隊列中的任務            while (!eventQueue.isEmpty()) {                postSingleEvent(eventQueue.remove(0), postingState);            }        } finally {            postingState.isPosting = false;            postingState.isMainThread = false;        }    }}

這裡遇到一個類：PostingThreadState,用來記錄當前線程的狀態和任務隊列，EventBus用postSingleEvent來處理任務隊列中的每個任務，代碼如下：

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {    Class eventClass = event.getClass();    boolean subscriptionFound = false;    if (eventInheritance) {        List> eventTypes = lookupAllEventTypes(eventClass);        int countTypes = eventTypes.size();        //擷取事件類型，包括基類和介面        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));        }    }}

這個函數的主要功能是擷取Event的類型，如果eventInheritance為True,則會擷取Event的父類，和介面，然後對每一種類型調用一次 postSingleEventForEventType(Object event, PostingThreadState postingState, Class

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class eventClass) {    CopyOnWriteArrayList subscriptions;    synchronized (this) {        //擷取相應事件類型為eventClass的訂閱者        subscriptions = subscriptionsByEventType.get(eventClass);    }    if (subscriptions != null && !subscriptions.isEmpty()) {        for (Subscription subscription : subscriptions) {            //初始化發送事件線程的狀態            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;}

在擷取到訂閱者，方法和發送事件的線程類型之後，EventBus調用 postToSubscription(Subscription subscription, Object event, boolean isMainThread)來通知訂閱者進行事件處理，該函數代碼如下：

    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {        switch (subscription.subscriberMethod.threadMode) {            case PostThread:                //直接調用Event處理事件                invokeSubscriber(subscription, event);                break;            case MainThread:                if (isMainThread) {                    //發送線程是主線程，直接調用                    invokeSubscriber(subscription, event);                } else {                    mainThreadPoster.enqueue(subscription, event);                }                break;            case BackgroundThread:                if (isMainThread) {                    backgroundPoster.enqueue(subscription, event);                } else {                    //發送線程是非主線程，直接調用                    invokeSubscriber(subscription, event);                }                break;            case Async:                asyncPoster.enqueue(subscription, event);                break;            default:                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);        }    }

該函數根據不同的執行緒模式來採用不同的方法來通知訂閱者，下面分別討論之：

1 PostThread模型

直接調用invokeSubscriber(Subscription subscription, Object event)，該方法利用反射原理來調用接受者的事件處理方法

2 MainThread模型

若發送線程為主線程，則直接調用訂閱者事件處理方法。
若發送線程為非主線程，則將事件發送到mainThreadPoster中進行處理。mainThreadPoster的類型為HandlerPoster,繼承自Handler。HandlerPoster內部維護一個隊列（PendingPostQueue）來存取訂閱者和對應的事件回應程式法。每次往HandlerPoster中插入資料時， HandlerPoster便發一個訊息，告知Handler來從PendingPostQueue中取出資料，然後調用訂閱者的事件處理方法，代碼如下：

void enqueue(Subscription subscription, Object event) {    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);    synchronized (this) {        queue.enqueue(pendingPost);        if (!handlerActive) {            handlerActive = true;            //若looper不在運行，則發訊息通知它            if (!sendMessage(obtainMessage())) {                throw new EventBusException("Could not send handler message");            }        }    }}@Overridepublic 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;                    }                }            }            //取出隊列中的資料，調用訂閱者的方法            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;    }}

3 BackgroundThread模型

若發送線程為非主線程，則直接呼叫事件響應函數。
若發送線程為主線程，則將訊息發送到backgroundPoster中。backgroundPoster的類型為BackgroundPoster,繼承自Runnable。和HandlerPoster類型，該對象也維護一個事件隊列：PendingPosteQueue,每次插入資料時，BackgroundPoster會將自己放入到EventBus的線程池中等待調度，完整代碼如下：

public void enqueue(Subscription subscription, Object event) {    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);    synchronized (this) {        queue.enqueue(pendingPost);        //若線程池中的任務已經運行，則立即返回，        if (!executorRunning) {            executorRunning = true;            //擷取到EventBus的線程池            eventBus.getExecutorService().execute(this);        }    }}@Overridepublic void run() {    try {        try {            while (true) {                //等待一秒後取出任務                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;    }}

4 Async模型

該模型直接將事件發送到asyncPoster中進行處理。asyncPoster的類型為AsyncPoster，繼承自Runnable。與BackgroundPoster不同的是，AsyncPoster將資料插入隊列後，直接將自己放入線程池中處理，完成代碼如下：

public void enqueue(Subscription subscription, Object event) {    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);    queue.enqueue(pendingPost);    eventBus.getExecutorService().execute(this);}@Overridepublic void run() {    PendingPost pendingPost = queue.poll();    if(pendingPost == null) {        throw new IllegalStateException("No pending post available");    }    eventBus.invokeSubscriber(pendingPost);}

Async與BackgroundPoster的不同之處為前者每次加入訂閱者和事件後，便立即將自己放進線程池中，確保每一個任務都處於不同的線程中。而BackgroundPoster會根據線程池中的BackgroundPoster類型的任務是否處於運行狀態來往線程池中加任務，這樣就確保所有處於BackgroundThread模型中的事件處理任務都在同一個後台線程中運行。