Android EventBus3.0使用及原始碼解析

阿新 • • 發佈：2019-02-13

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叨了個叨

最近因為換工作的一些瑣事搞的我一個頭兩個大，也沒怎麼去學新東西，實在是有些愧疚。新專案用到了EventBus3.0，原來只是聽說EventBus的鼎鼎大名，一直沒仔細研究過。趁著週末有些時間，研究下程式碼，也算沒有虛度光陰。

EventBus3.0簡介

EventBus是greenrobot出品的一個用於Android中事件釋出/訂閱的庫。以前傳遞物件可能通過介面、廣播、檔案等等，尤其像同一個Activity兩個Fragment之間採用介面傳遞物件，十分的麻煩，而且耦合度較高。使用EventBus

之後，這些將不再是問題。盜用GiHub上EventBus的一張圖。

可以看到，釋出者(Publisher)使用post()方法將Event傳送到Event Bus，而後Event Bus自動將Event傳送到多個訂閱者(Subcriber)。這裡需要注意兩個地方：（1）一個釋出者可以對應多個訂閱者。（2）3.0以前訂閱者的訂閱方法為onEvent()、onEventMainThread()、onEventBackgroundThread()和onEventAsync()。在Event Bus3.0之後統一採用註解@Subscribe的形式，具體實現方式見下文。

EventBus3.0的使用

新建兩個Activity,花3s掃一下即可。程式碼如下：

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // 註冊EventBus
        EventBus.getDefault().register(this);
        startActivity(new Intent(this,SecondActivity.class));
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // 反註冊EventBus
        EventBus.getDefault().unregister(this);
    }

    // 主執行緒呼叫
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void eventBusMain(String str){
        Log.i("TAG", "MAIN:"+str+" Thread="+Thread.currentThread().getId());
    }

    // 1.釋出執行緒為主執行緒，新開執行緒呼叫
    // 2.釋出執行緒為子執行緒，釋出執行緒呼叫
    @Subscribe(threadMode = ThreadMode.BACKGROUND)
    public void eventBusBg(String str){
        Log.i("TAG", "BACKGROUND:"+str+" Thread="+Thread.currentThread().getId());
    }

    // 在釋出執行緒呼叫，預設值
    @Subscribe(threadMode = ThreadMode.POSTING)
    public void eventBusPosting(String str){
        Log.i("TAG", "POSTING:"+str+" Thread="+Thread.currentThread().getId());
    }

    // 每次都新開執行緒呼叫
    @Subscribe(threadMode = ThreadMode.ASYNC)
    public void eventBusAsync(String str){
        Log.i("TAG", "ASYNC:"+str+" Thread="+Thread.currentThread().getId());
    }
}

public class SecondActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        EventBus.getDefault().post("from second activity mainThread: info");
        Log.i("TAG", "Post thread="+Thread.currentThread().getId());
        new Thread(new Runnable() {
            @Override
            public void run() {
                EventBus.getDefault().post("from second activity childThread: info");
                Log.i("TAG", "Post thread="+Thread.currentThread().getId());
            }
        }).start();
    }
}

在MainActivity的onCreate()/onDestroy()中分別註冊/反註冊EventBus。然後寫了四個測試ThreadMode的方法，呼叫時機註釋的很清楚，就不贅述了。最後在SecondActivity的主執行緒和子執行緒中分別呼叫Post()方法，注意，這裡Post()方法的引數為Object型別，這也就意味著我們傳遞任何物件都是可以的，例如JavaBean、List<E>等等都是可以的，這裡為了方便演示直接傳遞了String。Log資訊如下：
main

child
第一張圖中釋出者傳送執行緒為主執行緒，即Post thread = 1，在訂閱者收到訊息時，ThreadMode = Main和ThreadMode = Posting的方法都在主執行緒呼叫，ASYNC和BACKGROUND都新開了執行緒。圖二對比註釋同理。

除此之外，Subscribe註解還支援priority和sticky屬性。priority設定接收者的優先順序，預設值為0。優先順序高的方法先被呼叫，在方法呼叫完成後可以呼叫EventBus.getDefault().cancelEventDelivery(event) ;終止優先順序低的方法的呼叫。sticky為粘性事件，預設為關閉狀態。能夠收到訂閱之前傳送到的最後一條訊息，並且傳送的方法不再是post()而是postSticky()。

EventBus3.0原始碼解析

EventBus是Very的好用。耦合度大大的降低，而且程式碼十分優雅。它是怎麼就做到了這麼優雅的呢？知其然，知其所以然。下面就開始一步步的分析。

註解標籤`Subscribe`

對註解不瞭解的同學可以看下這篇部落格。

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Subscribe {
    ThreadMode threadMode() default ThreadMode.POSTING;

    /**
     * If true, delivers the most recent sticky event (posted with
     * {@link EventBus#postSticky(Object)}) to this subscriber (if event available).
     */
    boolean sticky() default false;

    /** Subscriber priority to influence the order of event delivery.
     * Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before
     * others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of
     * delivery among subscribers with different {@link ThreadMode}s! */
    int priority() default 0;
}

public enum ThreadMode {

    POSTING,

    MAIN,

    BACKGROUND,

    ASYNC
}

註解Subscribe在執行時解析，且只能加在METHOD上。其中有三個方法，threadMode()返回型別ThreadMode為列舉型別，預設值為POSTING，sticky()預設返回false,priority()預設返回0。

1. `Register`流程

`EventBus#getDefault()`

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

EventBus採用雙重校驗鎖設計為一個單例模式，奇怪的在於雖然設計為單例模式，但是構造方法確實public型別，這不是坑爹嘛！難道greenrobot在設計EventBus獲取例項方法的時候在打LOL，一不小心打錯了？原來啊，EventBus預設支援一條事件匯流排，通常是通過getDefault()方法獲取EventBus例項，但也能通過直接new EventBus這種最簡單的方式獲取多條事件匯流排，彼此之間完全分開。設計之思想不禁讓人拍案叫絕。

`EventBus#register()`

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

首先得到訂閱者的報名.類名，即哪個具體類註冊。然後呼叫subscriberMethodFinder.findSubscriberMethods(subscriberClass)，從方法名和返回值來看，findSubscriberMethods()的作用應該是遍歷查詢訂閱者中所有的訂閱方法。

`SubscriberMethodFinder#findSubscriberMethods()`

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    // 查詢快取
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        // 快取中有則直接返回
        return subscriberMethods;
    }
    // 預設false
    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 {
        // 加入快取
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

注意subscriberMethods.isEmpty()，如果註冊了EventBus，但卻沒有使用註解Subscribe是會出現EventBusException異常的。下面跟進findUsingInfo()方法。

`SubscriberMethodFinder#findUsingInfo()`

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    // 從FIND_STATE_POOL陣列中查詢FindState，命中返回，否則直接new
    FindState findState = prepareFindState();
    // 初始化FindState
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        findState.subscriberInfo = getSubscriberInfo(findState);
        // findState.subscriberInfo預設null
        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 {
            findUsingReflectionInSingleClass(findState);
        }
        // 將findState.clazz變為改類的父類
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

// SubscriberMethodFinder$FindState#initForSubscriber()
void initForSubscriber(Class<?> subscriberClass) {
    this.subscriberClass = clazz = subscriberClass;
    skipSuperClasses = false;
    subscriberInfo = null;
}

findState.subscriberInfo預設null，那麼就進入到findUsingReflectionInSingleClass(findState)，先看下這個方法，等下還要返回來看。

`SubscriberMethodFinder#findUsingReflectionInSingleClass()`

private static final int MODIFIERS_IGNORE = Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;

private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        // This is faster than getMethods, especially when subscribers are fat classes like Activities
        // 獲取到類中所有的方法
        methods = findState.clazz.getDeclaredMethods();
    } catch (Throwable th) {
        // Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
        methods = findState.clazz.getMethods();
        findState.skipSuperClasses = true;
    }
    for (Method method : methods) {
        // 獲取方法的修飾符
        int modifiers = method.getModifiers();
        // 必須被public修飾，而且不能為MODIFIERS_IGNORE
        // MODIFIERS_IGNORE定義為ABSTRACT、STATIC、BRIDGE、SYNTHETIC。
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
            // 獲取方法所有引數型別
            Class<?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length == 1) {
                // 是否有Subscribe註解標籤
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) {
                    // 帶有Subscribe註解標籤的方法的第一個引數型別
                    Class<?> eventType = parameterTypes[0];
                    // 關聯method, eventType到anyMethodByEventType
                    if (findState.checkAdd(method, eventType)) {
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        // 構造SubscriberMethod，並且新增到findState.subscriberMethods
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    }
                }
            // strictMethodVerification 預設為false，不會丟擲異常，但還是建議符合規範
            } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                throw new EventBusException("@Subscribe method " + methodName +
                        "must have exactly 1 parameter but has " + parameterTypes.length);
            }
        } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
            String methodName = method.getDeclaringClass().getName() + "." + method.getName();
            throw new EventBusException(methodName +
                    " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
        }
    }
}

// `SubscriberMethodFinder#checkAdd()`
boolean checkAdd(Method method, Class<?> eventType) {
    // 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
    // Usually a subscriber doesn't have methods listening to the same event type.
    Object existing = anyMethodByEventType.put(eventType, method);
    if (existing == null) {
        return true;
    } else {
         ...
}

接下來返回SubscriberMethodFinder#findUsingInfo()接著看，在findUsingInfo()中迴圈執行完後return getMethodsAndRelease(findState)

static class FindState {
    final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
    ...
}

private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
    List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
    // 置空findState
    findState.recycle();
    synchronized (FIND_STATE_POOL) {
        for (int i = 0; i < POOL_SIZE; i++) {
            if (FIND_STATE_POOL[i] == null) {
                FIND_STATE_POOL[i] = findState;
                break;
            }
        }
    }
    return subscriberMethods;
}

在getMethodsAndRelease()中將findState置空，存放進FIND_STATE_POOL陣列，最後返回findState.subscriberMethods。返回EventBus#register()。

`EventBus#register()`

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

呼叫SubscriberMethodFinder#findSubscriberMethods()後，以List<SubscriberMethod>形式返回了訂閱者所有的訂閱事件。然後遍歷執行subscribe()方法。看樣子應該是遍歷List<SubscriberMethod>，然後將訂閱者和訂閱事件繫結。沒撒好說的，跟進subscribe()

`EventBus#subscribe()`

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    // 獲取訂閱事件的型別，即訂閱方法中的唯一引數型別
    Class<?> eventType = subscriberMethod.eventType;
    // 用訂閱者和訂閱方法構造一個Subscription物件
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    // 查詢所有的訂閱了訂閱事件的訂閱者
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    // 沒有訂閱者訂閱過則新建個CopyOnWriteArrayList<subscriptions>，並put進subscriptionsByEventType PS:CopyOnWriteArrayList支援併發讀寫
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        // 訂閱者List不為空，而且已經包含了newSubscription，則會丟擲異常。即：訂閱者不能重複訂閱同一事件
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }

    int size = subscriptions.size();
    // 根據訂閱者優先順序，增加到訂閱者列表subscriptions的相應位置
    for (int i = 0; i <= size; i++) {
        if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }
    // 獲取訂閱者所有訂閱事件的列表，預設為null
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    // 將訂閱事件新增進對應訂閱者的訂閱列表
    subscribedEvents.add(eventType);
    // sticky預設為false
    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);
        }
    }
}

EventBus#Register()其實只做了三件事：

1. 查詢訂閱者所有的訂閱事件

2. 將訂閱事件作為key，所有訂閱了此訂閱事件的訂閱者作為value存放進subscriptionsByEventType

3. 將訂閱者作為key，訂閱者的所有訂閱事件作為value存放進typesBySubscriber

至此，EventBus.getDefault().register(this)流程完畢。

2. `Post`流程

`EventBus#getDefault()`

獲取EventBus例項。和Register流程中一樣，不再贅述。

`EventBus#post()`

/** Posts the given event to the event bus. */
public void post(Object event) {
    // 依據不同的執行緒獲取相應的剛初始化的PostingThreadState
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    // 將event加入到postingState.eventQueue
    eventQueue.add(event);
    // isPosting預設false
    if (!postingState.isPosting) {
        // 判斷是否是主執行緒
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            // 遍歷傳送eventQueue中的event
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

上面程式碼中currentPostingThreadState為ThreadLocal<PostingThreadState>物件，對ThreadLocal<>機制不瞭解的同學，可以檢視這篇部落格。下面跟進postSingleEvent()方法。

`EventBus#postSingleEvent()`

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    // 獲取event的型別
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    // eventInheritance預設為true
    if (eventInheritance) {
        // 依據訂閱事件型別，將訂閱事件型別及所有父類新增進eventTypes。詳情見下文EventBus.lookupAllEventTypes()分析
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
        int countTypes = eventTypes.size();
        // 遍歷countTypes，通過呼叫postSingleEventForEventType()方法通知所有訂閱者
        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));
        }
    }
}

`EventBus#lookupAllEventTypes()`

private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
    synchronized (eventTypesCache) {
        // 根據訂閱事件查詢所有自身及父類，eventTypes預設為null
        List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
        if (eventTypes == null) {
            eventTypes = new ArrayList<>();
            Class<?> clazz = eventClass;
            while (clazz != null) {
                // 將訂閱事件新增進eventTypes
                eventTypes.add(clazz);
                // 遍歷訂閱事件的所有父類，依次新增進eventTypes
                addInterfaces(eventTypes, clazz.getInterfaces());
                clazz = clazz.getSuperclass();
            }
            // 將訂閱事件和包含訂閱事件自身及所有父類的eventTypes新增進eventTypesCache
            eventTypesCache.put(eventClass, eventTypes);
        }
        return eventTypes;
    }
}

現在假設傳遞的資料為Person類，而Person類實現了IPerson介面。通過上面的分析可以得出結論：在傳遞物件（Person）的時候，訂閱事件中引數為被傳遞物件的所有父類訂閱事件（IPerson）也都會被呼叫。筆者已經驗證通過，感興趣的同學可以再驗證一下。

`EventBus#postSingleEventForEventType()`

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
        // 根據訂閱事件查詢所有已經註冊過的訂閱者
        // 注意：這裡第一次傳遞進來的是訂閱事件，之後會逐個傳遞進來訂閱事件的父類
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        for (Subscription subscription : subscriptions) {
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            try {
                // 引數解釋：subscription-被遍歷到的訂閱者;event-訂閱事件引數(子類);
                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EventBus#register()最後總結道：將訂閱事件作為key，所有訂閱了此訂閱事件的訂閱者作為value存放進subscriptionsByEventType。這裡就依據訂閱事件然後查詢對應所有的訂閱者。注意：由於遍歷訂閱事件引數所有父類的原因，一個訂閱事件的Post第一次執行postToSubscription()時，subscription引數，遍歷時為訂閱事件的訂閱者。之後再呼叫postToSubscription()時，subscription引數都為訂閱時間父類的訂閱者。而event引數則一直是訂閱事件中的唯一引數（最底層子類）。

`EventBus#postToSubscription()`

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);
    }
}

看到這裡差不多可以鬆口氣，終於要分發呼叫訂閱者的訂閱事件了！寫了整整一下午，容我抽支菸再。

首先根據ThreadMode確定分發型別。這裡以最常用的Main為例，其餘兩個Poster同理。如果是isMainThread=true，那麼直接呼叫invokeSubscriber()，否則呼叫mainThreadPoster.enqueue()。下面分別解釋這兩種情況。

`EventBus#invokeSubscriber()`

void invokeSubscriber(Subscription subscription, Object event) {
    try {
        // 反射呼叫
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}

沒撒好說的，直接反射呼叫訂閱者的訂閱事件。注意：引數event是子類物件，就算呼叫訂閱事件中唯一引數是引數的父類，那麼傳遞的仍然是子類物件。筆者已經驗證，感興趣的同學可以自行驗證。然後檢視HandlerPoster#enqueue()。

`HandlerPoster#enqueue()`

final class HandlerPoster extends Handler {

    ...

    void enqueue(Subscription subscription, Object event) {
        // 嘗試從pendingPostPool中獲取pendingPost，沒有則直接new PendingPost
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            // 加入佇列
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                // 傳送空訊息 呼叫handleMessage
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
    }

    @Override
    public void handleMessage(Message msg) {
        // 已經切換到主執行緒 
        while (true) {
            // 遍歷獲取queue中的PendingPost物件
           PendingPost pendingPost = queue.poll();
            ...
            // 呼叫eventBus.invokeSubscriber
            eventBus.invokeSubscriber(pendingPost);
            ...
        }
    }
}

`EventBus#invokeSubscriber()`

void invokeSubscriber(PendingPost pendingPost) {
    // 提取訂閱事件
    Object event = pendingPost.event;
    // 提取訂閱者
    Subscription subscription = pendingPost.subscription;
    // 釋放pendingPost
    PendingPost.releasePendingPost(pendingPost);
    if (subscription.active) {
        // 反射呼叫訂閱者的訂閱事件
        invokeSubscriber(subscription, event);
    }
}

至此，訂閱者在相應執行緒呼叫訂閱事件完成，EventBus.getDefault().Post()流程完畢。

EventBus#Post()也只做了三件事

1. 根據訂閱事件在subscriptionsByEventType中查詢相應的訂閱者

2. 分發訂閱者的訂閱事件呼叫執行緒

2. 通過反射呼叫訂閱者的訂閱事件

3. `unregister`流程

`EventBus#getDefault()`

獲取EventBus例項。和Register流程中一樣，不再贅述。

`EventBus#unregister()`

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

`EventBus#unsubscribeByEventType()`

private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
    List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions != null) {
        int size = subscriptions.size();
        for (int i = 0; i < size; i++) {
            Subscription subscription = subscriptions.get(i);
            if (subscription.subscriber == subscriber) {
                subscription.active = false;
                subscriptions.remove(i);
                i--;
                size--;
            }
        }
    }
}

在EventBus#register()最後總結道：

將訂閱事件作為key，所有訂閱了此訂閱事件的訂閱者作為value存放進subscriptionsByEventType。

將訂閱者作為key，訂閱者的所有訂閱事件作為value存放進typesBySubscriber。

現在要反註冊咯。移除相應的key、value即可。EventBus3.0的使用及原始碼解析到此結束，Have a nice day~

Android EventBus3.0使用及原始碼解析

叨了個叨

EventBus3.0簡介

EventBus3.0的使用

EventBus3.0原始碼解析

註解標籤Subscribe

1. Register流程

EventBus#getDefault()

EventBus#register()

SubscriberMethodFinder#findSubscriberMethods()

SubscriberMethodFinder#findUsingInfo()

SubscriberMethodFinder#findUsingReflectionInSingleClass()

EventBus#register()

EventBus#subscribe()

2. Post流程

EventBus#getDefault()

EventBus#post()

EventBus#postSingleEvent()

EventBus#lookupAllEventTypes()

EventBus#postSingleEventForEventType()

EventBus#postToSubscription()

EventBus#invokeSubscriber()

HandlerPoster#enqueue()

EventBus#invokeSubscriber()

3. unregister流程

EventBus#getDefault()

EventBus#unregister()

EventBus#unsubscribeByEventType()

相關推薦

註解標籤`Subscribe`

1. `Register`流程

`EventBus#getDefault()`

`EventBus#register()`

`SubscriberMethodFinder#findSubscriberMethods()`

`SubscriberMethodFinder#findUsingInfo()`

`SubscriberMethodFinder#findUsingReflectionInSingleClass()`

`EventBus#register()`

`EventBus#subscribe()`

2. `Post`流程

`EventBus#getDefault()`

`EventBus#post()`

`EventBus#postSingleEvent()`

`EventBus#lookupAllEventTypes()`

`EventBus#postSingleEventForEventType()`

`EventBus#postToSubscription()`

`EventBus#invokeSubscriber()`

`HandlerPoster#enqueue()`

`EventBus#invokeSubscriber()`

3. `unregister`流程

`EventBus#getDefault()`

`EventBus#unregister()`

`EventBus#unsubscribeByEventType()`