項(xiàng)目地址：EventBus，本文分析版本: 3.1.1

一、概述

EventBus 是一個(gè) Android 事件發(fā)布/訂閱框架，通過(guò)解耦發(fā)布者和訂閱者簡(jiǎn)化 Android 事件傳遞，這里的事件可以理解為消息，本文中統(tǒng)一稱為事件。事件傳遞既可用于 Android 四大組件間通訊，也可以用戶異步線程和主線程間通訊等等。
傳統(tǒng)的事件傳遞方式包括：Handler、BroadCastReceiver、Interface 回調(diào)，相比之下 EventBus 的優(yōu)點(diǎn)是代碼簡(jiǎn)潔，使用簡(jiǎn)單，并將事件發(fā)布和訂閱充分解耦。

• 事件(Event)：又可稱為消息，本文中統(tǒng)一用事件表示。其實(shí)就是一個(gè)對(duì)象，可以是網(wǎng)絡(luò)請(qǐng)求返回的字符串，也可以是某個(gè)開(kāi)關(guān)狀態(tài)等等。事件類型(EventType)指事件所屬的 Class。
  • 事件分為一般事件和 Sticky 事件，相對(duì)于一般事件，Sticky 事件不同之處在于，當(dāng)事件發(fā)布后，再有訂閱者開(kāi)始訂閱該類型事件，依然能收到該類型事件最近一個(gè) Sticky 事件（所謂「最近一個(gè)」指的就是該類型事件「最后一次發(fā)出」）。
• 訂閱者(Subscriber)：訂閱某種事件類型的對(duì)象。當(dāng)有發(fā)布者發(fā)布這類事件后，EventBus 會(huì)執(zhí)行訂閱者的 onEvent 函數(shù)，這個(gè)函數(shù)叫事件響應(yīng)函數(shù)。訂閱者通過(guò) register 接口訂閱某個(gè)事件類型，unregister 接口退訂。訂閱者存在優(yōu)先級(jí)，優(yōu)先級(jí)高的訂閱者可以取消事件繼續(xù)向優(yōu)先級(jí)低的訂閱者分發(fā)，默認(rèn)所有訂閱者優(yōu)先級(jí)都為 0。
• 發(fā)布者(Publisher)：發(fā)布某事件的對(duì)象，通過(guò) post 接口發(fā)布事件。

二、如何使用

2.1 添加依賴

方式一，運(yùn)行期處理注解

在app 的 build.gradle 文件中添加依賴

dependencies {
    implementation 'org.greenrobot:eventbus:3.1.1'
}

方式二，編譯期預(yù)處理注解

Android Studio 3.0 及以上

在 app 的 build.gradle 文件中添加

android {
    //……
    defaultConfig {
        //……
        javaCompileOptions {
            annotationProcessorOptions {
                arguments = [eventBusIndex: 'org.greenrobot.eventbusperf.MyEventBusIndex']
            }
        }
    }
}

dependencies {
    implementation 'org.greenrobot:eventbus:3.1.1'
    annotationProcessor  'org.greenrobot:eventbus-annotation-processor:3.0.1'
}

build 之后，會(huì)生成一個(gè) MyEventBusIndex.java類。

然后在使用 EventBus 實(shí)例之前，又有兩種方式可以將配置MyEventBusIndex.java配置到類中是喲經(jīng)。

方式一 在構(gòu)造 EventBus 時(shí)傳入我們自定義的 EventBusIndex，

EventBus mEventBus = EventBus.builder().addIndex(new MyEventBusIndex()).build();

方式二 將索引應(yīng)用到默認(rèn)的單例中

使用 EventBus 之前，先調(diào)用下面的代碼初始化 EventBus。

EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();

2.2 定義事件類

public class CustomEvent {
    private String mEventName;

    public CustomEvent() {
    }

    public String getEventName() {
        return mEventName;
    }

    public void setEventName(String eventName) {
        mEventName = eventName;
    }
}

2.3 注冊(cè)為監(jiān)聽(tīng)者

在合適的地方（比如 Activity#onCreate、Fragment#onCreateView）通過(guò)下方代碼進(jìn)行注冊(cè)

EventBus.getDefault().register(this);

2.4 編寫響應(yīng)事件的訂閱方法

@Subscribe(threadMode = ThreadMode.BACKGROUND, sticky = true, priority = 100)
public void onMessage(CustomEvent event) {
    Log.d(TAG, "onMessage: " + event.getEventName());
}

使用編譯期注解處理的情況下，訂閱方法的訪問(wèn)控制權(quán)限必須是 非 private 并且非 static 的

使用運(yùn)行期反射處理的情況下，訂閱方法的訪問(wèn)控制權(quán)限必須是 public 的

• 通過(guò) ThreadMode 可以指定訂閱方法在哪個(gè)線程執(zhí)行，有四種選擇
  • ThreadMode.MAIN 事件訂閱方法會(huì)在 UI 線程中執(zhí)行
    • 使用此模式的事件訂閱方法必須快速返回以避免阻塞主線程。
  • ThreadMode.POSTING （默認(rèn)的模式）表示事件在哪個(gè)線程中發(fā)布出來(lái)的，事件訂閱方法就會(huì)在這個(gè)線程中運(yùn)行；
    • 該模式避免了線程切換，適用于那些在很短的時(shí)間內(nèi)完成的簡(jiǎn)單任務(wù)，無(wú)需主線程。使用這種模式的事件訂閱方法必須快速返回以避免阻塞發(fā)布線程（發(fā)布線程可能是主線程）。
  • ThreadMode.MAIN_ORDERED
    • 在 Android 上，訂閱方法將在 Android 的主線程中被調(diào)用。事件將會(huì)排隊(duì)等待交付，這確保了 post 調(diào)用是非阻塞的。
  • ThreadMode.BACKGROUND 子線程執(zhí)行，如果本來(lái)就在子線程，直接在該子線程執(zhí)行
    • EventBus 使用一個(gè)后臺(tái)線程，將按順序發(fā)送所有事件。使用這種模式的訂閱方法應(yīng)該盡快返回以避免阻塞后臺(tái)線程。
      • 注意：「一個(gè)后臺(tái)線程」所指的并不是 Executors.newSingleThreadPool()，而是使用 EventBus 在實(shí)例化時(shí)創(chuàng)建的 cacheThreadPool 中的某一個(gè)線程。
  • ThreadMode.ASYNC 新建子線程執(zhí)行。適用于耗時(shí)操作
    • 發(fā)布事件永遠(yuǎn)不會(huì)等待使用此模式的訂閱方法。適用于比較耗時(shí)的訂閱方法，比如用于網(wǎng)絡(luò)請(qǐng)求。使用時(shí)應(yīng)該避免同時(shí)觸發(fā)大量長(zhǎng)時(shí)間運(yùn)行的異步訂閱方法來(lái)限制并發(fā)線程的數(shù)量。 EventBus 使用線程池有效地重用已完成的異步用戶通知中的線程。
• 通過(guò) sticky 指定是否接收粘性事件，默認(rèn)為 false
• 通過(guò) priority 設(shè)置接收訂閱方法的優(yōu)先級(jí)，相同的事件，優(yōu)先級(jí)越高的訂閱方法 越早收到事件

2.5 發(fā)送事件

通過(guò)EventBus的post()方法來(lái)發(fā)送事件, 發(fā)送之后就會(huì)執(zhí)行注冊(cè)過(guò)這個(gè)事件的對(duì)應(yīng)類的方法. 或者通過(guò)postSticky()來(lái)發(fā)送一個(gè)粘性事件。

2.6 解除注冊(cè)

在合適的地方（比如 Activity#onDestroy）使用下面的代碼進(jìn)行解除注冊(cè) EventBus.getDefault().unregister(this);

2.7 小結(jié)

要實(shí)現(xiàn)訂閱，需要進(jìn)行注冊(cè)，以及解注冊(cè)，訂閱方法以「目標(biāo)事件」作為方法的參數(shù)， 使用 Subscribe 注解，可以指定訂閱方法執(zhí)行的線程、是否接收 sticky 事件、訂閱方法的優(yōu)先級(jí)。

至于發(fā)送方，只需要?jiǎng)?chuàng)建相應(yīng)的 事件實(shí)例，然后調(diào)用 post 或者 postSticky 將事件發(fā)送出去即可。

三、實(shí)現(xiàn)

3.1 初始化 EventBus

開(kāi)發(fā)者通常是調(diào)用 EventBus#getDefault 方法獲取 EventBus 實(shí)例。

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

getDefault 通過(guò)雙重校驗(yàn)鎖的方式來(lái)實(shí)現(xiàn)單例

構(gòu)造方法

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

public EventBus() {
    this(DEFAULT_BUILDER);
}

通過(guò) getDefault 獲取的 EventBus 對(duì)象是通過(guò)默認(rèn)的 EventBusBuilder 構(gòu)造而成的。

private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();//默認(rèn)為 CachedThreadPool，不限制線程數(shù)

boolean logSubscriberExceptions = true;
boolean logNoSubscriberMessages = true;
boolean sendSubscriberExceptionEvent = true;
boolean sendNoSubscriberEvent = true;
boolean throwSubscriberException;

boolean strictMethodVerification;//
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;//默認(rèn)的線程池
List<Class<?>> skipMethodVerificationForClasses;
List<SubscriberInfoIndex> subscriberInfoIndexes;
MainThreadSupport mainThreadSupport;

EventBus(EventBusBuilder builder) {
    //……
    subscriptionsByEventType = new HashMap<>();
    typesBySubscriber = new HashMap<>();
    stickyEvents = new ConcurrentHashMap<>();
    //……
    eventInheritance = builder.eventInheritance;
    executorService = builder.executorService;
}

主要看以下幾個(gè)單例的實(shí)現(xiàn)。

boolean eventInheritance = true;//是否允許事件繼承
boolean ignoreGeneratedIndex;//是否忽略 生成的 index，默認(rèn)為 false，也就是會(huì)先嘗試尋找編譯期注解生成的訂閱方法信息，找不到再使用反射去獲取。

private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
private final Map<Object, List<Class<?>>> typesBySubscriber;
private final Map<Class<?>, Object> stickyEvents;

private final HandlerPoster mainThreadPoster;
private final BackgroundPoster backgroundPoster;
private final AsyncPoster asyncPoster;

• subscriptionsByEventType ，key 是事件類型，value 為 訂閱了該事件的方法列表
• typesBySubscriber，key 為訂閱者，value 某個(gè)訂閱者訂閱的事件列表
• stickyEvents，key 為事件類型，value 為具體的事件實(shí)例
• mainThreadPoster 主線程分發(fā)
• backgroundPoster 后臺(tái)線程分發(fā)
• asyncPoster 異步線程分發(fā)

3.2 注冊(cè)訂閱

org.greenrobot.eventbus.EventBus#register

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();//獲取訂閱者的 class 對(duì)象
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);//查找訂閱者中所有的訂閱方法
    synchronized (this) {
        //迭代遍歷訂閱者中所有的訂閱方法
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

EventBus#findSubscriberMethods

找出給定 class 中所有的訂閱方法

private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap();//以 class 為 key，方法列表為 value 的，Map 作為緩存

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    List subscriberMethods = (List)METHOD_CACHE.get(subscriberClass);//緩存中獲取
    if(subscriberMethods != null) {
        return subscriberMethods;//緩存命中，直接返回
    } else {
        if(this.ignoreGeneratedIndex) {//忽略編譯期生成的 訂閱方法信息
            subscriberMethods = this.findUsingReflection(subscriberClass);//通過(guò)反射獲取訂閱方法信息
        } else {
            //獲取編譯期生成的 訂閱方法信息
            subscriberMethods = this.findUsingInfo(subscriberClass);//
        }

        if(subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            METHOD_CACHE.put(subscriberClass, subscriberMethods);//添加到緩存中
            return subscriberMethods;//返回
        }
    }
}

EventBus#subscribe()

//必須從同步塊中調(diào)用
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;//事件類型
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);//新建一個(gè) Subscription，存儲(chǔ)訂閱的對(duì)象以及 響應(yīng)的方法
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);//Map<Class<?>, CopyOnWriteArrayList<Subscription>> 從 map 中獲取相應(yīng)訂閱類型的 列表
    if (subscriptions == null) {//如果沒(méi)有則新建一個(gè)
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);//沒(méi)有則拋出異常。
        }
    }

    int size = subscriptions.size();
    //遍歷監(jiān)聽(tīng)列表，將新的 subscription 插入到正確位置。列表按照優(yōu)先級(jí)遞減的順序排序
    for (int i = 0; i <= size; i++) {
        if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }

    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    //如果觸發(fā)的方法 要接收「粘性事件」，獲取相應(yīng)類型的 Event 并觸發(fā)相應(yīng)的方法
    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);
        }
    }
}

該方法會(huì)將相應(yīng)的事件插入對(duì)應(yīng)事件的列表中。如果在方法注解中聲明了 sticky，還會(huì)馬上調(diào)用該方法。

檢測(cè) stick 事件，如果相應(yīng)的事件定義有子類的話，會(huì)遍歷事件的事件子類逐一通知該方法。

3.2.1 通過(guò)反射處理注解

SubscriberMethodFinder#findUsingReflection

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        findUsingReflectionInSingleClass(findState);
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

SubscriberMethodFinder#findUsingReflectionInSingleClass

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            methods = findState.clazz.getDeclaredMethods();//獲取的是類的所有公有方法，這就包括自身 和從基類繼承的、從接口實(shí)現(xiàn)的所有 public 方法。
            //getDeclareMethods 返回的是該類中定義的「所有方法」，但是不包括從父類繼承而來(lái)的方法
        } catch (Throwable th) {
            methods = findState.clazz.getMethods();
            findState.skipSuperClasses = true;
        }
        for (Method method : methods) {//遍歷方法
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {//參數(shù)列表長(zhǎng)度為 0
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            //將訂閱方法信息添加到 findState 中
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } 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");
            }
        }
    }

ignoreGeneratedIndex 是什么?

由于反射成本高,而且 EventBus 3.0 引入了 EventBusAnnotationProcessor,故默認(rèn) ignoreGeneratedIndex 為 false,需要注意的是,如果設(shè)置 ignoreGeneratedIndex 為 true,則前面使用的 MyEventBusIndex 無(wú)效,還是會(huì)走反射解析的分支。

3.2.2 使用編譯期生成的訂閱方法信息

網(wǎng)上有很多介紹 EventBus 的文章,但是幾乎沒(méi)有提到 EventBusAnnotationProcessor 的。在 3.0 版本開(kāi)始，EventBus提供了一個(gè)EventBusAnnotationProcessor注解處理器來(lái)在編譯期通過(guò)讀取@Subscribe()注解并解析,處理其中所包含的信息,然后生成java類來(lái)保存所有訂閱者關(guān)于訂閱的信息,這樣就比在運(yùn)行時(shí)使用反射來(lái)獲得這些訂閱者的信息速度要快.我們可以參考EventBus項(xiàng)目里的EventBusPerformance這個(gè)例子,編譯后我們可以在build文件夾里找到這個(gè)類,MyEventBusIndex 類,當(dāng)然類名是可以自定義的.我們大致看一下生成的MyEventBusIndex類是什么樣的:

訂閱者中的 訂閱方法

public class ReceiveEventFragment extends Fragment {
    //……
    
    @Subscribe(threadMode = ThreadMode.MAIN, priority = 0)
    public void onReceive(MsgEvent event) {
        Log.d(TAG, "onReceive: " + event);
        mTvMsg.setText(String.format("msgId:%d\nmsg:%s", event.msgId, event.msg));
    }

    @Subscribe(threadMode = ThreadMode.MAIN, priority = 10000)
    public void ShowToast(MsgEvent event) {
        Toast.makeText(getActivity(), String.format("msgId:%d\nmsg:%s", event.msgId, event.msg), Toast.LENGTH_SHORT).show();
    }
}

/** This class is generated by EventBus, do not edit. */
public class MyEventBusIndex implements SubscriberInfoIndex {
    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

    static {
        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();//以訂閱者為 key，以訂閱者中的 訂閱方法列表為 value 的 map
        //將 ReceiveEventFragment 的訂閱信息存儲(chǔ)到 map 中
        putIndex(new SimpleSubscriberInfo(com.test.commentdemo.eventbusdemo.ReceiveEventFragment.class,
                true, new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("onReceive", com.test.commentdemo.eventbusdemo.MsgEvent.class,
                    ThreadMode.MAIN),
            new SubscriberMethodInfo("ShowToast", com.test.commentdemo.eventbusdemo.MsgEvent.class,
                    ThreadMode.MAIN, 10000, false),
        }));//訂閱者中所有的訂閱方法
        
        //……代碼省略（存儲(chǔ)其他訂閱者的訂閱信息到 map 中）
    }

    private static void putIndex(SubscriberInfo info) {
        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
    }

    @Override
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);//根據(jù)類型從 Map 獲取訂閱者的訂閱信息
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

繼續(xù)前面 ignoreGeneratedIndex 為 false 時(shí)，會(huì)執(zhí)行以下分支。

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        findState.subscriberInfo = getSubscriberInfo(findState);
        if (findState.subscriberInfo != null) {
            SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();//獲取訂閱的方法信息
            for (SubscriberMethod subscriberMethod : array) {
                if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                    findState.subscriberMethods.add(subscriberMethod);
                }
            }
        } else {
            findUsingReflectionInSingleClass(findState);//如果找不到相應(yīng)的訂閱方法信息（可能使用 EventBus 實(shí)例之前，沒(méi)有將MyEventBusIndex ），需要通過(guò)反射獲取訂閱方法信息
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

findUsingInfo()方法,其無(wú)非就是通過(guò)查找我們前面所說(shuō)的MyEventBusIndex類中的信息,來(lái)轉(zhuǎn)換成List<SubscriberMethod>從而獲得訂閱類的相關(guān)訂閱函數(shù)的各種信息

SubscriberMethodFinder#getSubscriberInfo

private SubscriberInfo getSubscriberInfo(SubscriberMethodFinder.FindState findState) {
    if(findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) 
        SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();//獲取父類的訂閱方法信息
        if(findState.clazz == superclassInfo.getSubscriberClass()) {//
            return superclassInfo;
        }
    }

    if(this.subscriberInfoIndexes != null) {
        Iterator superclassInfo1 = this.subscriberInfoIndexes.iterator();
        //遍歷，從 index 中獲取 訂閱信息
        while(superclassInfo1.hasNext()) {
            SubscriberInfoIndex index = (SubscriberInfoIndex)superclassInfo1.next();
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);//從自動(dòng)生成的 MyEventBusIndex 類中的 SUBSCRIBER_INDEX 里面獲取訂閱方法信息
            if(info != null) {
                return info;
            }
        }
    }
    return null;
}

3.3 解注冊(cè)

/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);//獲取訂閱者訂閱的事件列表
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {//遍歷訂閱者所訂閱的事件
            //從事件對(duì)應(yīng)的訂閱列表中 訂閱者的訂閱信息
            unsubscribeByEventType(subscriber, eventType);
        typesBySubscriber.remove(subscriber);//移除訂閱者
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
    List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    //從事件對(duì)應(yīng)的訂閱列表中 訂閱者的訂閱信息
    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--;
            }
        }
    }
}

3.4 發(fā)送事件

EventBus#post

1. 獲取訂閱列表

public class EventBus {

    private final ThreadLocal<EventBus.PostingThreadState> currentPostingThreadState;//線程私有變量——當(dāng)前發(fā)布線程狀態(tài)

    public void post(Object event) {
        EventBus.PostingThreadState postingState = (EventBus.PostingThreadState)this.currentPostingThreadState.get();//記錄發(fā)布線程的狀態(tài)（比如是否是主線程）
        List eventQueue = postingState.eventQueue;//從發(fā)布狀態(tài)中獲取事件隊(duì)列
        eventQueue.add(event);//添加進(jìn)事件隊(duì)列的隊(duì)尾
        if(!postingState.isPosting) {//當(dāng)前不是處于分發(fā)狀態(tài)
            postingState.isMainThread = this.isMainThread();//是否在主線程
            postingState.isPosting = true;//將狀態(tài)改為發(fā)布中
            if(postingState.canceled) {//取消發(fā)布
                throw new EventBusException("Internal error. Abort state was not reset
            }

            try {
                while(!eventQueue.isEmpty()) {//只要事件隊(duì)列非空，就一直往外取出事件并發(fā)布
                    this.postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }
                                            
    static final class PostingThreadState {
        final List<Object> eventQueue = new ArrayList();//事件隊(duì)列
        boolean isPosting;
        boolean isMainThread;
        Subscription subscription;
        Object event;
        boolean canceled;

        PostingThreadState() {
        }
    }                                                                   
}

final static class PostingThreadState {
    final List<Object> eventQueue = new ArrayList<>();//事件隊(duì)列
    boolean isPosting;//是否正在分發(fā)中
    boolean isMainThread;//是否在主線程
    Subscription subscription;
    Object event;//事件
    boolean canceled;//取消
}

private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
};//currentPostingThreadState 是一個(gè)線程局部變量


/** Posts the given event to the event bus. */
public void post(Object event) {
    PostingThreadState postingState = currentPostingThreadState.get();//獲取當(dāng)前線程事件分發(fā)狀態(tài)
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);//添加到事務(wù)隊(duì)列中

    if (!postingState.isPosting) {//如果當(dāng)前不是在分發(fā)狀態(tài),則進(jìn)入分發(fā)狀態(tài)
        postingState.isMainThread = isMainThread();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            //編譯事件隊(duì)列，逐一進(jìn)行分發(fā)
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

EventBus#postSingleEvent

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    if (eventInheritance) {//如果事件支持子類型，查找該事件的所有子類型
        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));
        }
    }
}

EventBus#postSingleEventForEventType，

 //查找 事件所對(duì)應(yīng)的訂閱者列表，然后迭代列表，切換到目標(biāo)線程執(zhí)行
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
        subscriptions = subscriptionsByEventType.get(eventClass);//查找 事件所對(duì)應(yīng)的訂閱者列表
    }
    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) {//事件取消了
                break;
            }
        }
        return true;
    }
    return false;
}

2.切換到指定線程

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 MAIN_ORDERED:
            if (mainThreadPoster != null) {
                mainThreadPoster.enqueue(subscription, event);
            } else {
                // temporary: technically not correct as poster not decoupled from subscriber
                invokeSubscriber(subscription, event);
            }
            break;
        case BACKGROUND://后臺(tái)線程
            if (isMainThread) {
                backgroundPoster.enqueue(subscription, event);
            } else {
                invokeSubscriber(subscription, event);
            }
            break;
        case ASYNC://新建一個(gè)子線程處理
            asyncPoster.enqueue(subscription, event);
            break;
        default:
            throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    }
}

BACKGROUND

public void enqueue(Subscription subscription, Object event) {
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);//包裝為一個(gè) PendingPost
    synchronized (this) {//加鎖，進(jìn)入同步塊
        queue.enqueue(pendingPost);//添加到隊(duì)列中
        if (!executorRunning) {//如果現(xiàn)在沒(méi)有在執(zhí)行 后臺(tái)任務(wù)，則獲取一個(gè)新線程執(zhí)行任務(wù)
            executorRunning = true;
            eventBus.getExecutorService().execute(this);
        }
    }
}

@Override
public void run() {
    try {
        try {
            while (true) {//循環(huán)，直到隊(duì)列為空
                PendingPost pendingPost = queue.poll(1000);//獲取 PendingPost，最多等到 1 秒

                if (pendingPost == null) {
                    //前面取不到 PendingPost，下面進(jìn)行雙重校驗(yàn)檢查
                    synchronized (this) {
                        // Check again, this time in synchronized
                        pendingPost = queue.poll();//
                        if (pendingPost == null) {
                            //隊(duì)列確實(shí)為空，停止運(yùn)行
                            executorRunning = false;
                            return;
                        }
                    }
                }
                eventBus.invokeSubscriber(pendingPost);//通過(guò)反射調(diào)用相應(yīng)的方法
            }
        } catch (InterruptedException e) {
            eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
        }
    } finally {
        executorRunning = false;
    }
}

MAIN

mainThreadPoster.enqueue(subscription, event);

HandlerPoster#enqueue

public void enqueue(Subscription subscription, Object event) {
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) {//加鎖
        queue.enqueue(pendingPost);//加入 pendingQueue 中。該隊(duì)列會(huì)在 handleMessage 方法中調(diào)用
        if (!handlerActive) {//如果當(dāng)前 handler 不是處于活躍狀態(tài)，則退出
            handlerActive = true;
            if (!sendMessage(obtainMessage())) {//發(fā)送信息，提示
                throw new EventBusException("Could not send handler message");
            }
        }
    }
}

org.greenrobot.eventbus.HandlerPoster#handleMessage

@Override
public 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;
                    }
                }
            }
            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;
    }
}

在 handleMessage 方法內(nèi)部停留時(shí)間不能大于 10 毫秒，從

MAIN_ORDERED

新增加的模式，但是當(dāng)前版本的實(shí)現(xiàn)還不完善。

ASYN

將事件添加 cachedThreadPool 中執(zhí)行(如果當(dāng)前有空閑線程，則復(fù)用空閑線程，如果沒(méi)有就創(chuàng)建新線程)

public void enqueue(Subscription subscription, Object event) {
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    queue.enqueue(pendingPost);
    eventBus.getExecutorService().execute(this);//放到默認(rèn)的 cachedThreaPool 中執(zhí)行
}

@Override
public void run() {
    PendingPost pendingPost = queue.poll();
    if(pendingPost == null) {
        throw new IllegalStateException("No pending post available");
    }
    eventBus.invokeSubscriber(pendingPost);
}

3.反射調(diào)用訂閱方法

org.greenrobot.eventbus.EventBus#invokeSubscriber(org.greenrobot.eventbus.PendingPost)

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

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);
    }
}

切換到指定線程之后，通過(guò)反射的方法，調(diào)用訂閱方法。

四、總結(jié)

EventBus 的實(shí)現(xiàn)原理可以歸結(jié)為以下三點(diǎn)

• 注冊(cè)—>掃描訂閱方法，添加到訂閱方法列表中
• 發(fā)送事件—>根據(jù)事件的類型，遍歷方法列表，反射調(diào)用訂閱方法
• 解注冊(cè)—>從訂閱方法列表中移除相應(yīng)的訂閱方法

EventBus 雖然不是標(biāo)準(zhǔn)的觀察者模式的實(shí)現(xiàn), 但是它的整體就是一個(gè)發(fā)布 / 訂閱框架, 也擁有觀察者模式的優(yōu)點(diǎn), 比如: 發(fā)布者和訂閱者的解耦。

五、參考資料與學(xué)習(xí)資源推薦

• EventBus 源碼解析
• EventBus 高效使用及源碼解析-知乎專欄
• EventBus 3.0 源碼分析
• EventBus 3.0 源碼分析（二）注解處理器的使用

由于本人水平有限，可能出于誤解或者筆誤難免出錯(cuò)，如果發(fā)現(xiàn)有問(wèn)題或者對(duì)文中內(nèi)容存在疑問(wèn)歡迎在下面評(píng)論區(qū)告訴我。謝謝！