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前面在 EventBus設(shè)計(jì)與實(shí)現(xiàn)分析——特性介紹 一文中介紹了EventBus的基本用法，及其提供的大多數(shù)特性的用法，這讓我們對(duì)EventBus為用戶提供的主要功能有了大體的了解，為我們后續(xù)理解EventBus的設(shè)計(jì)決策提供了良好的基礎(chǔ)。這里我們就開始深入到EventBus的實(shí)現(xiàn)細(xì)節(jié)，先來(lái)了解其中的訂閱者注冊(cè)的過(guò)程。

訂閱者的注冊(cè)

EventBus使用了一種類似于閉包的機(jī)制來(lái)描述訂閱者。即對(duì)于EventBus而言，一個(gè)訂閱者由一個(gè)方法＋該方法綁定的上下文（也就是該方法綁定的對(duì)象）＋訂閱者感興趣的事件的類型來(lái)描述。具體到實(shí)現(xiàn)，即是通過(guò)SubscriberMethod＋SubscriberMethod這樣幾個(gè)類來(lái)描述。

我們可以看一下這幾個(gè)類的定義，首先是Subscription類：

package org.greenrobot.eventbus;final class Subscription {final Object subscriber;final SubscriberMethod subscriberMethod;/*** Becomes false as soon as {@link EventBus#unregister(Object)} is called, which is checked by queued event delivery* {@link EventBus#invokeSubscriber(PendingPost)} to prevent race conditions.*/volatile boolean active;Subscription(Object subscriber, SubscriberMethod subscriberMethod) {this.subscriber = subscriber;this.subscriberMethod = subscriberMethod;active = true;}@Overridepublic boolean equals(Object other) {if (other instanceof Subscription) {Subscription otherSubscription = (Subscription) other;return subscriber == otherSubscription.subscriber&& subscriberMethod.equals(otherSubscription.subscriberMethod);} else {return false;}}@Overridepublic int hashCode() {return subscriber.hashCode() + subscriberMethod.methodString.hashCode();} }

subscriberMethod即是我們前面提到的方法，而subscriber則是上下文object。

然后是SubscriberMethod類：

package org.greenrobot.eventbus;import java.lang.reflect.Method;/** Used internally by EventBus and generated subscriber indexes. */ public class SubscriberMethod {final Method method;final ThreadMode threadMode;final Class<?> eventType;final int priority;final boolean sticky;/** Used for efficient comparison */String methodString;public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) {this.method = method;this.threadMode = threadMode;this.eventType = eventType;this.priority = priority;this.sticky = sticky;}@Overridepublic boolean equals(Object other) {if (other == this) {return true;} else if (other instanceof SubscriberMethod) {checkMethodString();SubscriberMethod otherSubscriberMethod = (SubscriberMethod)other;otherSubscriberMethod.checkMethodString();// Don't use method.equals because of http://code.google.com/p/android/issues/detail?id=7811#c6return methodString.equals(otherSubscriberMethod.methodString);} else {return false;}}private synchronized void checkMethodString() {if (methodString == null) {// Method.toString has more overhead, just take relevant parts of the methodStringBuilder builder = new StringBuilder(64);builder.append(method.getDeclaringClass().getName());builder.append('#').append(method.getName());builder.append('(').append(eventType.getName());methodString = builder.toString();}}@Overridepublic int hashCode() {return method.hashCode();} }

訂閱者的注冊(cè)過(guò)程，即是構(gòu)造SubscriberMethod，繼而構(gòu)造Subscription，并對(duì)這些對(duì)象進(jìn)行存儲(chǔ)管理的過(guò)程。我們可以通過(guò)EventBus.register()方法的定義具體看一下在EventBus中是怎么做：

/*** Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they* are no longer interested in receiving events.* <p/>* Subscribers have event handling methods that must be annotated by {@link Subscribe}.* The {@link Subscribe} annotation also allows configuration like {@link* ThreadMode} and priority.*/public void register(Object subscriber) {Class<?> subscriberClass = subscriber.getClass();List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);synchronized (this) {for (SubscriberMethod subscriberMethod : subscriberMethods) {subscribe(subscriber, subscriberMethod);}}}// Must be called in synchronized blockprivate void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {Class<?> eventType = subscriberMethod.eventType;Subscription newSubscription = new Subscription(subscriber, subscriberMethod);CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);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);}}int size = subscriptions.size();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);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);}}}

通過(guò)EventBus.register()和EventBus.subscribe()可以看到，register的過(guò)程主要做了這樣一些事情：

查找訂閱者對(duì)象的所有訂閱者方法。方法隸屬于類，不以對(duì)象的改變而改變，應(yīng)用程序的一次生命周期中，這些方法總是不變的。

構(gòu)造Subscription來(lái)描述每一個(gè)訂閱者，也就是訂閱者對(duì)象＋一個(gè)訂閱者方法。

建立事件類型與訂閱者之間的映射關(guān)系，這主要通過(guò)subscriptionsByEventType數(shù)據(jù)結(jié)構(gòu)來(lái)描述。對(duì)于每一個(gè)事件類型，其訂閱者會(huì)依優(yōu)先級(jí)而排序。不難看出，這個(gè)結(jié)構(gòu)主要用于事件的發(fā)布，在事件發(fā)布時(shí)，可以通過(guò)這個(gè)結(jié)構(gòu)快速地找到每一個(gè)訂閱者。
這個(gè)地方查找正確的插入位置是用的順序查找的方法，估計(jì)是考慮到對(duì)于一個(gè)事件類型，訂閱者一般不會(huì)太多，而訂閱者的優(yōu)先級(jí)一般不會(huì)很分散的緣故。

建立訂閱者對(duì)象與該對(duì)象監(jiān)聽的事件類型之間的映射關(guān)系，這主要由typesBySubscriber數(shù)據(jù)結(jié)構(gòu)來(lái)描述。這個(gè)結(jié)構(gòu)則主要用于注銷一個(gè)對(duì)象對(duì)事件的監(jiān)聽。注銷一個(gè)對(duì)象對(duì)事件得監(jiān)聽時(shí)，可以通過(guò)這個(gè)結(jié)構(gòu)快速地將監(jiān)聽者對(duì)象得信息從EventBus中整個(gè)的移除出去。

處理對(duì)Sticky事件的監(jiān)聽。如我們前面所見(jiàn)，這種類型的事件在發(fā)布時(shí)會(huì)通知到每個(gè)監(jiān)聽者，同時(shí)還會(huì)被保存起來(lái)，當(dāng)后面再次注冊(cè)對(duì)這種事件的監(jiān)聽時(shí)，監(jiān)聽者會(huì)直接得到通知。

EventBus使用SubscriberMethodFinder來(lái)查找一個(gè)類中所有的監(jiān)聽者方法，主要即是findSubscriberMethods()方法。

如我們前面所見(jiàn)，要使一個(gè)類的某個(gè)方法成為監(jiān)聽者方法，需要給該方法添加Subscribe annotation，在SubscriberMethodFinder.findSubscriberMethods()中會(huì)查找加了這個(gè)annotation的方法。但老版本的EventBus并不是通過(guò)annotation來(lái)標(biāo)識(shí)一個(gè)監(jiān)聽者方法的，而是通過(guò)命名模式等方法，因而這里可以看到一種更為靈活的設(shè)計(jì)，來(lái)給用戶提供空間，以支持不同的標(biāo)識(shí)監(jiān)聽者方法的方法：

class SubscriberMethodFinder {/** In newer class files, compilers may add methods. Those are called bridge or synthetic methods.* EventBus must ignore both. There modifiers are not public but defined in the Java class file format:* http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.6-200-A.1*/private static final int BRIDGE = 0x40;private static final int SYNTHETIC = 0x1000;private static final int MODIFIERS_IGNORE = Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();private List<SubscriberInfoIndex> subscriberInfoIndexes;private final boolean strictMethodVerification;private final boolean ignoreGeneratedIndex;private static final int POOL_SIZE = 4;private static final FindState[] FIND_STATE_POOL = new FindState[POOL_SIZE];SubscriberMethodFinder(List<SubscriberInfoIndex> subscriberInfoIndexes, boolean strictMethodVerification,boolean ignoreGeneratedIndex) {this.subscriberInfoIndexes = subscriberInfoIndexes;this.strictMethodVerification = strictMethodVerification;this.ignoreGeneratedIndex = ignoreGeneratedIndex;}List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);if (subscriberMethods != null) {return subscriberMethods;}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 {METHOD_CACHE.put(subscriberClass, subscriberMethods);return subscriberMethods;}}

由于在應(yīng)用程序的整個(gè)生命周期中，一個(gè)類的監(jiān)聽者方法總是不會(huì)變的，因而可以看到在SubscriberMethodFinder中有創(chuàng)建一個(gè)緩存METHOD_CACHE，以監(jiān)聽者類為key，以監(jiān)聽者方法的列表為值。在SubscriberMethodFinder.findSubscriberMethods()方法中會(huì)首先檢查這個(gè)緩存中是否已經(jīng)有了對(duì)應(yīng)類的監(jiān)聽者方法，如果有的話會(huì)直接返回給調(diào)用者，沒(méi)有的時(shí)候才會(huì)通過(guò)反射等機(jī)制來(lái)查找。

ignoreGeneratedIndex用來(lái)標(biāo)記是否要忽略用戶定制的監(jiān)聽者方法標(biāo)識(shí)方法。若忽略則直接利用反射機(jī)制，也就是Subscribe annotation機(jī)制來(lái)查找監(jiān)聽者方法，即調(diào)用findUsingReflection()方法；否則會(huì)先嘗試使用用戶定制的機(jī)制來(lái)查找，查找失敗時(shí)才會(huì)使用反射機(jī)制來(lái)查找，這也就是調(diào)用findUsingInfo()方法。

在findUsingReflection()方法中實(shí)現(xiàn)了如我們前面提到的用annotation標(biāo)識(shí)監(jiān)聽者方法的方式下，查找監(jiān)聽者方法的過(guò)程：

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);while (findState.clazz != null) {findUsingReflectionInSingleClass(findState);findState.moveToSuperclass();}return getMethodsAndRelease(findState);}private void findUsingReflectionInSingleClass(FindState findState) {Method[] methods;try {// This is faster than getMethods, especially when subscribers are fat classes like Activitiesmethods = findState.clazz.getDeclaredMethods();} catch (Throwable th) {// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149methods = findState.clazz.getMethods();findState.skipSuperClasses = true;}for (Method method : methods) {int modifiers = method.getModifiers();if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {Class<?>[] parameterTypes = method.getParameterTypes();if (parameterTypes.length == 1) {Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);if (subscribeAnnotation != null) {Class<?> eventType = parameterTypes[0];if (findState.checkAdd(method, eventType)) {ThreadMode threadMode = subscribeAnnotation.threadMode();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");}}}

在findUsingReflection()中，會(huì)借助于FindState來(lái)查找所有的監(jiān)聽者方法。FindState類的角色主要有三個(gè)，一是迭代器，用于在監(jiān)聽者類的整個(gè)繼承體系中遍歷每一個(gè)類；二是容器，用來(lái)存放找到的每一個(gè)監(jiān)聽者方法；三是對(duì)找到的方法做驗(yàn)證。

findUsingReflectionInSingleClass()方法找到一個(gè)特定類中所有的監(jiān)聽者方法。這里可以清楚地看到EventBus對(duì)于監(jiān)聽者方法的聲明的完整限制。一個(gè)方法被認(rèn)定為監(jiān)聽者方法的條件為：

方法的可見(jiàn)性被聲明為全局可見(jiàn)public，同時(shí)沒(méi)有abstract、static、bridge及synthetic修飾。

方法的參數(shù)只有一個(gè)。

有annotation Subscribe的聲明。

FindState.checkAdd()會(huì)實(shí)施一些額外的限制。主要是關(guān)于，類中聲明了多個(gè)方法監(jiān)聽相同事件的處理；及方法override的處理，即在類的繼承體系中，有多個(gè)類聲明了相同的方法，對(duì)相同的事件進(jìn)行監(jiān)聽。

strictMethodVerification是一個(gè)調(diào)試開關(guān)，如果開關(guān)打開，則當(dāng)一個(gè)類聲明了annotation Subscribe，但modifier或參數(shù)列表不滿足要求時(shí)，會(huì)直接拋出異常。

我們?cè)賮?lái)詳細(xì)地看一下FindState的定義：

static class FindState {final List<SubscriberMethod> subscriberMethods = new ArrayList<>();final Map<Class, Object> anyMethodByEventType = new HashMap<>();final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();final StringBuilder methodKeyBuilder = new StringBuilder(128);Class<?> subscriberClass;Class<?> clazz;boolean skipSuperClasses;SubscriberInfo subscriberInfo;void initForSubscriber(Class<?> subscriberClass) {this.subscriberClass = clazz = subscriberClass;skipSuperClasses = false;subscriberInfo = null;}void recycle() {subscriberMethods.clear();anyMethodByEventType.clear();subscriberClassByMethodKey.clear();methodKeyBuilder.setLength(0);subscriberClass = null;clazz = null;skipSuperClasses = false;subscriberInfo = null;}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 {if (existing instanceof Method) {if (!checkAddWithMethodSignature((Method) existing, eventType)) {// Paranoia checkthrow new IllegalStateException();}// Put any non-Method object to "consume" the existing MethodanyMethodByEventType.put(eventType, this);}return checkAddWithMethodSignature(method, eventType);}}private boolean checkAddWithMethodSignature(Method method, Class<?> eventType) {methodKeyBuilder.setLength(0);methodKeyBuilder.append(method.getName());methodKeyBuilder.append('>').append(eventType.getName());String methodKey = methodKeyBuilder.toString();Class<?> methodClass = method.getDeclaringClass();Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass);if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {// Only add if not already found in a sub classreturn true;} else {// Revert the put, old class is further down the class hierarchysubscriberClassByMethodKey.put(methodKey, methodClassOld);return false;}}void moveToSuperclass() {if (skipSuperClasses) {clazz = null;} else {clazz = clazz.getSuperclass();String clazzName = clazz.getName();/** Skip system classes, this just degrades performance. */if (clazzName.startsWith("java.") || clazzName.startsWith("javax.") || clazzName.startsWith("android.")) {clazz = null;}}}}

moveToSuperclass()是迭代器方法，用于幫助SubscriberMethodFinder在訂閱者類的整個(gè)繼承層次中進(jìn)行遍歷。這里可以看到，這個(gè)遍歷過(guò)程一般不會(huì)遍歷到Object才停止，如果訂閱者類直接或間接繼承了java標(biāo)準(zhǔn)庫(kù)或android框架層的類的話，遍歷到那些類的時(shí)候就會(huì)終止了。checkAdd(Method method, Class<?> eventType)方法施加我們前面提到的額外限制，這個(gè)過(guò)程大體為：

訂閱者類的整個(gè)繼承層次中只有一個(gè)方法訂閱了某個(gè)特定類型的事件，則該方法直接被認(rèn)定為訂閱者方法。

繼承層次中對(duì)同一個(gè)事件類型有多個(gè)訂閱者方法的，只要不同方法的方法名不同，則都會(huì)被認(rèn)定為訂閱者方法。

繼承層次中某個(gè)被override的方法被聲明了訂閱者方法，則繼承層次中深度最深的類的方法被認(rèn)定為訂閱者方法。子類比父類在繼承層次中的深度要深。

SubscriberMethodFinder中FindState對(duì)象并不會(huì)總是被重新創(chuàng)建，而是有一個(gè)緩沖池，需要的時(shí)候從這個(gè)池子中獲取所需得對(duì)象，而在對(duì)象用完之后，又會(huì)被重新歸還到池子中，這些操作的詳細(xì)過(guò)程可以在prepareFindState()和getMethodsAndRelease()這兩個(gè)方法中看到：

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

findUsingInfo()和getSubscriberInfo()方法則允許用戶插入自定義的訂閱者方法聲明機(jī)制：

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();for (SubscriberMethod subscriberMethod : array) {if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {findState.subscriberMethods.add(subscriberMethod);}}} else {findUsingReflectionInSingleClass(findState);}findState.moveToSuperclass();}return getMethodsAndRelease(findState);} ......private SubscriberInfo getSubscriberInfo(FindState findState) {if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();if (findState.clazz == superclassInfo.getSubscriberClass()) {return superclassInfo;}}if (subscriberInfoIndexes != null) {for (SubscriberInfoIndex index : subscriberInfoIndexes) {SubscriberInfo info = index.getSubscriberInfo(findState.clazz);if (info != null) {return info;}}}return null;}

所謂的用戶自定義訂閱者方法聲明機(jī)制，需要通過(guò)實(shí)現(xiàn)SubscriberInfoIndex和SubscriberInfo接口來(lái)實(shí)現(xiàn)，并通過(guò)EventBusBuilder在創(chuàng)建EventBus對(duì)象時(shí)傳遞進(jìn)來(lái)。SubscriberInfoIndex和SubscriberInfo接口的定義為：

package org.greenrobot.eventbus.meta;/*** Interface for generated indexes.*/ public interface SubscriberInfoIndex {SubscriberInfo getSubscriberInfo(Class<?> subscriberClass); }

和

package org.greenrobot.eventbus.meta;import org.greenrobot.eventbus.SubscriberMethod;/** Base class for generated index classes created by annotation processing. */ public interface SubscriberInfo {Class<?> getSubscriberClass();SubscriberMethod[] getSubscriberMethods();SubscriberInfo getSuperSubscriberInfo();boolean shouldCheckSuperclass(); }

至此，EventBus中，注冊(cè)訂閱者的過(guò)程就分析完了。

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