相關(guān)文章
Android事件總線（一）EventBus3.0用法全解析
前言
上一篇我們講到了EventBus3.0的用法，這一篇我們來講一下EventBus3.0的源碼以及它的利與弊。
1.構(gòu)造函數(shù)
當(dāng)我們要調(diào)用EventBus的功能時(shí)，比如注冊(cè)或者發(fā)送事件，總會(huì)調(diào)用EventBus.getDefault()來獲取EventBus實(shí)例：
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
很明顯這是一個(gè)單例模式，采用了雙重檢查模式 （DCL），不了解的同學(xué)可以查看設(shè)計(jì)模式（二）單例模式的七種寫法這篇文章。
接下來看看new EventBus()做了什么：
public EventBus() {
this(DEFAULT_BUILDER);
}
這里DEFAULT_BUILDER是默認(rèn)的EventBusBuilder，用來構(gòu)造EventBus：
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
this調(diào)用了EventBus另一個(gè)構(gòu)造函數(shù)：
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;
}
2.訂閱者注冊(cè)
獲取到EventBus后，便可以將訂閱者注冊(cè)到EventBus中，下面來看一下register方法：
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
// 用 subscriberMethodFinder 提供的方法，找到在 subscriber 這個(gè)類里面訂閱的內(nèi)容。
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
查找訂閱方法
findSubscriberMethods找出一個(gè)SubscriberMethod的集合，也就是傳進(jìn)來的訂閱者所有的訂閱的方法，接下來遍歷訂閱者的訂閱方法來完成訂閱者的訂閱操作。對(duì)于SubscriberMethod（訂閱方法）類中，主要就是用保存訂閱方法的Method對(duì)象、線程模式、事件類型、優(yōu)先級(jí)、是否是粘性事件等屬性。下面就來看一下findSubscriberMethods方法：
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//從緩存中獲取SubscriberMethod集合
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//ignoreGeneratedIndex屬性表示是否忽略注解器生成的MyEventBusIndex
if (ignoreGeneratedIndex) {
//通過反射獲取subscriberMethods
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
//在獲得subscriberMethods以后，如果訂閱者中不存在@Subscribe注解并且為public的訂閱方法，則會(huì)拋出異常。
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;
}
}
首先從緩存中查找，如果找到了就立馬返回。如果緩存中沒有的話，則根據(jù) ignoreGeneratedIndex 選擇如何查找訂閱方法，ignoreGeneratedIndex屬性表示是否忽略注解器生成的MyEventBusIndex。不了解MyEventBusIndex的同學(xué)可以查看【Bugly干貨分享】老司機(jī)教你 “飆” EventBus 3這篇文章。最后，找到訂閱方法后，放入緩存，以免下次繼續(xù)查找。ignoreGeneratedIndex 默認(rèn)就是false，可以通過EventBusBuilder來設(shè)置它的值。我們?cè)陧?xiàng)目中經(jīng)常通過EventBus單例模式來獲取默認(rèn)的EventBus對(duì)象，也就是ignoreGeneratedIndex為false的情況，這種情況調(diào)用了findUsingInfo方法：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//獲取訂閱者信息，沒有配置MyEventBusIndex返回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 {
//通過反射來查找訂閱方法
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
通過getSubscriberInfo方法來獲取訂閱者信息。在我們開始查找訂閱方法的時(shí)候并沒有忽略注解器為我們生成的索引MyEventBusIndex，如果我們通過EventBusBuilder配置了MyEventBusIndex，便會(huì)獲取到subscriberInfo，調(diào)用subscriberInfo的getSubscriberMethods方法便可以得到訂閱方法相關(guān)的信息，這個(gè)時(shí)候就不在需要通過注解進(jìn)行獲取訂閱方法。如果沒有配置MyEventBusIndex，便會(huì)執(zhí)行findUsingReflectionInSingleClass方法，將訂閱方法保存到findState中。最后再通過getMethodsAndRelease方法對(duì)findState做回收處理并反回訂閱方法的List集合。
對(duì)于MyEventBusIndex的配置它是一個(gè)可選項(xiàng)，所以在這里就不在進(jìn)行詳細(xì)的介紹，下面就來看一下findUsingReflectionInSingleClass的執(zhí)行過程：
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();
} 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) {
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");
}
}
}
在這里主要是使用了Java的反射和對(duì)注解的解析。首先通過反射來獲取訂閱者中所有的方法。并根據(jù)方法的類型，參數(shù)和注解來找到訂閱方法。找到訂閱方法后將訂閱方法相關(guān)信息保存到FindState當(dāng)中。
訂閱者的注冊(cè)過程
在查找完所有的訂閱方法以后便開始對(duì)所有的訂閱方法進(jìn)行注冊(cè)：
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
//根據(jù)訂閱者和訂閱方法構(gòu)造一個(gè)訂閱事件
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//獲取當(dāng)前訂閱事件中Subscription的List集合
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
//該事件對(duì)應(yīng)的Subscription的List集合不存在，則重新創(chuàng)建并保存在subscriptionsByEventType中
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//訂閱者已經(jīng)注冊(cè)則拋出EventBusException
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//遍歷訂閱事件，找到比subscriptions中訂閱事件小的位置，然后插進(jìn)去
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;
}
}
//通過訂閱者獲取該訂閱者所訂閱事件的集合
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
//將當(dāng)前的訂閱事件添加到subscribedEvents中
subscribedEvents.add(eventType);
if (subscriberMethod.sticky) {
if (eventInheritance) {
//粘性事件的處理
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);
}
}
}
訂閱的代碼主要就做了兩件事，第一件事是將我們的訂閱方法和訂閱者封裝到subscriptionsByEventType和typesBySubscriber中，subscriptionsByEventType是我們投遞訂閱事件的時(shí)候，就是根據(jù)我們的EventType找到我們的訂閱事件，從而去分發(fā)事件，處理事件的；typesBySubscriber在調(diào)用unregister(this)的時(shí)候，根據(jù)訂閱者找到EventType，又根據(jù)EventType找到訂閱事件，從而對(duì)訂閱者進(jìn)行解綁。第二件事，如果是粘性事件的話，就立馬投遞、執(zhí)行。
3.事件的發(fā)送
在獲取到EventBus對(duì)象以后，可以通過post方法來進(jìn)行對(duì)事件的提交：
public void post(Object event) {
//PostingThreadState保存著事件隊(duì)列和線程狀態(tài)信息
PostingThreadState postingState = currentPostingThreadState.get();
//獲取事件隊(duì)列，并將當(dāng)前事插入到事件隊(duì)列中
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(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 {
//處理隊(duì)列中的所有事件
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
首先從PostingThreadState對(duì)象中取出事件隊(duì)列，然后再將當(dāng)前的事件插入到事件隊(duì)列當(dāng)中。最后將隊(duì)列中的事件依次交由postSingleEvent方法進(jìn)行處理，并移除該事件。來看看postSingleEvent方法里做了什么：
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//eventInheritance表示是否向上查找事件的父類,默認(rèn)為true
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);
}
//找不到該事件時(shí)的異常處理
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));
}
}
}
eventInheritance表示是否向上查找事件的父類,它的默認(rèn)值為true，可以通過在EventBusBuilder中來進(jìn)行配置。當(dāng)eventInheritance為true時(shí)，則通過lookupAllEventTypes找到所有的父類事件并存在List中，然后通過postSingleEventForEventType方法對(duì)事件逐一處理，接下來看看postSingleEventForEventType方法：
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
//取出該事件對(duì)應(yīng)的Subscription集合
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
//將該事件的event和對(duì)應(yīng)的Subscription中的信息（包擴(kuò)訂閱者類和訂閱方法）傳遞給postingState
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
//對(duì)事件進(jìn)行處理
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;
}
同步取出該事件對(duì)應(yīng)的Subscription集合并遍歷該集合將事件event和對(duì)應(yīng)Subscription傳遞給postingState并調(diào)用postToSubscription方法對(duì)事件進(jìn)行處理，接下來看看postToSubscription方法：
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);
}
}
取出訂閱方法的線程模式，之后根據(jù)線程模式來分別處理。舉個(gè)例子，如果線程模式是MAIN，提交事件的線程是主線程的話則通過反射，直接運(yùn)行訂閱的方法，如果不是主線程，我們需要mainThreadPoster將我們的訂閱事件入隊(duì)列，mainThreadPoster是HandlerPoster類型的繼承自Handler，通過Handler將訂閱方法切換到主線程執(zhí)行。
4.訂閱者取消注冊(cè)
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());
}
}
typesBySubscriber我們?cè)谟嗛喺咦?cè)的過程中講到過這個(gè)屬性，他根據(jù)訂閱者找到EventType，然后根據(jù)EventType和訂閱者來得到訂閱事件來對(duì)訂閱者進(jìn)行解綁。
5.核心架構(gòu)與利弊
核心架構(gòu)
從EventBus作者提供的圖我們可以看到EventBus的核心架構(gòu)，其實(shí)就是基于觀察者模式來實(shí)現(xiàn)的，關(guān)于觀察者模式可以查看設(shè)計(jì)模式（五）觀察者模式這篇文章。
利與弊
EventBus好處比較明顯，它能夠解耦和，將業(yè)務(wù)和視圖分離，代碼實(shí)現(xiàn)比較容易。而且3.0后，我們可以通過apt預(yù)編譯找到訂閱者，避免了運(yùn)行期間的反射處理解析，大大提高了效率。當(dāng)然EventBus也會(huì)帶來一些隱患和弊端，如果濫用的話會(huì)導(dǎo)致邏輯的分散并造成維護(hù)起來的困難。另外大量采用EventBus代碼的可讀性也會(huì)變差。
參考資料
EventBus3.0源碼
EventBus3.0 源碼解析
EventBus 利弊與源碼解析
EventBus3.0源碼解讀
【Bugly干貨分享】老司機(jī)教你 “飆” EventBus 3
