看了別人的原始碼文章，一開始有點懵，還要複習下反射和註解器的知識。然後自己也做了下筆記，加深印象。
參考文章：
描述
EventBus是Android和Java的釋出/訂閱事件匯流排。
優缺點：
- 簡化了元件之間的通訊，將事件釋出者和訂閱者分離
- 程式碼簡單
- 包體積小缺點：利用反射，效能可能會差一點。可以使用EventBus annotation processor（EventBus註解處理器），在編譯期間建立了訂閱者索引。（提升EventBus效能：greenrobot.org/eventbus/do…
三要素：
- Event：自定義事件型別
- Subscriber：事件訂閱者。在EventBus3.0之前我們必須定義以onEvent開頭的那幾個方法，分別是onEvent、onEventMainThread、onEventBackgroundThread和onEventAsync，而在3.0之後事件處理的方法名可以隨意取，不過需要加上註解@subscribe()，並且指定執行緒模型，預設是POSTING。
- Publisher：事件釋出者。可以在任意執行緒裡釋出事件，一般情況下，使用EventBus.getDefault()就可以得到一個EventBus物件，然後再呼叫post(Object)方法即可。
4個執行緒：
- POSTING:在釋出執行緒的事件執行
- MAIN：在主執行緒執行。
- BACKGROUND：如果釋出事件的執行緒為主執行緒則新建一個執行緒執行，否則在釋出事件的執行緒執行。
- ASYNC：在新的執行緒執行
利用Handler切換到主執行緒。BACKGROUND執行緒，使用到了執行緒池ExecutorService。
說說EventBus裡面用到的知識點
- 註解處理器：
使用EventBus annotation processor（EventBus註解處理器），在編譯期間建立了訂閱者索引。（提升EventBus效能：greenrobot.org/eventbus/do…）。可以看到，會在build包下面生成一個EventBus的索引類。SUBSCRIBER_INDEX是一個Map型別資料，存放著每一個類裡面的註解資訊
/** 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>();
putIndex(new SimpleSubscriberInfo(com.example.camera_learning.eventbus.EventBusActivity.class, true,
new SubscriberMethodInfo[] {
new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent.class),
new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent2.class),
}));
}
private static void putIndex(SubscriberInfo info) {
SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
}
@Override
public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
if (info != null) {
return info;
} else {
return null;
}
}
}
複製程式碼- 反射
如果索引類中有快取相關的註解資訊的話，那就會直接用索引類裡面的註解資訊。如果沒有的話，就需要利用反射去獲取類中的所有方法，然後拿到有@Subscribe的方法。
post方法最後需要利用invoke來呼叫訂閱者物件的訂閱方法（具體程式碼可以看下面）
- 設計模式：單例模式、建造者模式、觀察者模式
- ThreadLocal：EventBus會通過ThreadLocal為每個執行緒維護一個PostingThreadState物件。
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
final static class PostingThreadState {
//分發佇列
final List<Object> eventQueue = new ArrayList<>();
//是否正在分發
boolean isPosting;
//是否是主執行緒
boolean isMainThread;
//訂閱關係
Subscription subscription;
//當前正在分發的事件
Object event;
//是否取消
boolean canceled;
}
複製程式碼EventBus裡面的一些資料結構
比較重要的就是下面兩個map集合。
//key為事件型別，value值為Subscription的集合。這樣可以根據事件的型別，直接獲取訂閱該事件的所有類中的所有方法
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//key為訂閱物件，value為該類中的所有訂閱事件型別的集合。
private final Map<Object, List<Class<?>>> typesBySubscriber;
//粘性事件
private final Map<Class<?>, Object> stickyEvents;
//Subscription類，表示一個訂閱關係，包含訂閱物件和訂閱方法
final class Subscription {
final Object subscriber;
final SubscriberMethod subscriberMethod;
}
複製程式碼閱讀原始碼
getDefault()方法
- 利用雙重校驗鎖實現單例模式，保證執行緒安全
- 使用Builder建造者模式，將一個複雜物件的構建和它的表示分離。建造者模式一般用來建立複雜物件，使用者可以不用關心其建造過程和細節。在EventBus中，指的是EventBusBuilder類，可以使用自定義引數建立EventBus例項。
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
public EventBus() {
this(DEFAULT_BUILDER);
}
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
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;
}
public class EventBusBuilder {
private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();
boolean logSubscriberExceptions = true;
boolean logNoSubscriberMessages = true;
boolean sendSubscriberExceptionEvent = true;
boolean sendNoSubscriberEvent = true;
boolean throwSubscriberException;
boolean eventInheritance = true;
boolean ignoreGeneratedIndex;
boolean strictMethodVerification;
ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
List<Class<?>> skipMethodVerificationForClasses;
List<SubscriberInfoIndex> subscriberInfoIndexes;
Logger logger;
MainThreadSupport mainThreadSupport;
EventBusBuilder() {
}
/** Builds an EventBus based on the current configuration. */
public EventBus build() {
return new EventBus(this);
}
複製程式碼register方法
register的最終目的是，將註解的資訊儲存到對應的map中。而獲取的過程，優先通過註解器生成的註解資訊獲取，如果沒有的話，再利用反射獲取註解方法。
public void register(Object subscriber) {
//拿到class物件
Class<?> subscriberClass = subscriber.getClass();
//利用SubscriberMethodFinder類來找出這個類中的所有訂閱方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
//拿到類中所有的訂閱方法後，呼叫subscribe方法
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
private 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);
}
}
}
複製程式碼下面看一下SubscriberMethodFinder類。
//這個map物件，key值為訂閱者類，value為該類中的所有訂閱方法的list集合。已經register過的話，相應的資料已經在map中存在了，這個時候就可以直接從map中讀取就行了，不用再次利用反射進行多餘的遍歷類中的方法。有點空間換時間的思想。
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
//通過這個方法，遍歷出某個類中的所有訂閱方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//根據快取值，判斷是否註冊過EventBus
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//預設為false，所以一般是先走findUsingInfo方法，通過自動生成的索引類比如MyEventBusIndex來獲取到被註解的辦法。這種辦法比通過findUsingReflection（）利用反射去獲取方法的效能好一點。如果沒有獲取到索引中儲存的訂閱者Subscriber資訊的話，才去走利用反射的辦法。
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
//如果訂閱者類和父類中，沒有@Subscribe利用監聽事件的方法，就會拋這個錯誤。
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
//獲取到訂閱者類中的所有註解方法後，儲存到cache中
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
//利用註解處理器生成的索引類，來獲取類中的訂閱方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//在生成的索引類中，獲取儲存的訂閱資訊
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
//獲取拿到這個訂閱類中的所有的訂閱方法
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
//遍歷，檢查後，新增到list集合
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
//如果在索引類中找不到訂閱資訊，那就通過反射的辦法來獲取
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
//在生成的索引類中，獲取儲存的訂閱資訊
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;
}
}
//如果subscriberInfoIndexes為空，也就是沒有利用生成索引的辦法來提高EventBus效能，直接返回null
//不為空的話，遍歷，拿到這個訂閱類中的所有註解方法
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}
//使用subscribe方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//訂閱方法的事件型別
Class<?> eventType = subscriberMethod.eventType;
//建立一個Subscription，包含訂閱類物件和訂閱方法
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//獲取到這種事件型別的list集合
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//儲存到subscriptionsByEventType裡面，key為事件型別，value為Subscription
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//處理重複註冊的情況
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
//這部分的作用，是根據事件優先順序進行排序
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;
}
}
//根據訂閱物件，獲取訂閱的所有的事件型別
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//處理粘性事件
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);
}
}
}
複製程式碼post方法
post方法最後會走到postSingleEventForEventType方法,拿到註冊這個事件的訂閱者資訊，利用反射呼叫訂閱方法的執行。
public void post(Object event) {
//獲取當前執行緒的PostingThreadState物件
PostingThreadState postingState = currentPostingThreadState.get();
//獲取當前的分發佇列
List<Object> eventQueue = postingState.eventQueue;
//將事件新增到佇列中
eventQueue.add(event);
if (!postingState.isPosting) {
postingState.isMainThread = isMainThread();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//當事件佇列不為空的時候，一個一個進行分發
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
複製程式碼private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//去map集合中，根據事件型別，獲取到所有的訂閱方法
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
//遍歷訂閱該事件的list集合
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) {
break;
}
}
return true;
}
return false;
}
//根據執行緒進行分發事件
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:
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);
}
}
//最後是利用反射，呼叫訂閱者物件的訂閱方法
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);
}
}
複製程式碼unregister方法
unregister方法就是要把這個類中的註解資訊都給移除掉。
public synchronized void unregister(Object subscriber) {
//獲取這個訂閱者種的所有訂閱事件型別list
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
//遍歷事件型別list，移除subscriptionsByEventType中包含這個訂閱者的所有訂閱關係
unsubscribeByEventType(subscriber, eventType);
}
//移除這個訂閱物件
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
//移除subscriptionsByEventType中包含這個訂閱者的所有訂閱關係
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--;
}
}
}
}
複製程式碼