EventBus
EventBus是一個為Android和Java平臺設計的發(fā)布/訂閱事件總線
EventBus有以下特點:
- 簡化組件之間的通信
- 將事件發(fā)送方和事件接收方解耦
- 很好的使用于
Activitis
、Fragments
和后臺線程
- 避免復雜廓奕、易出錯的依賴
- 簡化你的代碼
- 快
- 體積小(~50K jar)
- 有100,000,000+次相關APP安裝幸斥,證明EventBus的可靠性
- 還有其他額外的功能
集成到項目
gradle配置:
implementation 'org.greenrobot:eventbus:3.1.1'
使用(只需3步)
-
定義事件
public static class MessageEvent { /* 如果有需要旱幼,可以聲明相應的屬性 */ }
-
聲明訂閱方法
@Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(MessageEvent event) {/* Do something */};
需要@Subscribe
注解,默認的threadMode
是ThreadMode.MAIN
注冊
和反注冊
訂閱舀射,比如在Activities
和Fragments
中根據(jù)生命周期方法去注冊和反注冊
@Override
public void onStart() {
super.onStart();
EventBus.getDefault().register(this);
}
@Override
public void onStop() {
super.onStop();
EventBus.getDefault().unregister(this);
}
-
發(fā)送事件
EventBus.getDefault().post(new MessageEvent());
源碼解析
register
/**
* 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);// -> 1
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);// -> 2
}
}
}
注釋:
注冊subscriber用來接收事件,當不再希望接收到事件發(fā)送時必須要調用unregister
取消注冊
subscriber必須包含帶Subscribe
注解的方法,Subscribe
注解可配置ThreadMode
和priority
-
注釋1處代碼:
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
根據(jù)sbuscriber
的class去查找?guī)в?code>@subscribe注解的方法陵吸,findSubscriberMethods
方法:
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);// -> 3
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {// -> 4
subscriberMethods = findUsingReflection(subscriberClass);// -> 5
} else {
subscriberMethods = findUsingInfo(subscriberClass);// -> 6
}
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;
}
}
- 注釋3處代碼
先從緩存中去查找該subscriber
中訂閱的方法,如果查到介牙,直接返回壮虫。
METHOD_CACHE
緩存中的數(shù)據(jù)是在register
后添加的,但是unregister
的時候并不會清掉环础,這樣當一個subscriber
注冊完囚似,然后反注冊,下次再注冊的時候线得,不需要再去查找其中訂閱的方法饶唤。
- 注釋4處的代碼
如果上一步?jīng)]有查到相應的方法,就會走到這一步框都,先根據(jù)ignoreGeneratedIndex
判斷是否忽略索引查找,如果忽略搬素,則執(zhí)行注釋5處的代碼,不管你是否添加了index
所有魏保,都利用反射去查找subscriber
中訂閱的方法列表熬尺;如果不忽略,則執(zhí)行6處的代碼谓罗,先從索引index
中去查找粱哼,如果沒有索引或者索引中沒有找到,再考慮利用反射的方式去查找檩咱。
關于索引index
的使用后面會提到揭措。
- 注釋5處的代碼
調用findUsingReflection
方法:
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {// -> 7
findUsingReflectionInSingleClass(findState);// -> 8
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 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");
}
}
}
- 注釋7處的代碼
這里之所以使用while
循環(huán)而不用if
判斷胯舷,是因為除了查找當前subscriber
還需要查找其向上的繼承鏈,findState.moveToSuperclass();
就是用來控制向上查找的绊含,EventBus中所有的繼承鏈查找都是用這種方式桑嘶,不要迷惑就好。
- 注釋8處的代碼
這里就是調用了findUsingReflectionInSingleClass
去用反射的方式查找subscriber
中訂閱的方法列表躬充,代碼很簡單逃顶,就是普通的反射使用,看不懂的問我吧充甚。
反射的邏輯就這些
再回到注釋4處的代碼繼續(xù)分析另外一種查找方法列表的方式:
- 注釋6處的代碼
調用findUsingInfo
方法:
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findState.subscriberInfo = getSubscriberInfo(findState);// -> 9
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);// -> 10
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
- 注釋9處代碼
調用getSubscriberInfo
方法:
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;
}
這里有兩個分支以政,第一次register
的時候,第一個分支條件是不會成立的伴找,我們看第二個分支:
這里的subscriberInfoIndexes
就是我們通過EventBus的注解生成的index
索引盈蛮,然后調用EventBus.builder().addIndex(new MyEventBusIndex()).build();
添加的,這個索引的作用就是把本來運行時通過反射查找訂閱方法的邏輯換成了編譯器自動生成文件技矮,然后運行時在該文件中直接去查找的方式抖誉,從而提高了效率,index
索引的方式后面會介紹衰倦。
- 注釋10處的代碼
如果我們沒有添加index
索引寸五,那么代碼9處得到的findState.subscriberInfo
就是null,代碼就會走到注釋10處耿币,注釋10處的邏輯和上面注釋8處的邏輯一樣,都是通過反射去查找訂閱方法韧拒。
到這里淹接,不管通過哪種方式,訂閱的方法列表已經(jīng)找到了叛溢,我們回到注釋1處接著往下看:
- 注釋2處的代碼:
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//獲取事件類型
Class<?> eventType = subscriberMethod.eventType;
// 將訂閱者和訂閱者中的訂閱方法打包成一個Subscription對象
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();
// 遍歷訂閱了該event類型的方法塑悼,根據(jù)priority把新訂閱的方法放到合適的位置
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);
}
}
}
至此,register
流程已經(jīng)走完楷掉。
post事件流程
post事件的流程很簡單厢蒜,就是通過反射調用相應的方法(訂閱的方法在上一步已經(jīng)找到并存到了相應的集合中)即可。
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
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 {
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);// -> 1
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
- 注釋1處代碼
調用postSingleEvent
方法:
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) { // -> 2
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); // -> 3
}
} else {
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass); // -> 4
}
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));
}
}
}
- 注釋2處代碼
eventInheritance
:該屬性可以在EventBus的Build中初始化烹植,默認是true斑鸦。
true的情況舉個例子:
A和B是兩個Event阳欲,并且A extends B
X和Y是兩個方法鲫构,X訂閱了事件A,Y訂閱了事件B
post A事件滤蝠,同時會post B事件墩虹,那么X和Y都會收到事件
fasle的情況就是:post哪個事件嘱巾,就只會發(fā)送這一個事件憨琳,不會發(fā)送它的父類事件,上面這個例子旬昭,如果eventInheritance為false篙螟,post A的時候Y就不會收到事件
- 注釋3和注釋4代碼
都調用了postSingleEventForEventType
方法:
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
postToSubscription(subscription, event, postingState.isMainThread);// -> 5
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}
- 注釋5處代碼:
這里就是遍歷到該event中的所有訂閱方法,挨個給他們發(fā)送事件问拘,具體的發(fā)送邏輯就是代碼5處的postToSubscription
方法:
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
switch (subscription.subscriberMethod.threadMode) {
case POSTING://直接在當前線程發(fā)送
invokeSubscriber(subscription, event);
break;
case MAIN:
if (isMainThread) {//如果發(fā)送事件是在主線程遍略,直接發(fā)送
invokeSubscriber(subscription, event);
} else {//如果發(fā)送事件不是在主線程,加入到主線程的事件隊列里面等待發(fā)送
mainThreadPoster.enqueue(subscription, event);
}
break;
case MAIN_ORDERED:// 沒太看懂和MAIN有啥區(qū)別
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) {// 如果發(fā)送事件是在主線程场梆,加入到子線程的事件隊列里面墅冷,等待發(fā)送
backgroundPoster.enqueue(subscription, event);
} else {// 如果發(fā)送事件不是在主線程,直接發(fā)送
invokeSubscriber(subscription, event);
}
break;
case ASYNC:// 不管發(fā)送線程是在哪個線程或油,都加入到異步線程的隊列里面等待發(fā)送
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}
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);
}
}
這里就是根據(jù)不同的ThreadMode
去執(zhí)行不同的邏輯了寞忿,invokeSubscriber
就是通過反射的方式去調用方法,沒什么可說的顶岸。
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 {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
當unregister
的時候腔彰,會把該subscriber
中的訂閱方法都從集合中移除掉,同時也會把該subscriber
從相應的集合中移除掉辖佣。
索引index
EventBus提供一個注解處理器霹抛,可以在編譯期把你項目中所有regisger
的class和所有的 @Subscribe
方法給整合起來,生成一個特殊的類卷谈,類似下面:
/** 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>();// key就是register的對象杯拐, value就是該對象中@subscribe的方法集合
putIndex(new SimpleSubscriberInfo(cn.jerry.webviewdemo.FirstActivity.class, true, new SubscriberMethodInfo[] {
new SubscriberMethodInfo("onEventArrived", cn.jerry.webviewdemo.CustomEvent.class),
}));
}
private static void putIndex(SubscriberInfo info) {
SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
}
@Override
// 根據(jù)class對象獲取該對象中@Subscribe注解的方法列表
public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
if (info != null) {
return info;
} else {
return null;
}
}
}
代碼運行的時候,再遇到register
邏輯世蔗,會直接從這個索引中查找相應的方法列表端逼,從而避免了相應的反射操作,上面提到的register
流程的注釋9處:
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;
}
}
// 這里就是從剛剛的index處去查找
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index : subscriberInfoIndexes) {
// 根據(jù)相應的class對象污淋,去查找該對象中@subscribe的方法列表
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}