本文主要通過源碼講解EventBus的內部實現,如果還沒有使用過該框架的朋友,可以先參考這篇文章
Android事件總線(一)EventBus3.0用法全解析
1. 前言
EventBus是一個基於觀察者模式的事件發佈/訂閱
框架,它能有效的解決android中組件的通信問題。理解框架的基本原理有助於更好的運用框架和查找問題,本文采用源碼+註釋+說明的形式帶領大家來解讀EventBus的原理,在下一篇會介紹如何優化EventBus的查找流程。
2. 源碼解讀
構造方法
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder(); //默認的構造器
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
public static EventBusBuilder builder() {
return new EventBusBuilder();
}
public EventBus() {
this(DEFAULT_BUILDER);
}
EventBus(EventBusBuilder builder) {
...
//初始化各種屬性
...
}
EventBus採用建造者模式來構建,通常我們通過getDefault來獲得一個單例的EventBus實例。EventBus有幾個比較重要的Map結構,我們來看看:
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
private final Map<Object, List<Class<?>>> typesBySubscriber;
private final Map<Class<?>, Object> stickyEvents;
需要注意的是,在EventBus中,事件類型是以事件類的Class對象來表示的,所以上面三個Map中的Class<?>都表示事件,接下來介紹下這三個屬性:
subscriptionsByEventType
存放所有的訂閱事件。因爲EventBus是通過方法去發佈事件的,可以理解爲最終接收到事件的是每個method。typesBySubscriber
存放每個訂閱者的訂閱的所有事件類型,用於解綁操作。stickyEvents
存放粘性事件的實例
註冊/訂閱
註冊/訂閱調用的是register方法:
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
}
}
}
- 在1處:找到這個訂閱者(例如:MainActivity)裏面所有的訂閱方法(用@Subscribe註解的方法)
- 在2處:遍歷SubscriberMethod數組,調用subscribe()
至於findSubscriberMethods()怎麼實現我們放到後面,通常情況下是通過反射來找到使用@Subscribe註解過的方法,我們來看看subscribe() 的實現:
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod); //-->1
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++) { //-->2
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) { //-->3
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
if (subscriberMethod.sticky) { //-->4
if (eventInheritance) {
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);
}
}
}
- 在1處:由subscriber和subscriberMethod組合成一個Subscription對象,也就是訂閱事件,找到
subscriptionsByEventType
對應eventType的Subscription集合,如果已經訂閱了該事件,則拋出異常。 - 在2處:根據優先級,將Subscription添加到Subscription集合中。
- 在3處:將event的類型添加到
typesBySubscriber
中,對應的key就是訂閱者。 - 在4處:如果訂閱方法是粘性的,查看有沒有這個類型的粘性事件,有的話就回調這個method。
總結一下EventBus的註冊過程:首先通過findSubscriberMethods找到訂閱者的所有訂閱方法,然後將訂閱每個方法(SubscriberMethod )和訂閱者組成一個訂閱事件根據優先級添加到subscriptionsByEventType
中,接着添加事件類型eventType到typesBySubscriber
中,最後處理粘性事件。
發佈事件
發佈事件有postSticky和post方法:
public void postSticky(Object event) {
synchronized (stickyEvents) {
stickyEvents.put(event.getClass(), event);
}
post(event);
}
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get(); //-->1
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event); //--2
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); //-->3
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
可以看到,postSticky只是將事件添加到stickyEvents
中,最終調用的還是post方法,post方法分爲以下幾步:
- 在1處:獲取當前線程的PostingThreadState對象,currentPostingThreadState是一個ThreadLocal示例,PostingThreadState主要包含事件集合和一些狀態
- 在2處:添加事件到事件集合中
- 在3處:循環調用postSingleEvent處理每個事件,直到eventQueue爲空
繼續看postSingleEvent:
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) { //-->1
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 { //-->2
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
if (!subscriptionFound) {
...
//沒有找到訂閱事件 的處理過程
...
}
}
在1處:判斷eventInheritance這個字段,這個字段表示是否支持事件的繼承,舉個例子,如果EventA繼承EventB,也實現EventC接口,那麼發送EventA的時候,訂閱了EventB和EventC的方法會被通知。這個值默認是true,在lookupAllEventTypes中找到這個事件的所有父類和接口,並循環調用postSingleEventForEventType方法。
在2處:如果eventInheritance爲false,直接調用postSingleEventForEventType方法。
可見在1和2處都是調用postSingleEventForEventType進行發佈,我們來看postSingleEventForEventType方法:
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass); //-->1
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) { //-->2
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
postToSubscription(subscription, event, postingState.isMainThread);//-->3
aborted = postingState.canceled; //-->4
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}
- 在1處:同步的方式獲取對應事件的Subscription對象
- 在2處:遍歷subscriptions數組,並設置postingState的event和subscription,設置這兩個值的作用會在取消事件下發的時候用到,最後調用postToSubscription發佈事件。
- 在3處:調用postToSubscription發佈事件
- 在4處:獲取postingState.canceled值,當canceled爲true時,表情事件被上游接收者取消掉了,退出循環。
上面說到,subscriptionsByEventType裏面的subscriptions是按照優先級排序的,優先級高的在前面,在發佈事件的時候會遍歷改事件的所有接收者,所以優先級高的接收者可以取消該事件的傳遞。我們來看看cancelEventDelivery方法:
public void cancelEventDelivery(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
if (!postingState.isPosting) {
throw new EventBusException(
"This method may only be called from inside event handling methods on the posting thread");
} else if (event == null) {
throw new EventBusException("Event may not be null");
} else if (postingState.event != event) {
throw new EventBusException("Only the currently handled event may be aborted");
} else if (postingState.subscription.subscriberMethod.threadMode != ThreadMode.POSTING) {
throw new EventBusException(" event handlers may only abort the incoming event");
}
postingState.canceled = true;
}
一目瞭然,postingState.canceled 被設置爲true。
到這一步我們應該知道,真正進行回調的過程是在postToSubscription裏面進行的(因爲是在回調方法裏面進行取消),我們來看一下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 MAIN_ORDERED:
//省略
break;
case BACKGROUND:
//省略
break;
case ASYNC:
//省略
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}
這裏通過註冊時的線程模式來區分處理,我們主要看默認的POSTING模式和常用的MAIN模式,其他的線程模式類似。點進去看invokeSubscriber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);
}
}
這裏就直接調用Method的invoke方法就ok了,不懂的可以看看反射的部分,發佈線程是主線程的話,同樣調用的是invokeSubscriber,如果發佈線程不是主線程的話調用的是 mainThreadPoster.enqueue(subscription, event)。直接看mainThreadPoster的初始化:
//EventBus類
EventBus(EventBusBuilder builder) {
//....省略無關代碼
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
//....省略無關代碼
}
//EventBusBuilder類
MainThreadSupport getMainThreadSupport() {
if (mainThreadSupport != null) {
return mainThreadSupport;
} else if (Logger.AndroidLogger.isAndroidLogAvailable()) {
Object looperOrNull = getAndroidMainLooperOrNull();
return looperOrNull == null ? null :
new MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull);
} else {
return null;
}
}
mainThreadSupport 的類型是MainThreadSupport,它的初始值是由EventBusBuilder的getMainThreadSupport方法獲得,getMainThreadSupport主要通過判斷當前環境是否是adnroid環境,如果是android環境,獲取到主線程的Looper對象,然後調用new MainThreadSupport.AndroidHandlerMainThreadSupport()構造MainThreadSupport,MainThreadSupport是一個接口,如下:
public interface MainThreadSupport {
boolean isMainThread();
Poster createPoster(EventBus eventBus);
class AndroidHandlerMainThreadSupport implements MainThreadSupport {
private final Looper looper;
public AndroidHandlerMainThreadSupport(Looper looper) {
this.looper = looper;
}
@Override
public boolean isMainThread() {
return looper == Looper.myLooper();
}
@Override
public Poster createPoster(EventBus eventBus) {
return new HandlerPoster(eventBus, looper, 10);
}
}
}
mainThreadPoster 正是調用AndroidHandlerMainThreadSupport的createPoster創建的,createPoster創建了一個HandlerPoster對象,傳入的是主線程的Looper對象和EventBus實例,由名字就可以看出,裏面是維護了一套Handler的消息發送過程,裏面的過程就不再闡述,最終調用的還是invokeSubscriber方法,只不過是在主線中調用而已。
解綁
解綁事件調用的是unregister方法,解綁之後,該對象將接受不到任何事件。來看一下unregister方法:
public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber); //-->1
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType); //-->2
}
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
上文提到subscribedTypes 存放的是每個訂閱者訂閱的所有事件類型。
在1處:得到該訂閱者的所有事件
在2處:循環調用unsubscribeByEventType移除此訂閱者的訂閱事件
unsubscribeByEventType方法:
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--;
}
}
}
}
unsubscribeByEventType方法將存放在subscriptionsByEventType裏面和訂閱者相關的訂閱事件移除。
到這裏,EventBus大致的功能就講清楚了。
3.總結
可以看到,EventBus的源碼並不算複雜,前面說到,EventBus默認是通過反射來添加訂閱事件的,在性能上有一定的損耗,在下一篇中會講到如何優化EventBus的一個查找功能。