EventBus 及一些思考
EventBus 是 Android 开发的一种常用框架,其解耦的思维令人赞叹
从特性上来讲,其与 Android SDK中的BroadcastReceiver很像,二者都是注册,发送事件,反注册,都可以定义事件的优先级,且都支持粘性(sticky)事件,只是EventBus 使用起来简单得多,而且不能跨进程
Android SDK其实也有一个不能跨进程的BroadcastReceiver机制——LocalBroadcastManager,其发送和接受的广播只能在本进程,相比传统的 registerBroadcastReceiver,其有着更高的安全性,与EventBus的相似度也更高
关于 EventBus 基础,请参考:
这里结合源码,记录几个在 EventBus 学习和使用中值得思考和注意的地方:
注册
- 哪些方法会被注册 ?
- 多次注册同一对象会如何?
- 注册对象没有被 Subscribe 注解的方法会如何 ?
- 注册对象时父类中被 Subscribe 注解的方法会被注册吗?
- 如果 子类B 重写了 父类A 的方法 fun,注册子类B 的对象会发生什么?调用时会调用哪个类的方法?
上述问题,我们需要分析 register方法:
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
// 1. 找 subscriber 中被需要被注册的方法
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
// 2. 注册这些方法
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
先来看上面代码第一点;
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
// 缓存中取
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) {
// 通过反射去找
subscriberMethods = findUsingReflection(subscriberClass);
} else {
// 通过 Subscriber Index 去找,这里如果没找到,也会通过反射去找
subscriberMethods = findUsingInfo(subscriberClass);
}
// 如果没有 找到被 Subscribe 注解的方法,抛出异常 ———— 问题 3
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;
}
}
// 继续分析注册方法的查找过程findUsingReflection
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findUsingReflectionInSingleClass(findState);
// 继续去 subscriberClass 的父类中找,但是这里有异常情况(几乎不会发生),往下看 ———— 问题 4
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
// findState.moveToSuperclass();
void moveToSuperclass() {
if (skipSuperClasses) { // 异常情况,至于在哪里设置该标志位呢,继续往下看
clazz = null;
}
// ...
}
// 继续分析注册方法的查找过程 findUsingReflectionInSingleClass
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
// 先通过getDeclaredMethods查找,再通过getMethods查找
try {
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
methods = findState.clazz.getMethods();
// ...
// 问题 4 的异常情况,在 getDeclaredMethods 发生异常时,跳过父类的注册方法查找
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)) {
// 方法被注册的条件: public,被 Subscribe 注解,参数列表只有1个参数(也就是事件类型) ———— 问题 1
ThreadMode threadMode = subscribeAnnotation.threadMode();
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
// 在设置了 strictMethodVerification 时,如果 Subscribe 注解的方法参数个数不是 1,抛出异常,默认 strictMethodVerification标志是false,可以通过 EventBusBuilder 设置
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)) {
// 在设置了 strictMethodVerification 时,如果被 Subscribe 注解的方法不是 public,抛出异常
String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException(methodName +
" is a illegal @Subscribe method: must be public, non-static, and non-abstract");
}
}
}
再来看 register方法中的第2 点:
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 {
// 已经注册过了该 subscriber,再次注册抛出异常 ———— 问题 2
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
// ...
}
上述问题 5 ,代码中没写,但是其实在回答问题 4 时已经回答了,在寻找注册方法时,会注册父类的合格的方法,那么在post 调用时,调用的是哪个方法呢,EventBus 通过反射调用方法,自然也就是实现类的方法;
事件触发
- post时,如果没有找到 eventType 对应的注册方法会如何?
- 事件的 eventType 调用时符合多态吗?
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
// 设置了 eventInheritance 标志,查找 eventType 的 父类 和 父接口 的对应的事件类型,eventInheritance 标志默认为 true ———— 问题 2
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);
}
if (!subscriptionFound) {
// 没有找到对应 eventType 的注册方法,先打个日志
if (logNoSubscriberMessages) {
logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
}
// 如果设置了 sendNoSubscriberEvent 标志,post 一个 NoSubscriberEvent 事件 ———— 问题 1
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
反注册
- 多次反注册同一对象,或者反注册一个未被注册过的对象会如何?
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 {
// 没找到,就打了个日志 ———— 问题 1
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
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