浅谈EventBus的使用原理
2018-05-21 本文已影响53人
niknowzcd
EventBus这是一个目前被广泛使用的,用于在不同的界面(Activity/Fragment)之间传递数据的三方库,其简单的使用深受广大开发者喜欢。
相比起Bundle或者接口回调,EventBus使用起来更加简单快捷,但有一点需要注意,再使用EventBus的时候,你需要对自己业务中的通知分发有很清晰的了解，不然很容易导致分发过多的无用通知,导致性能的消耗.
本文会对EventBus做一个简单的介绍
简单的使用
1.oncreate()中注册
EventBus.getDefault().register(this);
2.onDestroy()销毁
EventBus.getDefault().unregister(this);
3.使用@Subscribe注解方法,用来接收事件通知
@Subscribe
public void onMainEvent(String str){
System.out.println("event = "+str);
}
4.发送通知
EventBus.getDefault().post("12456");
上述4个步骤就能完成一次简单的事件分发,这也是EventBus被广泛使用的部分原因。
源码解读
在深入源码之前先解释几个主要的成员
Subscription.class (订阅信息)
final class Subscription {
//订阅者对象,一般情况下多为activity/fragment等
final Object subscriber;
//订阅者方法对象,主要包括订阅的
final SubscriberMethod subscriberMethod;
}
SubscriberMethod.class (订阅者方法对象)
public class SubscriberMethod {
//方法
final Method method;
//线程模式
final ThreadMode threadMode;
//事件类型
final Class<?> eventType;
//优先级
final int priority;
//是否是粘性事件
final boolean sticky;
}
FindState.class (查找状态,主要用来保存查找过程中的变量和结果)
static class FindState {
final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
//事件type为key,method为value
final Map<Class, Object> anyMethodByEventType = new HashMap<>();
//method为key,订阅者class对象为value
final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
final StringBuilder methodKeyBuilder = new StringBuilder(128);
}
另外还有几个比较重要的Map对象
-
subscriptionsByEventType type为key,
CopyOnWriteArrayList<Subscription>为value,从这里可以看出EventBus中一个type类型可以对应很多个订阅者。 - typesBySubscriber 则刚好相反,Subscription为key,Subscription内的所有事件type为value
-
METHOD_CACHE class类对象为key,List<SubscriberMethod>为value,注意这里的是SubscriberMethod,而上述的是 Subscription，
其中值得注意的是METHOD_CACHE采用的是ConcurrentHashMap()这个数据模型,对于多并发做了一定的优化。
接下来我们以前看看这个强大的函数库的内部实现原理。
register()
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
register过程的主要工作
1.获取订阅者的类名 (比如MainActivity)
2.通过findSubscriberMethods方法查找订阅者的订阅方法
(@Subscribe注解的并且是Public修饰的)
2.1 查找订阅方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//ignoreGeneratedIndex属性表示是否忽略注解器生成的MyEventBusIndex,默认情况下为false
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
//如果订阅者类中没有被 @Subscribe且public声明的方法就会报异常
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;
}
}
findUsingInfo()
List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != 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 {
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
其中以线程池的形式获取FindState对象,并初始化Subscriber订阅者对象
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
正常情况下第一次使用if (findState.subscriberInfo != null)这个判断会为false,接下来进入findUsingReflectionInSingleClass(findState);流程
另外需要注意的是每一次的循环都会调用findState.moveToSuperclass()检索父类的方法,所以对于一个订阅者来说,子类和父类中的方法都会被检索到,顺序是子类->父类
findUsingReflectionInSingleClass()
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");
}
}
}
- 通过放射获取所有的类中所有的方法数
- 过滤方法 1.public修饰 2.非静态，非抽象 3.一个参数 4.函数拥有@Subscribe的注解
-
findState.checkAdd(method, eventType)校验是否可以加入到list中
findState.checkAdd(method, eventType)这里分两种情况
-
anyMethodByEventType()中没有的直接返回true -
anyMethodByEventType()中有的,做2次校验,
这次根据 方法名>参数名进行完整校验,因为同一个类中同名同参的函数是不存在的,而同名不同参的在前一步已经被过滤了，所以这里确保在一个类中不会重复注册.
但如果子类重写父类的方法的话,就会出现相同的methodKey。这时EventBus会做一次验证,并保留子类的订阅信息。由于扫描是由子类向父类的顺序，故此时应当保留methodClassOld而忽略methodClass。如果代码上的注释 Revert the put
if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
// Only add if not already found in a sub class
return true;
} else {
// Revert the put, old class is further down the class hierarchy
subscriberClassByMethodKey.put(methodKey, methodClassOld);
return false;
}
checkAdd() 之后
ThreadMode threadMode = subscribeAnnotation.threadMode();
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
添加到findState中,以一个arrayList保存
接下来回到 getMethodsAndRelease()方法中,return 一个方法集合
List<SubscriberMethod>
private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
findState.recycle();
synchronized (FIND_STATE_POOL) {
for (int i = 0; i < POOL_SIZE; i++) {
if (FIND_STATE_POOL[i] == null) {
FIND_STATE_POOL[i] = findState;
break;
}
}
}
return subscriberMethods;
}
3.回到开头的 register()方法中
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
相对而言注册流程还是比较简单的,主要是让最开始提到的两个map(subscriptionsByEventType和typesBySubscriber)里面填加数据,留着事件分发时候使用。最多加一些优先级/粘性事件的判断。
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);
}
}
}
post流程
同样先介绍几个重要的类
PostingThreadState.class (事件分发状态)
final static class PostingThreadState {
//事件队列
final List<Object> eventQueue = new ArrayList<>();
//是否正在分发,防止并发状态下同一个事件发出多次
boolean isPosting;
//是否在主线程
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
ThreadLocal.class (线程数据的存储类)
在指定线程存储数据,数据存储之后，只有在指定线程才能获取到之前存储过的数据。
post流程
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);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
核心部分只有下面这句,遍历发送eventQueue队列中的event,并且移除已经发送的event
postSingleEvent(eventQueue.remove(0), postingState);
postSingleEvent.class
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
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) {
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));
}
}
}
eventInheritance变量默认情况下被赋值为true,其中lookupAllEventTypes函数会遍历eventclass,得到其父类和接口的class类,事件派发的核心部分在postSingleEventForEventType()
postSingleEventForEventType.class
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);
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}
这个函数的逻辑也是十分简单,根据eventClass从subscriptionsByEventType列表中获取订阅者列表,接着遍历订阅者列表,以此将event回调。读了这里时,发现将eventClass作为key,而不是event作为key,估计是因为class对象能追溯到其父类和接口实现吧。
到此,post流程也结束了,比起注册流程还要简单。
线程调度 postToSubscription()
几种poster类型
1.mainThreadPoster 创建于HandlerPoster
HandlerPoster.class
主要变量
//队列
private final PendingPostQueue queue;
//最大存在秒数 通常为10s,超过则会报错,这就跟广播的onReciver回调10s没处理完成就会报ANR错误有些类似
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
//标志是否活动可用的
private boolean handlerActive;
核心逻辑 handleMessage
@Override
public void handleMessage(Message msg) {
boolean rescheduled = false;
try {
long started = SystemClock.uptimeMillis();
while (true) {
PendingPost pendingPost = queue.poll();
if (pendingPost == null) {
synchronized (this) {
// Check again, this time in synchronized
pendingPost = queue.poll();
if (pendingPost == null) {
handlerActive = false;
return;
}
}
}
eventBus.invokeSubscriber(pendingPost);
long timeInMethod = SystemClock.uptimeMillis() - started;
if (timeInMethod >= maxMillisInsideHandleMessage) {
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
rescheduled = true;
return;
}
}
} finally {
handlerActive = rescheduled;
}
}
遍历队列,执行eventBus.invokeSubscriber(pendingPost)方法
void invokeSubscriber(Subscription subscription, Object event) {
try {
//通过反射的方式，直接调用订阅该事件方法。
subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
} catch (InvocationTargetException e) {
handleSubscriberException(subscription, event, e.getCause());
} catch (IllegalAccessException e) {
throw new IllegalStateException("Unexpected exception", e);
}
}
2.BackgroundPoster:实现 Runnable,会将PendingPostQueue队列中所有的事件分发出去
3.AsyncPoster:同样实现 Runnable,只会分发一个PendingPostQueue队列中的事件
postToSubscription.class
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);
}
}
逻辑也很好理解,相同线程时直接invokeSubscriber反射回调,不同线程则发到相同线程去回调。
unregister流程
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());
}
}
主要工作分两步
- 移除订阅者的所有订阅信息
- 移除订阅者和其所有Event的关联
另外看过的一篇博文上写了有关于EventBus的优化操作,记录下来
- eventInheritance默认为true,会遍历class对应的父类和接口对象,如果程序中没有使用到事件的继承关系,可以将该值设为false
- ignoreGeneratedIndex表示是否使用生成的APT代码去优化查找事件的过程,如果项目中没有接入EventBus的APT，也可以将其设置为false
补充
hashMap.put()
hashMap.put() 会根据key返回对应的value值
如果put的时候没有对应的key值,则添加到map中,如果有 先返回后添加
如
map.put("222", "222");
String value2=map.put("222","333");
此时的value2为"222"，而map中 key="222"的value为"333"
instanceof, isinstance,isAssignableFrom的区别
instanceof运算符 只被用于对象引用变量，检查左边的被测试对象 是不是 右边类或接口的 实例化。如果被测对象是null值，则测试结果总是false。
形象地：自身实例或子类实例 instanceof 自身类 返回true
String s=new String("javaisland");
System.out.println(s instanceof String); //true
Class类的isInstance(Object obj)方法，obj是被测试的对象，如果obj是调用这个方法的class或接口 的实例，则返回true。这个方法是instanceof运算符的动态等价。
形象地：自身类.class.isInstance(自身实例或子类实例) 返回true
String s=new String("javaisland");
System.out.println(String.class.isInstance(s)); //true
Class类的isAssignableFrom(Class cls)方法，如果调用这个方法的class或接口 与 参数cls表示的类或接口相同，或者是参数cls表示的类或接口的父类，则返回true。
形象地：自身类.class.isAssignableFrom(自身类或子类.class) 返回true
例：
System.out.println(ArrayList.class.isAssignableFrom(Object.class)); //false
System.out.println(Object.class.isAssignableFrom(ArrayList.class)); //true
