本文发表于KuTear's Blog,转载请注明
最简单的例子说起
先从一个简单的栗子出发，看看EventBus的功能是什么。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
EventBus.getDefault().register(this);
findViewById(R.id.say_hello).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
EventBus.getDefault().post(new EventBean(1,"Hello"));
}
});
}
@Subscribe(threadMode = ThreadMode.MAIN) //在ui线程执行
public void onEvent(EventBean event) {
System.out.println(Thread.currentThread().getName());
}
@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}
上面的代码是最简单的一个事件，当点击按钮之后回调onEvent()方法。下面就着重看看这个过程的实现。类似的代码我们见得很多，比如App存在一个UserManager,有一个用户状态的分发，很多类在这里注册了用户状态的监听回调，当用户登陆，所有的注册了监听的类都会收到这个消息。其实EventBus的实现也是类似的，只是不存在接口。
看看上面的代码，我们可能会对是怎样回调onEvent()感到一丝的困惑。下面进入源码的世界。
EventBus 源码分析
先看看一些有用的字段
//K->方法参数的类型 V->K的所有父类的结合（包括本身） 用作缓存
private static final Map<Class<?>, List<Class<?>>> eventTypesCache = new HashMap<>();
//K->方法参数的类型 V->所有参数类型的K的订阅函数的集合 主要是消息发送使用
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//K->注册的类，如Activity V-> 注册类的注册函数的参数的集合 主要是注册/解绑使用
private final Map<Object, List<Class<?>>> typesBySubscriber;
//粘性事件 K->发出的事件的参数类型 V->事件的值
private final Map<Class<?>, Object> stickyEvents;
一切的开始-事件的订阅
//EventBus
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
//获取该类的所有的能接受事件的函数，也就是上面说的`onEvent(...)`
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
如何才能找到注册的方法呢,这就要看看SubscriberMethodFinder的具体实现了.
//SubscriberMethodFinder
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//提升速度，优先重缓存中取,支持并发操作
//声明为 Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
if (ignoreGeneratedIndex) { //默认为false
subscriberMethods = findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}
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;
}
}
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
//从复用池中取回一个FindState
FindState findState = prepareFindState();
//为findState设置clazz等参数
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findState.subscriberInfo = getSubscriberInfo(findState);
//通常为NULL
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);
}
//设置findState.clazz为刚刚clazz的父类
findState.moveToSuperclass();
}
//获取findState.subscriberMethods
return getMethodsAndRelease(findState);
}
再看findUsingReflectionInSingleClass()之前,线看看FindState的一部分实现
//FindState
static class FindState {
//订阅者的方法的列表
final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
//以EventType为key，method为value
final Map<Class, Object> anyMethodByEventType = new HashMap<>();
//以method的名字生成一个methodKey为key，该method的类(订阅者)为value
final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
//构建methodKey的StringBuilder
final StringBuilder methodKeyBuilder = new StringBuilder(128);
//订阅者
Class<?> subscriberClass;
//当前类
Class<?> clazz;
//是否跳过父类
boolean skipSuperClasses;
//SubscriberInfo
SubscriberInfo subscriberInfo;
void initForSubscriber(Class<?> subscriberClass) {
//clazz为当前类
this.subscriberClass = clazz = subscriberClass;
skipSuperClasses = false;
subscriberInfo = null;
}
boolean checkAdd(Method method, Class<?> eventType) { //带检测方法，和他的参数类型
// 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
// Usually a subscriber doesn't have methods listening to the same event type.
Object existing = anyMethodByEventType.put(eventType, method);
if (existing == null) {
return true;
} else {
if (existing instanceof Method) {
if (!checkAddWithMethodSignature((Method) existing, eventType)) { //检测函数的签名
// Paranoia check
throw new IllegalStateException();
}
// Put any non-Method object to "consume" the existing Method
anyMethodByEventType.put(eventType, this);
}
return checkAddWithMethodSignature(method, eventType);
}
}
private boolean checkAddWithMethodSignature(Method method, Class<?> eventType) {
methodKeyBuilder.setLength(0);
methodKeyBuilder.append(method.getName());
methodKeyBuilder.append('>').append(eventType.getName());
String methodKey = methodKeyBuilder.toString();
Class<?> methodClass = method.getDeclaringClass(); //找到声明该方法的类
Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass);
if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
//判断old是否为methodClass的父类
// 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;
}
}
}
Ok,下面看看findUsingReflectionInSingleClass()的实现吧.
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();//返回该类的所有方法，但是不包括继承得来的
} 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(); //方法的修饰符 public|protected|private|default(package)
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) { //大体上就是要求public,非static|abstract
Class<?>[] parameterTypes = method.getParameterTypes(); //获取参数类型数组
if (parameterTypes.length == 1) { //只允许有一个参数
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) { //函数必须包含注解
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");
}
}
}
现在函数调用栈退回到了最开始的register(),接着看subscribe(...)方法。
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType; //参数类型
//将注册类和方法打包为Subscription
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//同一个事件不再多次注册，所以每次使用后一定要解绑，不解绑还会内存泄露
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++) {
//根据优先级放入合适的位置
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//typesBySubscriber K->注册的类，如Activity V-> 注册类的注册函数的参数的集合
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//注册的时候判断是否有粘性事件，如有就执行咯
if (subscriberMethod.sticky) {
if (eventInheritance) { //Default true
// 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();
//判断eventType是否为candidateEventType的父类，即所有的子类都能收到消息
if (eventType.isAssignableFrom(candidateEventType)) {
Object stickyEvent = entry.getValue();
//发送消息，这里先不讲，在后面也会说的
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
上面注册的过程基本已经说完，下面将讲事件的发送过程。根据最上面的栗子，我们知道入口是post()函数，下面就入手post()函数。
事件的发送
//保证每个线程取得的PostingThreadState不同，但是相同线程取得的相同，本质就是HashMap<Thread,PostingThreadState>
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
final static class PostingThreadState {
final List<Object> eventQueue = new ArrayList<Object>();
boolean isPosting;
boolean isMainThread;
Subscription subscription;
Object event;
boolean canceled;
}
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
//添加到队列
eventQueue.add(event);
//如果队列没有在分发事件就开始分发
if (!postingState.isPosting) {
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
//循环执行分发
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass(); //事件参数类型
boolean subscriptionFound = false;
if (eventInheritance) { //default true
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
//只要有一个成功就返回成功
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
//对错误的处理，可以自己注册`NoSubscriberEvent`来捕获
if (!subscriptionFound) {
if (logNoSubscriberMessages) {
Log.d(TAG, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
//eventTypesCache K->事件参数的类型 V->K的所有父类的结合（包括本身）
//发送一个子类型的事件，父类型的也要求收到该事件
synchronized (eventTypesCache) {
List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
if (eventTypes == null) {
eventTypes = new ArrayList<>();
Class<?> clazz = eventClass;
while (clazz != null) {
eventTypes.add(clazz);
//递归添加所有的父类/接口
addInterfaces(eventTypes, clazz.getInterfaces());
clazz = clazz.getSuperclass();
}
eventTypesCache.put(eventClass, eventTypes);
}
return eventTypes;
}
}
根据上面的代码查看，知道所有的事件发送都是通过函数postSingleEventForEventType()发送。下面看看具体的实现。
/**
*
* @param event 事件数据
* @param postingState
* @param eventClass 事件参数类型
* @return
*/
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//subscriptionsByEventType K->方法参数的类型 V->所有参数类型的K的订阅函数的集合
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;
}
/**
* @param subscription 订阅者
* @param event 数据
* @param isMainThread 当前线程是否为主线程
*/
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 = new HandlerPoster(this/*eventBus*/, Looper.getMainLooper(), 10);
// HandlerPoster extends Handler
mainThreadPoster.enqueue(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);
}
}
这里就是EventBus对四种线程模式的不同处理，这里只拿出其中一个来讲。对于MAIN，如果当前POST线程就是主线程，那么当然就是直接调对应的函数就OK,如果当前POST不是主线程，那么就要用Handler发送到主线程。下面看看实现。
void enqueue(Subscription subscription, Object event) {
//类似android源码中的Message的获取方式
PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
synchronized (this) {
//待发送的消息加入队列
queue.enqueue(pendingPost);
if (!handlerActive) {
handlerActive = true;
if (!sendMessage(obtainMessage())) {
throw new EventBusException("Could not send handler message");
}
}
}
}
@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); //使用EventBus回调方法
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;
}
}
在EventBus中，调用注册的方法。
void invokeSubscriber(PendingPost pendingPost) {
Object event = pendingPost.event;
Subscription subscription = pendingPost.subscription;
PendingPost.releasePendingPost(pendingPost);
if (subscription.active) {
invokeSubscriber(subscription, event);
}
}
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);
}
}
可以看出，只是很简单用反射调用了要调了方法，到此对普通事件的分析就完了，下面看看粘性事件。
//EventBus
public void postSticky(Object event) {
synchronized (stickyEvents) {
stickyEvents.put(event.getClass(), event);
}
// Should be posted after it is putted, in case the subscriber wants to remove immediately
post(event);
}
结合上面注册的时候的代码分析，我们知道postSticky()的事件会在postSticky()的时候发送一次，并在有新注册粘性事件的时候会再次匹配,最后就是看看事件的解绑。
事件的解绑
/**
* Unregisters the given subscriber from all event classes.
*/
public synchronized void unregister(Object subscriber) {
//注册类的注册函数的参数的集合
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
/**
* Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber.
*
* @param subscriber 注册类
* @param eventType 参数Type
*/
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
//取得参数类型为eventType的所有注册函数的集合
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--;
}
}
}
}
通过上面的代码，可以看出，其实事件的移除就是把它重List/HashMap中remove掉。
