EventBus是Android开发中常用的通知事件框架，使用EventBus非常简单只需3布即可，那么实际上EventBus是怎么实现订阅通知的？下面我们从源码开始分析
eventbus_img.png
二、源码分析
我们使用EventBus时，需要在生命周期中注册为订阅者 ，代码如下
@Override
public void onStart() {
super.onStart();
EventBus.getDefault().register(this);
}
@Override
public void onStop() {
super.onStop();
EventBus.getDefault().unregister(this);
}
那么我们就从这里开始分析
2.1 EventBus.getDefault()
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
EventBus.getDefault()是使用单例模式，获取一个EventBus的实例
2.2EventBus.getDefault().register(this)
public void register(Object subscriber) {
//通过反射获取类名
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
可以看到重要的方法为
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
2.3 findSubscriberMethods
根据他的名字，猜测是获取相关的订阅者，并返回列表集合
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
//从缓存中取出，有则直接返回
if (subscriberMethods != null) {
return subscriberMethods;
}
//// ignoreGeneratedIndex在SubscriberMethodFinder()的构造函数初始化的，默认值是 false
if (ignoreGeneratedIndex) {
//通过反射获取
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//通过插件生成代码，使用SubscriberInfo获取
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;
}
}
获取订阅者的步骤为
1.检查缓存，若缓存存在，则直接返回
2.根据配置参数，使用反射或SubscriberInfo获取，默认情况下不使用反射，减少反射消耗，提高运行效率。
2.4 findUsingReflection，prepareFindState
下面看看反射方法如何获取订阅者信息
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
FindState findState = prepareFindState(); //findState 用来做订阅者的保存和验证
findState.initForSubscriber(subscriberClass); //初始化findState
while (findState.clazz != null) {
//通过反射获取订阅方法
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
在查找订阅者之前，先做了FindState的准备工作，那先看看prepareFindState()
private FindState prepareFindState() {
synchronized (FIND_STATE_POOL) {
for (int i = 0; i < POOL_SIZE; i++) {
FindState state = FIND_STATE_POOL[i];
if (state != null) {
FIND_STATE_POOL[i] = null;
return state;
}
}
}
return new FindState();
}
prepareFindState()主要做了两步操作，1初始化FIND_STATE_POOL为null ，2返回一个FindState对象（FindState用来保存和校验订阅方法）
void initForSubscriber(Class<?> subscriberClass) {
this.subscriberClass = clazz = subscriberClass;
skipSuperClasses = false;
subscriberInfo = null;
}
赋值subscriberClass，clazz并设subscriberInfo为null
最后看看获取订阅方法的最主要的方法findUsingReflectionInSingleClass
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
methods = findState.clazz.getDeclaredMethods(); //获取订阅类中所有的非继承的方法
} catch (Throwable th) {
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
for (Method method : methods) { //遍历所有方法
int modifiers = method.getModifiers();
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//方法不是public,且不是abstract和static
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
//方法的参数为1个
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
//方法的注解是@Subscribe
Class<?> eventType = parameterTypes[0];//获取这个Event对象
if (findState.checkAdd(method, eventType)) {
ThreadMode threadMode = subscribeAnnotation.threadMode();
//实例化SubscriberMethod对象并添加到findState
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");
}
}
}
到这里所有的订阅类的订阅方法已经被保存通过 List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); 返回
2.5 subscribe(subscriber, subscriberMethod);
最后回到刚开始的register方法，看看subscribe的实现
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取订阅事件类型 （订阅方法的第一个参数）
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//根据订阅事件类型获取subscriptions（订阅者）
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//如果为空，则创建这个subscriptions，放到subscriptionsByEventType中
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();
//根据优先级进行排序
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//typesBySubscriber保存 订阅对象，和事件列表
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
//如果为空，则创建
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
if (subscriberMethod.sticky) { //是否粘性消息
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();
//发送事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
Object stickyEvent = stickyEvents.get(eventType);
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
主要做的事情为 subscriptionsByEventType用于维护事件(eventType)与订阅者(subscriptions)关系，
typesBySubscriber 用于维护 subscriber(订阅者) 与subscribedEvents（订阅事件）的关系，最后发送事件，发送事件与post类似，这个我们下面再看
用一张流程图总结一下注册的内容
img
2.6发布
发送事件给Eventbus
public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
//获取当前线程的eventQueue
List<Object> eventQueue = postingState.eventQueue;
//将要发送的event 添加到队列中
eventQueue.add(event);
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 {
while (!eventQueue.isEmpty()) {
//发送消息
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
最终调用
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 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);
}
}
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);
}
}
最终使用反射，调用订阅者的事件方法。
用一张流程图总结下发送流程
img
以上为EventBus的源码分析，当然还有很多地方没有分析到的，如果有不对的地方欢迎指出。
参考
