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本篇文章是
EventBus的源码分析，以了解其实现的精髓`。EventBus是针对Android优化的发布-订阅事件总线，简化了Android组件间的通信。Github地址：EventBus
一、注册 register 源码分析
我们在使用 EventBus 的时候，一般通过EventBus.getDefault().register(this); 进行注册，注册的内部实现是
public void register(Object subscriber) {
// 首先获得class对象
Class<?> subscriberClass = subscriber.getClass();
// 通过 subscriberMethodFinder 来找到订阅者订阅了哪些事件.返回一个 SubscriberMethod 对象的 List, SubscriberMethod
// 里包含了这个方法的 Method 对象,以及将来响应订阅是在哪个线程的 ThreadMode ,以及订阅的事件类型 eventType ,以及订阅的优
// 先级 priority ,以及是否接收粘性 sticky 事件的 boolean 值，其实就是解析这个类上的所有 Subscriber 注解方法属性。
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
// 订阅
subscribe(subscriber, subscriberMethod);
}
}
}再看到 subscribe(subscriber, subscriberMethod);
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
// 获取方法参数的 class
Class<?> eventType = subscriberMethod.eventType;
// 创建一个 Subscription
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
// 获取订阅了此事件类的所有订阅者信息列表
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
// 线程安全的 ArrayList
subscriptions = new CopyOnWriteArrayList<>();
// 添加
subscriptionsByEventType.put(eventType, subscriptions);
} else {
// 是否包含，如果包含再次添加抛异常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
// 处理优先级
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;
}
}
// 通过 subscriber 获取 List<Class<?>>
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
// 将此事件类加入 订阅者事件类列表中
subscribedEvents.add(eventType);
// 处理粘性事件
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);
}
}
}在代码中，我们可以看到获取订阅了此事件类的所有订阅者信息列表是通过 subscriptionsByEventType.get(eventType);来拿，那 subscriptionsByEventType 又是什么呢？追寻看到
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;是一个 HashMap，key 是 获取方法参数的 class，value 存放的是 Subscription 的集合列表，value 中的 CopyOnWriteArrayList 是一个线程安全的 ArrayList，数据类型是 Subscription，Subscription包含两个属性，一个是 subscriber 订阅者（反射执行对象），一个是 subscriberMethod 注解方法的所有属性参数值。
二、发送 post 源码分析
/** Posts the given event to the event bus. */
public void post(Object event) {
// currentPostingThreadState 是一个 ThreadLocal，
// 他的特点是获取当前线程一份独有的变量数据，不受其他线程影响。
PostingThreadState postingState = currentPostingThreadState.get();
// postingState 就是获取到的线程独有的变量数据
List<Object> eventQueue = postingState.eventQueue;
// 把 post 的事件添加到事件队列
eventQueue.add(event);
// 如果没有处在事件发布状态，那么开始发送事件并一直保持发布状态
if (!postingState.isPosting) {
// 是否是主线程
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
// isPosting = true
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;
}
}
}看到上面有个 postSingleEvent(eventQueue.remove(0), postingState);
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
// 得到事件的Class
Class<?> eventClass = event.getClass();
// 是否找到订阅者
boolean subscriptionFound = false;
// 如果支持事件继承，默认为支持
if (eventInheritance) {
// 查找 eventClass 的所有父类和接口
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
// 依次向 eventClass 的父类或接口的订阅方法发送事件
// 只要有一个事件发送成功，返回 true ，那么 subscriptionFound 就为 true
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
// 发送事件
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
// 如果没有订阅者
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));
}
}
}看到 postSingleEventForEventType(event, postingState, clazz);
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
// 得到Subscription 列表
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
// 遍历 subscriptions
for (Subscription subscription : subscriptions) {
//
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
// 发送事件
postToSubscription(subscription, event, postingState.isMainThread);
// 是否被取消了
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
// 如果被取消，则跳出循环
if (aborted) {
break;
}
}
return true;
}
return false;
}现在再看到发送事件 postToSubscription(subscription, event, postingState.isMainThread);
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
// 根据不同的线程模式执行对应
switch (subscription.subscriberMethod.threadMode) {
// 和发送事件处于同一个线程
case POSTING:
invokeSubscriber(subscription, event);
break;
// 主线程
case MAIN:
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
// 子线程
case BACKGROUND:
if (isMainThread) {
backgroundPoster.enqueue(subscription, event);
} else {
invokeSubscriber(subscription, event);
}
break;
// 和发送事件处于不同的线程
case ASYNC:
asyncPoster.enqueue(subscription, event);
break;
default:
throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
}
}可以看到，这里的会判断 threadMode，处理消息都是最终走 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);
}
}可以看到，它是利用调用反射 Method.invoke()，进行方法的执行。
三、解绑 unregister 源码分析
/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
// 获取订阅对象的所有订阅事件类列表
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
// 将订阅者的订阅信息移除
unsubscribeByEventType(subscriber, eventType);
}
// 将订阅者从列表中移除
typesBySubscriber.remove(subscriber);
} else {
Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}typesBySubscriber 是个 HashMap，key 是所有的订阅者，value 是所有订阅者里面方法的参数的class，在 一中的 subscribe 源码分析通过
typesBySubscriber.put(subscriber, subscribedEvents);来保存。再看到将订阅者的订阅信息移除 unsubscribeByEventType(subscriber, eventType);
/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
// 获取事件类的所有订阅信息列表，将订阅信息从订阅信息集合中移除，同时将订阅信息中的active属性置为FALSE
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) {
// 将订阅信息激活状态置为FALSE
subscription.active = false;
// 将订阅信息从集合中移除
subscriptions.remove(i);
i--;
size--;
}
}
}
}总结：以上便是 EventBus 的源码分析，EventBus 非常好用，希望我们在会用的同时了解好其原理。感谢辉哥。