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 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 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>
List> 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).
Set, Object>> entries = stickyEvents.entrySet();
for (Map.Entry, 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, CopyOnWriteArrayList> 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
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> 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 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> 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 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。
参考:https://www.jianshu.com/p/0b35f448acec