记得上次写EventBus还是在上次（一年前，哈哈），转眼间又是一年了,发现对于EventBus的源码细节有点模糊，挖个坑捋捋EventBus的源码
由于项目中使用且当前最新版本源码变化不大，本文贴出的源码基于EventBus3.0.0,关于EventBus的用法可以移步我之前的文章：
Android 轻量级线程间通信EventBus
Android EventBus保姆级源码解析（一）注册方法register
Android EventBus保姆级源码解析（二）发送事件post
Android EventBus保姆级源码解析（三）黏性事件原理
总结
先上总结(太长不看版)：
-
EventBus.register方法开始订阅，先通过反射获取注册类中使用@Subscribe注解的方法
- 遍历获取到的订阅方法集合，使用
EventBus.subscribe方法添加到subscriptionsByEventType列表中(<订阅class,<订阅类，订阅方法>>的一个Map)
- 如果订阅了黏性事件，则还会判断是否存在发送过这个事件，如果存在的话会对刚订阅的方法发送此黏性事件（刚订阅的方法就会被调用）
EventBus.register
首先，EventBus的原理是基于发布订阅模式，有些萌新开始会以为是观察者模式，其实它跟观察者模式的区别在于不需要观察者(订阅者)和目标(发布者)直接交互，而是通过调度中心来进行分发事件，实现了它们之间的解耦。
看源码首先可以从我们使用的第一步，注册订阅事件的方法register开始：
/**
* Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
* are no longer interested in receiving events.
* <p/>
* Subscribers have event handling methods that must be annotated by {@link Subscribe}.
* The {@link Subscribe} annotation also allows configuration like {@link
* ThreadMode} and priority.
*/
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
其中
-
SubscriberMethodFinder是EventBus用来管理订阅事件的方法类
-
SubscriberMethod是EventBus的订阅方法(使用@Subscribe注解的方法)，其中封装了订阅者的回调方法、线程模式、EventClass、是否为粘性事件等信息
EventBus通过传入的Object对象subscriber的class，构造出了一个SubscriberMethod数组，看看构造数组的findSubscriberMethods方法中有什么
SubscriberMethodFinder.findSubscriberMethods
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
//如果缓存里有的话直接使用
if (subscriberMethods != null) {
return subscriberMethods;
}
//针对是否忽略继承的索引，采用不同的查找策略
//通过反射获取Subscriber注解的方法list,这块就不往下跟了，各位有兴趣可以自己看看
if (ignoreGeneratedIndex) {
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 {
//如果构造出的subscriberMethods不为空，则放入缓存
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
简单说，findSubscriberMethods方法中主要是通过反射获取目标类中添加了@Subscriber注解的方法数组
然后register()中会遍历方法数组，调用subscribe方法订阅：
EventBus.subscribe
其中
-
Subscription是包裹订阅类和订阅者信息的包装类
-
SubscriberMethod是上边说过的EventBus的订阅方法，其中封装了订阅者的回调方法、线程模式、EventClass、是否为粘性事件等信息，为了防止各位弄混，再强调一下
-
typesBySubscriber是EventBus中存放的“订阅者-事件类型”列表
-
subscriptionsByEventType 是<订阅class,<订阅类，订阅方法>>的一个set列表
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
//获取事件类型(EventClass)对应的订阅（订阅者，订阅者方法）list
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
//指定的事件类型没有对应的观察对象的时候，初始化list再添加进去
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//如果同一个类重复订阅同一个事件，抛出EventBusException异常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
int size = subscriptions.size();
//遍历subscriptions，依据优先级，将新的订阅插入到list中
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
// 从“订阅者-事件类型”列表中尝试获取该订阅者对应的所有事件类型
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
//如果是订阅者的第一个订阅事件，构造一个typesBySubscriber put之后再添加
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);
//如果这个事件是黏性事件
if (subscriberMethod.sticky) {
//是否判断同类型的超类flag
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();
// post黏性事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
// 从黏性事件list中取出事件
Object stickyEvent = stickyEvents.get(eventType);
// post黏性事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
对于普通事件，添加到subscribedEvents后该时间就处理完毕了，对于黏性事件，则还会使用checkPostStickyEventToSubscription(newSubscription, stickyEvent);方法判断是否向该观察者post黏性事件
等等，我们的标题是什么，EventBus的register，注册事件，为什么注册事件会进行事件的分发，这个就是黏性事件的机制了，这个问题我们后面在黏性事件的一篇再分析，我们简单看一下里面的代码
//跳过stickyEvent为空的情况
private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
if (stickyEvent != null) {
// If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
// --> Strange corner case, which we don't take care of here.
postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
}
}
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);
}
}
到这里就是该post处理事件的逻辑了
到这里本篇EventBus.register注册方法的解析就结束了，详细的代码解析会放到后续的处理post事件解析
