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 EventBus你需要了解的都在这
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：
public void register(Object subscriber) {
Class subscriberClass = subscriber.getClass();
List 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.findSubscriberMethodsListfindSubscriberMethods(Class subscriberClass) { List 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
// 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 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> 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).
Set, Object>> entries = stickyEvents.entrySet();
for (Map.Entry, Object> entry : entries) {
Class candidateEventType = entry.getKey();
if (eventType.isAssignableFrom(candidateEventType)) {
Object stickyEvent = entry.getValue();
// post黏性事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
} else {
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，低于黏性事件，则还会使用checkPostStickyEventToSubscription(newSubscription, stickyEvent);方法继续向该观察者通知所有的黏性事件
//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);
}
}
这里就是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
