可用于应用内的消息事件传递，方便快捷，耦合性低
1.基本用法
public class EventBusMain extends AppCompatActivity {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.content_main);
EventBus.getDefault().register(this);
}
- 订阅的事件 onEvent1
@Subscribe
public void onEvent1(RemindBean bean){
}
- 订阅的事件 onEvent2
@Subscribe
public void onEvent2(UserInfo bean){
}
@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}
}
需要发送消息传递的时候：
EventBus.getDefault().post(new RemindBean())
2.源码解读
放上官网的一张原理图，感觉挺清晰的：
image.png
发布消息的一方(Publisher)，只需要 post 一个 event 之后就不用管了，EventBus 内部会将event逐一分发给订阅此 event 的订阅者（Subscriber）. 不错就是这样一个东西。
还记得以往我要实现两个不同的activity 之间要传递一些数据的时候，我都是通过定义一个interface的形式完成，时间一长，定义的接口一堆，在回顾查看代码也确实不够美观。好了话不多说，看下大家都在用的Eventbus.
3.首先
EventBus.getDefault().register(this);
getDefault():
EventBus 是一个单例模式，懒汉式，双重判断
/** Convenience singleton for apps using a process-wide EventBus instance. */
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
register 是什么意思呢，就是就跟你订阅报纸一样，报社需要确定几个重要的问题：
- 订阅者是谁（
Subscriber）？ - 订阅的什么报纸(
Event) ？
就是我认为比较重要的，那么register 这一步就是Subscriber 告诉 报社，订阅的event
public void register(Object subscriber) {
- 1.先拿到这个订阅者(subscriber)类的字节码
Class<?> subscriberClass = subscriber.getClass();
- 2. 通过这个类的字节码，拿到所有的订阅的 event,存放在List中
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
- 3. 循环遍历所有的订阅的方法，完成subscriber 和 subscriberMethod 的关联
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
我们看下这个如何根据subscriberClass 找到这个订阅的 method的，findSubscriberMethods:
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
- 1.先从缓存中取
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
- 2. 第一次肯定 null
if (subscriberMethods != null) {
return subscriberMethods;
}
- 3. 查找默认也是false，感兴趣的可以看下
if (ignoreGeneratedIndex) {
subscriberMethods = findUsingReflection(subscriberClass);
} else {
- 4. 所以是走这里
subscriberMethods = findUsingInfo(subscriberClass);
}
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
- 5. 找到之后添加到缓存中，key是 subscriber ;value 是：methods
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
看下：findUsingInfo(subscriberClass)
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
- 1. 我认为就是准备一个查找结果得存储对象
FindState findState = prepareFindState();
- 2. 将订阅者的subscriberClass 存储起来，保存在一个FindState 类中的subscriberClass
同时赋值给clazz变量中,以下代码能够看出
// void initForSubscriber(Class<?> subscriberClass) {
// this.subscriberClass = clazz = subscriberClass;
//}
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {进入循环中
//获取subscriberInfo 信息，返回null
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
- 3. 进入到这里了
findUsingReflectionInSingleClass(findState);
}
- 4. 查找父类中的方法
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
findUsingReflectionInSingleClass 如下：
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
- 1. 通过订阅者的字节码查找当前类中所有生命的方法
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
methods = findState.clazz.getMethods();
findState.skipSuperClasses = true;
}
- 2. 循环遍历所有的方法
for (Method method : methods) {
- 3. 获取方法的修饰符
int modifiers = method.getModifiers();
- 4.判断修饰符，订阅方法的修饰符不能是private，static
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
- 5. 获取方法的所有的参数
Class<?>[] parameterTypes = method.getParameterTypes();
- 6.判断参数的个数，只能有1个参数，订阅方法中
if (parameterTypes.length == 1) {
- 7.获取方法上具有subscribe 注解
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
- 8.含有subscribe注解的方法，就是该类订阅的方法，其它不符合的可能就是普通的方法
if (subscribeAnnotation != null) {
- 9. 获取第一个参数eventType
Class<?> eventType = parameterTypes[0];
if (findState.checkAdd(method, eventType)) {
- 10. 获取注解的mode，就是我们在注解上标识的，
有mainThread,Posting,background,async
ThreadMode threadMode = subscribeAnnotation.threadMode();
- 11. 将订阅方法的一系列信息（方法名称，threadMode,优先级，是否是粘性等）添加到集合subscriberMethods中去
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
- 12. 参数是多个的时候抛出异常
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)) {
- 13. 方法的修饰符不是public的，抛出异常
String methodName = method.getDeclaringClass().getName() + "." + method.getName();
throw new EventBusException(methodName +
" is a illegal @Subscribe method: must be public, non-static, and non-abstract");
}
}
}
这样我们将所有信息都保存到findState 类中去了。再回头看我们原先那个方法,到第三步了：
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
- 1. 我认为就是准备一个查找结果得存储对象
FindState findState = prepareFindState();
- 2. 将订阅者的subscriberClass 存储起来，保存在一个FindState 类中的subscriberClass
同时赋值给clazz变量中,以下代码能够看出
// void initForSubscriber(Class<?> subscriberClass) {
// this.subscriberClass = clazz = subscriberClass;
//}
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {进入循环中
//获取subscriberInfo 信息，返回null
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
- 3. 进入到这里了，上面已经分析所有信息保存到findState中
findUsingReflectionInSingleClass(findState);
}
- 4. 查找父类中的方法
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
在这个getMethodsAndRelease(findState):
private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
- 1. 取出里面的subscriberMethods
List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
findState.recycle();
synchronized (FIND_STATE_POOL) {
for (int i = 0; i < POOL_SIZE; i++) {
if (FIND_STATE_POOL[i] == null) {
FIND_STATE_POOL[i] = findState;
break;
}
}
}
- 2. 返回集合
return subscriberMethods;
}
至此，我们知道了根据订阅者（subscriber）的clazz 找到了所有订阅的方法事件
methods
回到最初的第一步register:
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
- 2. 完成
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
- 3.循环遍历所有的订阅方法和订阅者之间建立关联
subscribe(subscriber, subscriberMethod);
}
}
}
subscribe(subscriber, subscriberMethod) 方法：
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
- 1. 订阅方法的eventType的字节码
Class<?> eventType = subscriberMethod.eventType;
- 2. 订阅者和订阅方法封装成一个Subscription 对象
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
- 3. subscriptionsByEventType 第一次也是null ,根据eventType
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
- 4. 第一次肯定为null
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
- 5. key 为 eventType, value 是subscriptions对象
subscriptionsByEventType.put(eventType, subscriptions);
} else {
- 抛出异常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}
- 6. 获取所有添加的subscriptions
int size = subscriptions.size();
for (int i = 0; i <= size; i++) {
- 7. 会判断每个订阅方法的优先级，添加到这个 subscriptions中，按照优先级
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
- 8.获取订阅的方法集合
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
- 9. 为空添加到 typesBySubscriber
typesBySubscriber.put(subscriber, subscribedEvents);
}
- 10. 订阅事件添加到subscribedEvents集合中去
subscribedEvents.add(eventType);
- 11. 判断是否是粘性事件的关联
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);
}
}
}
到此，如果你跟着我一步步看到这里，应该大概明白一些了，还有一部分没完，就是register 前半部分完成订阅，存储等工作；剩下post(event) 方法就是将event 分发给相应订阅过此事件的订阅者了。
