前言
由于最近项目中频繁使用EventBus，所以想了解下它内部原理，我们得"知其然知其所以然"。在这里总结一下，方便自己复习。
欢迎start我的GitHub项目，里面会不定时更细Android 、java、设计模式相关知识！
EventBus的使用
项目配置
很简单只需要在build.gradle(app)下添加如下：
implementation 'org.greenrobot:eventbus:3.1.1'
项目中使用
普通使用：
1、发送消息：
EventBus.getDefault().post(事件);
2、在接受页面注册和反注册，并接收。
//注册
EventBus.getDefault().register(this);
//反注册
EventBus.getDefault().unregister(this);
//接收事件(getdata这名字是随便的，起什么都可以)
@Subscribe(threadMode = ThreadMode.MAIN)
public void getdata(事件) {
//在这里处理发送的事件
}
普通使用例子：
比如现在有这样一个需求，有两个页面分别是MainActivity和Main2Activity。需要MainActivity跳转到Main2Activity，点击Main2Activity中的按钮返回并携带数据。（当然我们也可以使用onActivityResult去实现 -。-）
MainActivity代码:
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//注册
EventBus.getDefault().register(this);
}
public void onClick(View view) {
switch (view.getId()) {
case R.id.main_tv:
Intent intent = new Intent(this, Main2Activity.class);
startActivity(intent);
break;
}
}
//订阅者方法
@Subscribe(threadMode = ThreadMode.MAIN)
public void getdata(Message message) {
Toast.makeText(this, "Type=" + message.getType(), Toast.LENGTH_SHORT).show();
}
@Override
protected void onDestroy() {
super.onDestroy();
//反注册
EventBus.getDefault().unregister(this);
}
}
Main2Activity代码：
public class Main2Activity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
}
public void onClick2(View view) {
//发送消息
EventBus.getDefault().post(new Message(1));
finish();
}
}
Message事件实体:
public class Message {
private int type;
public Message(int type) {
this.type = type;
}
public int getType() {
return type;
}
public void setType(int type) {
this.type = type;
}
}
效果图
EventBusPost.gif
可能有人会说，官网的是在onStart()/onStop()中注册和反注册，你怎么在onCreate方法中注册，在onDestory方法中反注册？这样的话有一些情况我们普通的方式去发送事件，是在需要接收消息页面会收不到，因为使用时需要往已经注册的页面发送消息，才能接受到。当我们发送消息时注册方法还没有被调用和关联订阅者，那么就无法接受事件。
粘性事件使用：
1、发送消息：
EventBus.getDefault().postSticky(事件);
2、在接受页面注册和反注册，并接收。
//注册
EventBus.getDefault().register(this);
//反注册
EventBus.getDefault().unregister(this);
//接收事件(getdata这名字是随便的，起什么都可以)
@Subscribe(threadMode = ThreadMode.MAIN,sticky = true)
public void getdata(事件) {
//在这里处理发送的事件
}
粘性事件例子;
有这样的一个需求，在MainActivity往Main2Activity（没有创建过）页面传递数据(当然我们也可以使用Intent携带Bundle传递)。
MainActivity代码：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void onClick(View view) {
switch (view.getId()) {
case R.id.main_tv:
//发送消息
EventBus.getDefault().postSticky(new Message(1));
Intent intent = new Intent(this, Main2Activity.class);
startActivity(intent);
break;
}
}
}
Main2Activity代码：
public class Main2Activity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
}
@Override
protected void onStart() {
super.onStart();
//注册
EventBus.getDefault().register(this);
}
@Override
protected void onStop() {
super.onStop();
//反注册
EventBus.getDefault().unregister(this);
}
public void onClick2(View view) {
finish();
}
//订阅者方法
@Subscribe(threadMode = ThreadMode.MAIN,sticky = true)
public void getdata(Message message) {
Toast.makeText(this, "MainActivity发送Type为" + message.getType(), Toast.LENGTH_SHORT).show();
}
}
效果图
EventBuspostSticky.gif
EventBus源码分析：
EventBus.getDefault()分析
我们在使用时不管操作什么都需要调用EventBus.getDefault()方法。
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
我们看上面源码可知，它用到了双重检锁获取到了EventBus的对象。
register注册分析
public void register(Object subscriber) {
//获取注册的对象
Class<?> subscriberClass = subscriber.getClass();
//找到订阅方法集合
List<SubscriberMethod>subscriberMethods=subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
//订阅
subscribe(subscriber, subscriberMethod);
}
}
}
解析1SubscriberMethod是存储订阅方法信息：
public class SubscriberMethod {
final Method method;
final ThreadMode threadMode;
final Class<?> eventType;
final int priority;
final boolean sticky;
/** Used for efficient comparison */
String methodString;
......
}
ThreadMode订阅者线程
eventType事件
priority优先级
sticky 是否粘性事件
Method接受事件的方法(比如例子中的getdata())
解析2findSubscriberMethods查找订阅者方法
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE=new ConcurrentHashMap<>();
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
//通过订阅者对象查找缓存（以ConcurrentHashMap存储的key为订阅者的类，value为订阅者方法集合）
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
//查到的缓存不为空直接返回
if (subscriberMethods != null) {
return subscriberMethods;
}
//是否忽略生成的索引 ，默认false
if (ignoreGeneratedIndex) {
//以反射的方式获取订阅者订阅方法
subscriberMethods = findUsingReflection(subscriberClass);
} else {
//以索引的形式获取
subscriberMethods = findUsingInfo(subscriberClass);
}
//判断内容是否为空，为空抛异常
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass + " and its super classes have no public methods with the @Subscribe annotation");
} else {
//缓存到ConcurrentHashMap中，以便于下次查找
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
解析2.1findUsingReflection
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
//创建FindState对象
FindState findState = prepareFindState();
//于订阅者类关联
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//使用反射获取订阅方法
findUsingReflectionInSingleClass(findState);
findState.moveToSuperclass();
}
//返回FindState中的订阅方法集合
return getMethodsAndRelease(findState);
}
解析2.1.1findUsingReflectionInSingleClass
private void findUsingReflectionInSingleClass(FindState findState) {
Method[] methods;
try {
// This is faster than getMethods, especially when subscribers are fat classes like Activities
//获取本类中的所有方法
methods = findState.clazz.getDeclaredMethods();
} catch (Throwable th) {
// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
//获取本类和父类的所有公共方法
methods = findState.clazz.getMethods();
//设置标记
findState.skipSuperClasses = true;
}
//遍历
for (Method method : methods) {
//返回方法的int值
int modifiers = method.getModifiers();
//忽略不是PUBLIC和static
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
//获取参数类型
Class<?>[] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
//获取Subscribe注解(里面有线程模型、是否粘性、优先级)
Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
if (subscribeAnnotation != null) {
Class<?> eventType = parameterTypes[0];
if (findState.checkAdd(method, eventType)) {
//获取线程模型
ThreadMode threadMode = subscribeAnnotation.threadMode();
//添加到订阅者集合
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
}
}
......
}
解析2.2findUsingInfo
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
//第一次会为空
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 {
//调用解析2.1.1反射获取
findUsingReflectionInSingleClass(findState);
}
//在父类中查找
findState.moveToSuperclass();
}
//返回订阅者集合
return getMethodsAndRelease(findState);
}
解析3subscribe
private final Map<Class<?>,CopyOnWriteArrayList<Subscription>>=subscriptionsByEventType;
private final Map<Object, List<Class<?>>> typesBySubscriber;
// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
//获取的事件
Class<?> eventType = subscriberMethod.eventType;
//存储订阅者和订阅者方法信息
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
CopyOnWriteArrayList<Subscription>subscriptions = subscriptionsByEventType.get(eventType);
//是否为空，并存储
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
//判断是否已经注册过，不能重复注册，抛异常
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "+ eventType);
}
}
int size = subscriptions.size();
//优先级排序
for (int i = 0; i <= size; i++) {
if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
subscriptions.add(i, newSubscription);
break;
}
}
//获取当前订阅者的事件
List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
//为空创建并添加缓存中
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
//添加当前事件
subscribedEvents.add(eventType);
//是否是粘性事件
if (subscriberMethod.sticky) {
//默认true
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 {
//获取粘性事件
Object stickyEvent = stickyEvents.get(eventType);
//发送事件
checkPostStickyEventToSubscription(newSubscription, stickyEvent);
}
}
}
注册总结
Eventbus注册流程图.png
注册时发送事件checkPostStickyEventToSubscription会调用这个方法postSingleEventForEventType
post普通发送分析
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
@Override
protected PostingThreadState initialValue() {
return new PostingThreadState();
}
};
使用ThreadLocal存储PostingThreadState,线程安全的
/** For ThreadLocal, much faster to set (and get multiple values). */
fin al static class PostingThreadState {
//事件队列集合
final List<Object> eventQueue = new ArrayList<>();
//是否发送
boolean isPosting;
//是否是主线程
boolean isMainThread;
//存储订阅信息的
Subscription subscription;
//事件
Object event;
//是否取消
boolean canceled;
}
/** Posts the given event to the event bus. */
public void post(Object event) {
//
PostingThreadState postingState = currentPostingThreadState.get();
//获取事件队列集合
List<Object> eventQueue = postingState.eventQueue;
//添加当前事件
eventQueue.add(event);
//判断是否发送
if (!postingState.isPosting) {
//是否是主线程
postingState.isMainThread = isMainThread();
//更改是否发送过默认值
postingState.isPosting = true;
//判断是否取消
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
while (!eventQueue.isEmpty()) {
//移除事件并传递
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
//恢复状态
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
postSingleEvent
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
//默认值为true
if (eventInheritance) {
//查找事件所有Class对象，包括超类和接口内部使用hashmap缓存
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
//遍历
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
//发送事件
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
//发送事件
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
//subscriptionFound为true是调用 也就是 subscriptions为空或者size为0时
if (!subscriptionFound) {
if (logNoSubscriberMessages) {
logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
}
if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
eventClass != SubscriberExceptionEvent.class) {
post(new NoSubscriberEvent(this, event));
}
}
}
postSingleEventForEventType发送事件
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
//存储订阅者信息
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
//获取缓存
subscriptions = subscriptionsByEventType.get(eventClass);
}
//如果不为空
if (subscriptions != null && !subscriptions.isEmpty()) {
//遍历
for (Subscription subscription : subscriptions) {
//设置订阅者的信息
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
} finally {
//清除状态
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}
postToSubscription判断哪个线程并通过反射调用订阅方法
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
//判断获取的线程模式
switch (subscription.subscriberMethod.threadMode) {
case POSTING:
//直接调用
invokeSubscriber(subscription, event);
break;
case MAIN:
//判断是否是主线程
if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;
case MAIN_ORDERED:
if (mainThreadPoster != null) {
mainThreadPoster.enqueue(subscription, event);
} else {
// temporary: technically not correct as poster not decoupled from subscriber
invokeSubscriber(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);
}
}
1、PostThread 默认的线程模式，直接调用invokeSubscriber方法。
发布事件和接收事件在同一个线程
2、MAIN判断是否在主线程，true直接调用invokeSubscriber方法,false使用mainThreadPoster处理，他的视线类是HandlerPoster。加入队列使用handler发送消息在handler的handleMessage中去执行invokeSubscriber方法。
事件的处理会在UI线程中执行
3、MAIN_ORDERED先判断mainThreadPoster是否为空，true调用mainThreadPoster排队等待传递非阻塞的，否则直接调用invokeSubscriber方法。
事件的处理会在UI线程中执行
4、BACKGROUND判断是否在主线程，true使用BackgroundPoster处理，false直接调用invokeSubscriber
事件是在UI线程中发布出来的，那么该事件处理函数就会在新的线程中运行，如果事件本来就是子线程中发布出来的，那么该事件处理函数直接在发布事件的线程中执行
5、ASYNC直接交由AsyncPoster处理
事件处理函数都会在新建的子线程中执行
invokeSubscriber通过反射调用订阅方法
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);
}
}
postSticky粘性事件
private final Map<Class<?>, Object> stickyEvents;
public void postSticky(Object event) {
synchronized (stickyEvents) {
stickyEvents.put(event.getClass(), event);
}
// Should be posted after it is putted, in case the subscriber wants to remove immediately
post(event);
}
先同步，缓存到stickyEvents（ConcurrentHashMap缓存的），之后调用post方法。之后流程跟普通发送事件一样。
unregister 反注册分析
/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
//获取订阅者集合
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
//遍历
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
//移除订阅者
typesBySubscriber.remove(subscriber);
} else {
logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
unsubscribeByEventType
/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
//获取订阅者信息集合
List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions != null) {
int size = subscriptions.size();
//遍历
for (int i = 0; i < size; i++) {
Subscription subscription = subscriptions.get(i);
//判断是否是当前订阅者
if (subscription.subscriber == subscriber) {
subscription.active = false;
//移除
subscriptions.remove(i);
i--;
size--;
}
}
}
}
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