EventBus

EventBus

EventBus is a publish/subscribe event bus optimized for Android.

EventBus介绍

  • simplifies the communication between components
    • decouples event senders and receivers
    • performs well with Activities, Fragments, and background threads
    • avoids complex and error-prone dependencies and life cycle issues
  • makes your code simpler
  • is fast
  • is tiny (~50k jar)
  • is proven in practice by apps with 100,000,000+ installs
  • has advanced features like delivery threads, subscriber priorities, etc.

添加EventBus支持

Gradle:

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gradle
compile 'org.greenrobot:eventbus:3.0.0'

download EventBus from Maven Central

EventBus 使用示例

定义事件

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  public class LoadEvent {
}

onCreate中注册事件

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EventBus.getDefault().register(this);

onDestroy中取消注册

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EventBus.getDefault().unregister(this);

订阅事件

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@Subscribe(threadMode = ThreadMode.MAIN, priority = 0, sticky = false)
public void onLoadEvent(LoadEvent event) {
Log.d(TAG, "onLoadEvent");
}
  • threadMode
    • POSTING 在调用post所在的线程执行回调,直接运行。
    • MAIN 在主线程回调,如果post所在线程为主线程则直接执行,否则则通过mainThreadPoster来调度。
    • BACKGROUND 如果post所在线程为非UI线程则直接执行,否则则通过backgroundPoster来调度,这里只适合执行时间比较短的任务。
    • ASYNC(交给线程池来管理):直接通过asyncPoster调度。
  • sticky 是否监听黏性事件

    If true, delivers the most recent sticky event (posted with EventBus#postSticky(Object)) to this subscriber (if event available).

发布事件

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EventBus.getDefault().post(new LoadEvent());

完整示例

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public class MainActivity extends AppCompatActivity {
public static final String TAG = "MainActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

EventBus.getDefault().register(this);

Button start = (Button) findViewById(R.id.start_load);
start.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
EventBus.getDefault().post(new LoadEvent());
}
});
}

@Override
protected void onDestroy() {
super.onDestroy();
EventBus.getDefault().unregister(this);
}

@Subscribe(threadMode = ThreadMode.MAIN, priority = 0, sticky = false)
public void onLoadEvent(LoadEvent event) {
Log.d(TAG, "onLoadEvent");
}

public static class LoadEvent {

}
}

EventBus的注解生成索引

项目的根目录build.gradle引入apt编译插件

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classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'

app的build.gradle 应用apt插件,并设置apt生成的索引的包名和类名

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apply plugin: 'com.neenbedankt.android-apt'
apt {
arguments {
eventBusIndex "com.android.gallery3d.MySubscriberInfoIndex"
}
}

app的dependencies中引入EventBusAnnotationProcessor:

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apt 'org.greenrobot:eventbus-annotation-processor:3.0.1'

把注解生成的索引添加到EventBus默认的单例中,注意installDefaultEventBus这个方法只能调用一次

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EventBus.builder().addIndex(new MySubscriberInfoIndex()).installDefaultEventBus();

EventBus原理浅析

EventBus.getDefault()方法

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public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}

EventBus.getDefault().register(this)方法

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public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
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private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;

private final Map<Object, List<Class<?>>> typesBySubscriber;

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
Class<?> eventType = subscriberMethod.eventType;
Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
if (subscriptions == null) {
subscriptions = new CopyOnWriteArrayList<>();
subscriptionsByEventType.put(eventType, subscriptions);
} else {
if (subscriptions.contains(newSubscription)) {
throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
+ eventType);
}
}

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;
}
}

List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
if (subscribedEvents == null) {
subscribedEvents = new ArrayList<>();
typesBySubscriber.put(subscriber, subscribedEvents);
}
subscribedEvents.add(eventType);

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);
}
}
}
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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());
}
}
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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);
}
}

EventBus.getDefault().post(new LoadEvent());

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public void post(Object event) {
PostingThreadState postingState = currentPostingThreadState.get();
List<Object> eventQueue = postingState.eventQueue;
eventQueue.add(event);

if (!postingState.isPosting) {
postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
postingState.isPosting = true;
if (postingState.canceled) {
throw new EventBusException("Internal error. Abort state was not reset");
}
try {
while (!eventQueue.isEmpty()) {
postSingleEvent(eventQueue.remove(0), postingState);
}
} finally {
postingState.isPosting = false;
postingState.isMainThread = false;
}
}
}
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private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
if (eventInheritance) {
List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
int countTypes = eventTypes.size();
for (int h = 0; h < countTypes; h++) {
Class<?> clazz = eventTypes.get(h);
subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
}
} else {
subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
}
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));
}
}
}
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  private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
CopyOnWriteArrayList<Subscription> subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}
if (subscriptions != null && !subscriptions.isEmpty()) {
for (Subscription subscription : subscriptions) {
postingState.event = event;
postingState.subscription = subscription;
boolean aborted = false;
try {
postToSubscription(subscription, event, postingState.isMainThread);
aborted = postingState.canceled;
} finally {
postingState.event = null;
postingState.subscription = null;
postingState.canceled = false;
}
if (aborted) {
break;
}
}
return true;
}
return false;
}
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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);
}
}
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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);
}
}

EventBus.getDefault().unregister(this);

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private final Map<Object, List<Class<?>>> typesBySubscriber;

public synchronized void unregister(Object subscriber) {
List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
if (subscribedTypes != null) {
for (Class<?> eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}
typesBySubscriber.remove(subscriber);
} else {
Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
}
}
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private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;

private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
List<Subscription> 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) {
subscription.active = false;
subscriptions.remove(i);
i--;
size--;
}
}
}
}

EventBus缺点

  • 耦合性太低
  • 订阅的事件是利用反射的机制执行的
  • 不能跨进程

用RxJava实现简易RxBus

实现RxBus

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public class RxBus {
private static volatile RxBus sInstance;
private final Subject subject;

private RxBus() {
subject = new SerializedSubject<>(PublishSubject.create());
}

public static RxBus getInstance() {
if (sInstance == null) {
synchronized (RxBus.class) {
if (sInstance == null) {
sInstance = new RxBus();
}
}
}
return sInstance;
}

public void post(Object event) {
subject.onNext(event);
}

public <T> Observable<T> toObservable(Class<T> eventType) {
return subject.ofType(eventType);
}
}

订阅事件

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RxBus.getInstance().toObservable(ForceUpdateEvent.class)
.observeOn(AndroidSchedulers.mainThread())
.compose(this.<ForceUpdateEvent>bindUntilEvent(ActivityEvent.DESTROY))
.subscribe(new Action1<ForceUpdateEvent>() {
@Override
public void call(ForceUpdateEvent forceUpdateEvent) {
//do some thing
}
});

发布事件

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RxBus.getInstance().post(new ForceUpdateEvent(parameter));
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public class ForceUpdateEvent {
}

参考链接

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