EventBus 是一个 Android 事件发布/订阅框架,通过解耦发布者和订阅者简化 Android 事件传递。传统的事件传递方式包括:Handler、BroadCastReceiver、Interface 回调,相比之下 EventBus 的优点是代码简洁,使用简单,并将事件发布和订阅充分解耦。
事件(Event):又可称为消息。其实就是一个对象,可以是网络请求返回的字符串,也可以是某个开关状态等等。 事件类型(EventType)
指事件所属的 Class。事件分为一般事件和 Sticky 事件,相对于一般事件,Sticky 事件不同之处在于,当事件发布后,再有订阅者开始订阅该类型事件,依然能收到该类型事件最近一个 Sticky 事件。
订阅者(Subscriber):订阅某种事件类型的对象。当有发布者发布这类事件后,EventBus 会执行订阅者的 onEvent 函数,这个函数叫 事件响应函数
。订阅者通过 register 接口订阅某个事件类型,unregister 接口退订。订阅者存在优先级,优先级高的订阅者可以取消事件继续向优先级低的订阅者分发,默认所有订阅者优先级都为 0。
发布者(Publisher):发布某事件的对象,通过 post 接口发布事件。
EventBus 负责存储订阅者、事件相关信息,订阅者和发布者都只和 EventBus 关联。
订阅者首先调用 EventBus 的 register 接口订阅某种类型的事件,当发布者通过 post 接口发布该类型的事件时,EventBus 执行调用者的事件响应函数。
EventBus 类负责所有对外暴露的 API,其中的 register()、post()、unregister() 函数配合上自定义的 EventType 及事件响应函数即可完成核心功能。
EventBus 默认可通过静态函数 getDefault()
获取单例,当然有需要也可以通过 EventBusBuilder 或 构造函数新建一个 EventBus,每个新建的 EventBus 发布和订阅事件都是相互隔离的,即一个 EventBus 对象中的发布者发布事件,另一个 EventBus 对象中的订阅者不会收到该订阅。
EventBus.getDefault().register(this);
register()方法解析:
public void register(Object subscriber) { Class<?> subscriberClass = subscriber.getClass(); // @Subscribe in anonymous classes is invisible to annotation processing, always fall back to reflection boolean forceReflection = subscriberClass.isAnonymousClass(); List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass, forceReflection);//主要查找又什么方法是在这个函数里面 for (SubscriberMethod subscriberMethod : subscriberMethods) { subscribe(subscriber, subscriberMethod); } }
通过一个findSubscriberMethods方法找到了一个订阅者中的所有订阅方法,返回一个 List
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, boolean forceReflection) { //类名 String key = subscriberClass.getName(); List<SubscriberMethod> subscriberMethods; synchronized (METHOD_CACHE) { //判断是否有缓存,有缓存直接返回缓存 subscriberMethods = METHOD_CACHE.get(key); } //第一次进来subscriberMethods肯定是Null if (subscriberMethods != null) { return subscriberMethods; } //INDEX是GeneratedSubscriberIndex if (INDEX != null && !forceReflection) { subscriberMethods = findSubscriberMethodsWithIndex(subscriberClass);//后面再来看这个函数 if (subscriberMethods.isEmpty()) { subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);//通过反射来找到方法 } } else { subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);//通过反射来找到方法 } if (subscriberMethods.isEmpty()) { throw new EventBusException("Subscriber " + subscriberClass + " and its super classes have no public methods with the @Subscribe annotation"); } else { synchronized (METHOD_CACHE) { METHOD_CACHE.put(key, subscriberMethods);//放入缓存 } return subscriberMethods; } }
findSubscriberMethodsWithReflection:
private List<SubscriberMethod> findSubscriberMethodsWithReflection(Class<?> subscriberClass) { List<SubscriberMethod> subscriberMethods = new ArrayList<SubscriberMethod>(); Class<?> clazz = subscriberClass; HashSet<String> eventTypesFound = new HashSet<String>(); StringBuilder methodKeyBuilder = new StringBuilder(); while (clazz != null) { String name = clazz.getName(); //过滤掉系统类 if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) { // Skip system classes, this just degrades performance break; } // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again) //通过反射,获取到订阅者的所有方法 Method[] methods = clazz.getDeclaredMethods(); for (Method method : methods) { int modifiers = method.getModifiers(); //判断是否是public,是否有修饰符 if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) { //获得订阅函数的参数 Class<?>[] parameterTypes = method.getParameterTypes(); //参数只能是1个 if (parameterTypes.length == 1) { //通过Annotation去拿一些数据 Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class); if (subscribeAnnotation != null) { String methodName = method.getName();//方法名字 Class<?> eventType = parameterTypes[0];//类型 //获取参数类型,其实就是接收事件的类型 methodKeyBuilder.setLength(0); methodKeyBuilder.append(methodName); methodKeyBuilder.append('>').append(eventType.getName()); String methodKey = methodKeyBuilder.toString(); if (eventTypesFound.add(methodKey)) { // Only add if not already found in a sub class ThreadMode threadMode = subscribeAnnotation.threadMode(); //封装一个订阅方法对象,这个对象包含Method对象,threadMode对象,eventType对象,优先级prority,sticky subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode, subscribeAnnotation.priority(), subscribeAnnotation.sticky())); } } } else if (strictMethodVerification) { if (method.isAnnotationPresent(Subscribe.class)) { String methodName = name + "." + method.getName(); throw new EventBusException("@Subscribe method " + methodName + "must have exactly 1 parameter but has " + parameterTypes.length); } } } else if (strictMethodVerification) { if (method.isAnnotationPresent(Subscribe.class)) { String methodName = name + "." + method.getName(); throw new EventBusException(methodName + " is a illegal @Subscribe method: must be public, non-static, and non-abstract"); } } } //再去查找父类 clazz = clazz.getSuperclass(); } return subscriberMethods; }
方法的讲解都在注释里面。
for (SubscriberMethod subscriberMethod : subscriberMethods) { subscribe(subscriber, subscriberMethod); }
每个订阅方法都调用subscribe方法:
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) { //从订阅方法中拿到订阅事件的类型 Class<?> eventType = subscriberMethod.eventType; //通过订阅事件类型,找到所有的订阅(Subscription),订阅中包含了订阅者,订阅方法 CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType); //创建一个新的订阅 Subscription newSubscription = new Subscription(subscriber, subscriberMethod); //将新建的订阅加入到这个事件类型对应的所有订阅列表 if (subscriptions == null) { //如果该事件目前没有订阅列表,那么创建并加入该订阅 subscriptions = new CopyOnWriteArrayList<Subscription>(); subscriptionsByEventType.put(eventType, subscriptions); } else { //如果有订阅列表,检查是否已经加入过 if (subscriptions.contains(newSubscription)) { throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event " + eventType); } } // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again) // subscriberMethod.method.setAccessible(true); // Got to synchronize to avoid shifted positions when adding/removing concurrently //根据优先级插入订阅 synchronized (subscriptions) { 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; } } } //将这个订阅事件加入到订阅者的订阅事件列表中 List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber); if (subscribedEvents == null) { subscribedEvents = new ArrayList<Class<?>>(); 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); } } }
第一步:通过 subscriptionsByEventType
得到该事件类型所有订阅者信息队列,根据优先级将当前订阅者信息插入到订阅者队列 subscriptionsByEventType
中;
第二步:在 typesBySubscriber
中得到当前订阅者订阅的所有事件队列,将此事件保存到队列 typesBySubscriber
中,用于后续取消订阅;
第三步:检查这个事件是否是 Sticky 事件,如果是则从 stickyEvents
事件保存队列中取出该事件类型最后一个事件发送给当前订阅者
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 PostThread: invokeSubscriber(subscription, event); break; case MainThread: if (isMainThread) { invokeSubscriber(subscription, event); } else { mainThreadPoster.enqueue(subscription, event); } break; case BackgroundThread: 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); } }
public void post(Object event) { //拿到PostingThreadState PostingThreadState postingState = currentPostingThreadState.get(); List<Object> eventQueue = postingState.eventQueue; //将事件放入队列 eventQueue.add(event); if (!postingState.isPosting) { postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper(); postingState.isPosting = true;//设置为正在post if (postingState.canceled) { throw new EventBusException("Internal error. Abort state was not reset"); } try { while (!eventQueue.isEmpty()) { //分发事件 postSingleEvent(eventQueue.remove(0), postingState); } } finally { postingState.isPosting = false; postingState.isMainThread = false; } } }
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error { Class<?> eventClass = event.getClass(); boolean subscriptionFound = false; if (eventInheritance) { //找到eventClass对应的事件,包含父类对应的事件和接口对应的事件 List<Class<?>> eventTypes = lookupAllEventTypes(eventClass); int countTypes = eventTypes.size(); for (int h = 0; h < countTypes; h++) { Class<?> clazz = eventTypes.get(h); //postSingleEventForEventType去查找,其中里面的数据都是通过subscribe()缓存进去的 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) { //如果没有订阅发现,那么会Post一个NoSubscriberEvent事件 post(new NoSubscriberEvent(this, event)); } } }
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) { CopyOnWriteArrayList<Subscription> subscriptions; synchronized (this) { //subscriptionsByEventType是从subscribe()缓存进去的 subscriptions = subscriptionsByEventType.get(eventClass); } if (subscriptions != null && !subscriptions.isEmpty()) { for (Subscription subscription : subscriptions) { postingState.event = event; postingState.subscription = subscription; boolean aborted = false; try { //对每个订阅调用该方法 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; }
post 函数会首先得到当前线程的 post 信息 PostingThreadState
,其中包含事件队列,将当前事件添加到其事件队列中,然后循环调用 postSingleEvent 函数发布队列中的每个事件。
postSingleEvent 函数会先去 eventTypesCache
得到该事件对应类型的的父类及接口类型,没有缓存则查找并插入缓存。循环得到的每个类型和接口,调用 postSingleEventForEventType 函数发布每个事件到每个订阅者。
postSingleEventForEventType 函数在 subscriptionsByEventType
查找该事件订阅者订阅者队列,调用 postToSubscription 函数向每个订阅者发布事件。
postToSubscription 函数中会判断订阅者的 ThreadMode,从而决定在什么 Mode 下执行事件响应函数。
PostThread
:默认的 ThreadMode,表示在执行 Post 操作的线程直接调用订阅者的事件响应方法,不论该线程是否为主线程(UI 线程)。当该线程为主线程时,响应方法中不能有耗时操作,否则有卡主线程的风险。适用场景: 对于是否在主线程执行无要求,但若 Post 线程为主线程,不能耗时的操作 ; MainThread
:在主线程中执行响应方法。如果发布线程就是主线程,则直接调用订阅者的事件响应方法,否则通过主线程的 Handler 发送消息在主线程中处理——调用订阅者的事件响应函数。显然, MainThread
类的方法也不能有耗时操作,以避免卡主线程。适用场景: 必须在主线程执行的操作 ; BackgroundThread
:在后台线程中执行响应方法。如果发布线程 不是 主线程,则直接调用订阅者的事件响应函数,否则启动 唯一的 后台线程去处理。由于后台线程是唯一的,当事件超过一个的时候,它们会被放在队列中依次执行,因此该类响应方法虽然没有 PostThread
类和 MainThread
类方法对性能敏感,但最好不要有重度耗时的操作或太频繁的轻度耗时操作,以造成其他操作等待。适用场景: 操作轻微耗时且不会过于频繁 ,即一般的耗时操作都可以放在这里; Async
:不论发布线程是否为主线程,都使用一个空闲线程来处理。和 BackgroundThread
不同的是, Async
类的所有线程是相互独立的,因此不会出现卡线程的问题。适用场景: 长耗时操作,例如网络访问 。 dependencies { compile fileTree(include: ['*.jar'], dir: 'libs') compile 'de.greenrobot:eventbus:3.0.0-beta1' }
Android -- EventBus使用: http://www.cnblogs.com/yydcdut/p/4290846.html