[Android] Otto源码简析
用例
本文主要按照如下例子展开：
//1. 新建bus对象，默认仅能在主线程上对消息进行调度Bus bus = new Bus(); // maybe singleton//2. 新建类A(subscriber)，answerAvailable()方法为事件回调，在主线程上运行class A { public A() { bus.register(this);} // 可见性为public，仅有一个Event类型的参数 @Subscribe public void answerAvailable(AnswerAvailableEvent event) { // process event }}//3. 往bus投递事件bus.post(new AnswerAvailableEvent(42));//4. 如果要A在注册时马上接收到一次回调，则可以新建类B(Producer)，produceAnswer()// 方法会在注册subscriber时，对每个订阅了AnswerAvailableEvent方法发送事件class B { public B() { bus.register(this);} //可见性为public，不带任何参数 @Produce public AnswerAvailableEvent produceAnswer() { return new AnswerAvailableEvent(); }}初始化
首先来看看Bus bus = new Bus()这一句，对应的源码如下所示：
public Bus() { this(DEFAULT_IDENTIFIER);}public Bus(String identifier) { this(ThreadEnforcer.MAIN, identifier);}public Bus(ThreadEnforcer enforcer, String identifier) { this(enforcer, identifier, HandlerFinder.ANNOTATED);}Bus(ThreadEnforcer enforcer, String identifier, HandlerFinder handlerFinder) { this.enforcer = enforcer; this.identifier = identifier; this.handlerFinder = handlerFinder;}默认参数为enforcer = ThreadEnforcer.MAIN，identifier = DEFAULT_IDENTIFIER，handlerFinder = HandlerFinder.ANNOTATED。我们来看看这些参数是什么意思。
ThreadEnforcer
ThreadEnforcer是一个接口，enforce()方法用于检查当前线程是否为指定的线程类型：
public interface ThreadEnforcer { ThreadEnforcer ANY = new ThreadEnforcer() {@Overridepublic void enforce(Bus bus) { // Allow any thread.} }; ThreadEnforcer MAIN = new ThreadEnforcer() {@Overridepublic void enforce(Bus bus) { if (Looper.myLooper() != Looper.getMainLooper()) { throw new IllegalStateException("Event bus " + bus +" accessed from non-main thread " + Looper.myLooper()); }} }; void enforce(Bus bus);}不带参数的构造函数bus()使用默认的ThreadEnforcer.MAIN，表示enforce()方法必须在主线程上执行。
identifier
identifier仅为bus的名字，debug用。
handlerFinder
HandlerFinder用于在注册/反注册的时候查找Subscriber和Producer，后文会对其展开源码级别的解析。不带参数的构造函数bus()使用默认的HandlerFinder.ANNOTATED，表示使用注解来进行查找。
除上述以外，bus类还有两个成员变量handlersByType和producersByType：
private final ConcurrentMap<Class<?>, Set<EventHandler>> handlersByType = new ConcurrentHashMap<Class<?>, Set<EventHandler>>();private final ConcurrentMap<Class<?>, EventProducer> producersByType = new ConcurrentHashMap<Class<?>, EventProducer>();分别用于通过event的类型（class类型）来查找event handle和event producer。
注册/反注册事件
如下所示，要A成为订阅者订阅AnswerAvailableEvent，只需将其注册到bus，然后使用@Subscribe注解标记回调方法即可。回调方法要求可见性为public，有且仅有一个参数，类型为订阅的event。
class A { public A() { bus.register(this);} @Subscribe public void answerAvailable(AnswerAvailableEvent event) { // process event }}@Subscribe
首先看一下@Subscribe注解：
@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.METHOD)public @interface Subscribe {}RetentionPolicy.RUNTIME表示它是运行时的注解，ElementType.METHOD表示用于注解方法。
bus.register
再看一下register流程：
public void register(Object object) { if (object == null) { throw new NullPointerException("Object to register must not be null."); } //1. 检查当前线程是否符合ThreadEnforcer的设置 enforcer.enforce(this); //2. 默认情况下，通过注解在object上找出所有Producer Map<Class<?>, EventProducer> foundProducers = handlerFinder.findAllProducers(object); for (Class<?> type : foundProducers.keySet()) { //2-1. 查一下object上的producer注册的event是否已经被别人注册过。 final EventProducer producer = foundProducers.get(type); EventProducer previousProducer = producersByType.putIfAbsent(type, producer); //checking if the previous producer existed if (previousProducer != null) {throw new IllegalArgumentException("Producer method for type " + type + " found on type " + producer.target.getClass() + ", but already registered by type " + previousProducer.target.getClass() + "."); } //2-2. 如果没有注册过，那么找出对应event的handler，触发一次回调。 Set<EventHandler> handlers = handlersByType.get(type); if (handlers != null && !handlers.isEmpty()) {for (EventHandler handler : handlers) { dispatchProducerResultToHandler(handler, producer);} } } //3. 找出object上用@Subscribe注解了的方法 Map<Class<?>, Set<EventHandler>> foundHandlersMap = handlerFinder.findAllSubscribers(object); for (Class<?> type : foundHandlersMap.keySet()) { Set<EventHandler> handlers = handlersByType.get(type); if (handlers == null) {//3-1. 该event是第一次注册，那么新建一个CopyOnWriteArraySet用来保存handler和event的对应关系（EventHandler）//concurrent put if absentSet<EventHandler> handlersCreation = new CopyOnWriteArraySet<EventHandler>();handlers = handlersByType.putIfAbsent(type, handlersCreation);if (handlers == null) { handlers = handlersCreation;} } //3-2. 保存object中新增的event-handler对应关系。 final Set<EventHandler> foundHandlers = foundHandlersMap.get(type); if (!handlers.addAll(foundHandlers)) {throw new IllegalArgumentException("Object already registered."); } } //4. 检查object上的event是否存在对应的Producer，有则触发一次回调 for (Map.Entry<Class<?>, Set<EventHandler>> entry : foundHandlersMap.entrySet()) { Class<?> type = entry.getKey(); EventProducer producer = producersByType.get(type); if (producer != null && producer.isValid()) {Set<EventHandler> foundHandlers = entry.getValue();for (EventHandler foundHandler : foundHandlers) { if (!producer.isValid()) { break; } if (foundHandler.isValid()) { dispatchProducerResultToHandler(foundHandler, producer); }} } }}总的来说register做了三件事情：触发新的Producer；注册新的event-handler关系；触发旧的Producer。另外有两点要注意一下：
由于在一般使用场景下，发送/处理event远比注册/反注册操作频繁，所以在保证线程安全的情况下，使用CopyOnWriteArraySet作为保存event和handler的容器，可以大大提高效率。
CopyOnWrite容器在读的时候不会加锁，写的时候先复制一份，写完再替换原容器。如果容器正在写操作时发生了读操作（或者正在读的时候发生了写操作），读操作的对象为容器的快照（snapshot）。由于register方法没有加锁，所以在3-1中，尽管已经检查了handlers是否存在，但仍需使用putIfAbsent来保存handler。
EventProducer和EventHandler
注意到bus通过HandlerFinder来查找object上的producer和subscriber，接下来看一下HandlerFinder的实现：
interface HandlerFinder { HandlerFinder ANNOTATED = new HandlerFinder() {@Overridepublic Map<Class<?>, EventProducer> findAllProducers( Object listener) { return AnnotatedHandlerFinder.findAllProducers(listener);}@Overridepublic Map<Class<?>, Set<EventHandler>> findAllSubscribers( Object listener) { return AnnotatedHandlerFinder.findAllSubscribers(listener);} }; Map<Class<?>, EventProducer> findAllProducers(Object listener); Map<Class<?>, Set<EventHandler>> findAllSubscribers(Object listener);}其中findAllProducers方法返回某event type对应的EventProducer，findAllSubscribers返回某event type对应的EventHandler集合。先看一下EventProducer和EventHandler。
EventProducer是一个producer方法的包装类，源码如下：
class EventProducer { final Object target; private final Method method; private final int hashCode; private boolean valid = true; EventProducer(Object target, Method method) { if (target == null) {throw new NullPointerException( "EventProducer target cannot be null."); } if (method == null) {throw new NullPointerException( "EventProducer method cannot be null."); } this.target = target; this.method = method; method.setAccessible(true); // 提前计算hashcode，以防每次调用hash()时消耗资源 final int prime = 31; hashCode = ((prime + method.hashCode()) * prime) + target.hashCode(); } public boolean isValid() { return valid; } // 应在object unregister时调用 public void invalidate() { valid = false; } public Object produceEvent() throws InvocationTargetException { if (!valid) {throw new IllegalStateException(toString() + " has been invalidated and can no longer produce events."); } try {return method.invoke(target); } catch (IllegalAccessException e) {throw new AssertionError(e); } catch (InvocationTargetException e) {if (e.getCause() instanceof Error) { throw (Error) e.getCause();}throw e; } }}其中produceEvent方法用于获得event。可以看出为什么Otto要求produce函数不能有参数。
与EventProducer类似，EventHandler是一个event handler方法（事件回调）的包装类，源码如下：
class EventHandler { private final Object target; private final Method method; private final int hashCode; private boolean valid = true; EventHandler(Object target, Method method) { if (target == null) {throw new NullPointerException( "EventHandler target cannot be null."); } if (method == null) {throw new NullPointerException( "EventHandler method cannot be null."); } this.target = target; this.method = method; method.setAccessible(true); // Compute hash code eagerly since we know it will be used frequently and we cannot estimate the runtime of the // target's hashCode call. final int prime = 31; hashCode = ((prime + method.hashCode()) * prime) + target.hashCode(); } public boolean isValid() { return valid; } public void invalidate() { valid = false; } public void handleEvent(Object event) throws InvocationTargetException { if (!valid) {throw new IllegalStateException(toString() + " has been invalidated and can no longer handle events."); } try {method.invoke(target, event); } catch (IllegalAccessException e) {throw new AssertionError(e); } catch (InvocationTargetException e) {if (e.getCause() instanceof Error) { throw (Error) e.getCause();}throw e; } }}其中handleEvent方法用于在object上调用handle方法（事件回调），传入event对象。可以看出为什么Otto要求event handler函数仅能有一个参数。
dispatchProducerResultToHandler
dispatchProducerResultToHandler方法用于将Producer产生的event分发给对应的handler。源码如下所示：
private void dispatchProducerResultToHandler(EventHandler handler, EventProducer producer) { Object event = null; try { event = producer.produceEvent(); } catch(InvocationTargetException e) { throwRuntimeException("Producer " + producer + " threw an exception.", e); } if (event == null) { return; } dispatch(event, handler);}protected void dispatch(Object event, EventHandler wrapper) { try { wrapper.handleEvent(event); } catch(InvocationTargetException e) { throwRuntimeException("Could not dispatch event: " + event.getClass() + " to handler " + wrapper, e); }}逻辑比较简单，主要是使用了Producer的produceEvent()方法获得event对象后，调用EventHandler的handleEvent()方法。
bus.unregister
Bus类的unregister方法用于解除目标对象和bus之间的关联关系，包括对象上的producer方法，subscriber方法，源码如下所示：
public void unregister(Object object) { if (object == null) { throw new NullPointerException("Object to unregister must not be null."); } //1. 检查当前线程是否符合ThreadEnforcer的设置 enforcer.enforce(this); //2. 默认情况下，通过注解在object上找出所有Producer，将其从producersByType中删除并标记为invalidate Map<Class<?>, EventProducer> producersInListener = handlerFinder.findAllProducers(object); for (Map.Entry<Class<?>, EventProducer> entry : producersInListener.entrySet()) { final Class<?> key = entry.getKey(); EventProducer producer = getProducerForEventType(key); EventProducer value = entry.getValue(); if (value == null || !value.equals(producer)) {throw new IllegalArgumentException("Missing event producer for an annotated method. Is " + object.getClass() + " registered?"); } producersByType.remove(key).invalidate(); } //3. 默认情况下，找出object上用@Subscribe注解了的handler，将其从event集合中删除并标记为invalidate Map<Class<?>, Set<EventHandler>> handlersInListener = handlerFinder.findAllSubscribers(object); for (Map.Entry<Class<?>, Set<EventHandler>> entry : handlersInListener.entrySet()) { Set<EventHandler> currentHandlers = getHandlersForEventType(entry.getKey()); Collection<EventHandler> eventMethodsInListener = entry.getValue(); if (currentHandlers == null || !currentHandlers.containsAll(eventMethodsInListener)) {throw new IllegalArgumentException("Missing event handler for an annotated method. Is " + object.getClass() + " registered?"); } for (EventHandler handler : currentHandlers) {if (eventMethodsInListener.contains(handler)) { handler.invalidate();} } currentHandlers.removeAll(eventMethodsInListener); }}投递事件
一次简单的事件投递操作如下所示：
bus.post(new AnswerAvailableEvent(42));我们来看一下post方法的源码实现：
public void post(Object event) { if (event == null) { throw new NullPointerException("Event to post must not be null."); } //1. 检查当前线程是否符合ThreadEnforcer的设置 enforcer.enforce(this); //2. 向上追溯event的所有父类 Set<Class<?>>dispatchTypes = flattenHierarchy(event.getClass()); //3. 当前event没有注册handler，则发送一个DeadEvent事件 boolean dispatched = false; for (Class<?>eventType: dispatchTypes) { Set<EventHandler> wrappers = getHandlersForEventType(eventType); if (wrappers != null && !wrappers.isEmpty()) {dispatched = true;for (EventHandler wrapper: wrappers) { //3-1 将事件和handler放到分发队列里 enqueueEvent(event, wrapper);} } } //4. 当前event没有注册handler，则发送一个DeadEvent事件 if (!dispatched && !(event instanceof DeadEvent)) { post(new DeadEvent(this, event)); } //5. 通知队列进行分发操作 dispatchQueuedEvents();}注意几点：
发送一个Event时，订阅了Event父类的Subscriber方法也会被调用。
事件会被放到调用者所在线程的队列里依次分发。
下面分点进行详述。
flattenHierarchy
进行post操作时，首先会通过flattenHierarchy方法获得event的父类或者接口：
Set<Class<?>>flattenHierarchy(Class<?>concreteClass) { Set<Class<?>>classes = flattenHierarchyCache.get(concreteClass); if (classes == null) { Set<Class<?>>classesCreation = getClassesFor(concreteClass); classes = flattenHierarchyCache.putIfAbsent(concreteClass, classesCreation); if (classes == null) {classes = classesCreation; } } return classes;}private Set<Class<?>> getClassesFor(Class<?> concreteClass) { List<Class<?>> parents = new LinkedList<Class<?>>(); Set<Class<?>> classes = new HashSet<Class<?>>(); parents.add(concreteClass); //深度优先遍历 while (!parents.isEmpty()) { Class<?> clazz = parents.remove(0); classes.add(clazz); Class<?> parent = clazz.getSuperclass(); if (parent != null) {parents.add(parent); } } return classes;}从上可知flattenHierarchy()通过getClassesFor()利用深度优先遍历导出了concreteClass的所有父类。
Dispatch Queue
通过post方法投递的event首先会放在当前线程所在的Dispatch Queue中，然后依次分发。Bus类有如下成员属性：
private final ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>> eventsToDispatch = new ThreadLocal<ConcurrentLinkedQueue<EventWithHandler>>() { @Override protected ConcurrentLinkedQueue<EventWithHandler> initialValue() {return new ConcurrentLinkedQueue<EventWithHandler>(); } };eventsToDispatch是一个ThreadLocal对象，通过initialValue()方法，eventsToDispatch每次在新的线程上调用的时候都会生成新的ConcurrentLinkedQueue实例。event是通过enqueueEvent(event, wrapper)方法放到queue中的，下面看看enqueueEvent()的实现：
protected void enqueueEvent(Object event, EventHandler handler) { eventsToDispatch.get().offer(new EventWithHandler(event, handler));}offer()方法会会将EventWithHandler对象放到当前线程的queue的尾部。offer方法和add方法的区别在于，当无法插入（例如空间不够）的情况发生时会发挥false，热不是抛出异常。EventWithHandler类对event和handler的关系进行了简单的包装，实现如下：
static class EventWithHandler { final Object event; final EventHandler handler; public EventWithHandler(Object event, EventHandler handler) { this.event = event; this.handler = handler; }}接下来看看dispatchQueuedEvents方法的实现：
protected void dispatchQueuedEvents() { // don't dispatch if we're already dispatching, that would allow reentrancy and out-of-order events. Instead, leave // the events to be dispatched after the in-progress dispatch is complete. //1. 不能重复分发，否则会导致event的分发次序混乱 if (isDispatching.get()) { return; } isDispatching.set(true); try { while (true) {//2. 依次取出EventWithHandler，并通过dispatch方法进行分发。EventWithHandler eventWithHandler = eventsToDispatch.get().poll();if (eventWithHandler == null) { break;}if (eventWithHandler.handler.isValid()) { dispatch(eventWithHandler.event, eventWithHandler.handler);} } } finally { isDispatching.set(false); }}值得注意的是，所有subscribe方法抛出的异常都会在这里捕获，捕获到异常以后event分发过程即停止，直到下一次在该线程上调用post为止。
结构图
综上，Otto的总体结构可用下图表示：
+-------------------------+|Bus(ThreadLocal) || +--------------+ || |EventProducers| || | +-------+ | register +-------+| | |Produce| <----+-------+Produce|| | +-------+ | | +-------+| | +-------+ | || | |Produce| | || | +-------+ | || +--------------+ ||||| event |||| post(event)| +-------v--------+ |+----------------> Dispatch Queue | || +-------+--------+ ||||| event ||||| +------v------+ || |EventHandlers| || | +---------+ | || | |Subscribe| | register +---------+| | +---------+ <-----+-------+Subscribe|| | +---------+ | | +---------+| | |Subscribe| | || | +---------+ | || +-------------+ || |+-------------------------+