[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() {            @Override            public void enforce(Bus bus) {                // Allow any thread.            }        };    ThreadEnforcer MAIN = new ThreadEnforcer() {            @Override            public 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 absent            Set<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() {            @Override            public Map<Class<?>, EventProducer> findAllProducers(                Object listener) {                return AnnotatedHandlerFinder.findAllProducers(listener);            }            @Override            public 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| |     |            |     | +---------+ |     |            |     +-------------+     |            |                         |            +-------------------------+

2015年11月11日发布

用例

初始化