[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| |     |
            |     | +---------+ |     |
            |     +-------------+     |
            |                         |
            +-------------------------+

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