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EventBus 源码分析

EventBus 源码分析

作者: 小神之路 | 来源:发表于2019-06-11 19:04 被阅读0次

    EventBus原理解析

    1. 注册EventBus

    将一个类注册为事件的订阅者分两步

    1. EventBus.getDefault().register(this)注册类为事件的订阅类
    2. 注解@Subscribe来定义真正的消费事件的订阅方法

    下面展示了如何注册EventBus以及订阅事件,MyEvent为我们自定义事件类,类似与Handler类中的消息载体Message对象

        public class EventActivity extends Activity(){
            onCreate(){
                EventBus.getDefault().register(this);
            }
            
            @Subscribe
            public void onChangeEvent(MyEvent event){
            
            }
        }
        
        public class MyEvent extends Event{
        
        }
    

    EventBus使用默认的EventBusBuilder进行了初始化,创建了跨进程级别的单例对象EventBus。其中构造方法中,我们需要关注的,是创建了SubscriberMethodFinder对象,对成员变量subscriberInfoIndexes与ignoreGeneratedIndex进行了赋值操作。后面解析注册类(EventActivity)中的订阅方法时,会用到这些变量

    public class EventBus {
    
         private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
            
         public static EventBus getDefault() {
            if (defaultInstance == null) {
                synchronized (EventBus.class) {
                    if (defaultInstance == null) {
                        defaultInstance = new EventBus();
                    }
                }
            }
            return defaultInstance;
        }
        
        public EventBus() {
            this(DEFAULT_BUILDER);
        }
        
         EventBus(EventBusBuilder builder) {
            ...
            subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                    builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        }
    }
    
    

    register()真正将一个类注册为事件的订阅类,首先获取到当前类的class文件,然后调用findSubscriberMethods解析出类中所有的订阅方法,然后调用subscribe()进行订阅操作

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

    具体看一下SubscriberMethodFinder类,是如何通过class字节码解析出类中订阅方法的。首先会通过findSubscriberMethods方法,去缓存中查看是否已经解析过当前class,若已经解析过则直接返回,否则会调用findUsingInfo(subscriberClass)去进行解析操作,然后更新订阅方法缓存METHOD_CACHE。
    注:因为默认DEFAULT_BUILDER中ignoreGeneratedIndex为false,所以不会执行findUsingReflection(subscriberClass);只有自定义EventBusBuilder创建EventBus,并主动设定ignoreGeneratedIndex = true,才会执行此方法。

        private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
    
        List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
            List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
            if (subscriberMethods != null) {
                return subscriberMethods;
            }
    
            if (ignoreGeneratedIndex) {
                subscriberMethods = findUsingReflection(subscriberClass);
            } else {
                subscriberMethods = findUsingInfo(subscriberClass);
            }
            if (subscriberMethods.isEmpty()) {
                throw new EventBusException("Subscriber " + subscriberClass
                        + " and its super classes have no public methods with the @Subscribe annotation");
            } else {
                METHOD_CACHE.put(subscriberClass, subscriberMethods);
                return subscriberMethods;
            }
        }
    

    这里我们先看一下FindState类, FindState对象内部以map形式存储了订阅类,订阅方法的各种信息,后续解析时都是以FindState为媒介进行的

        static class FindState {
            final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
            final Map<Class, Object> anyMethodByEventType = new HashMap<>();
            final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
            
            void initForSubscriber(Class<?> subscriberClass) {
                this.subscriberClass = clazz = subscriberClass;
                skipSuperClasses = false;
                subscriberInfo = null;
            }
        }
    

    findUsingInfo(subscriberClass)方法会先将创建FindState对象,并将订阅类存入成员变量,然后通过getSubscriberInfo(findState)方法,获取subscriberInfo,第一次调用时会返回null(详见下一步),会继续findUsingReflectionInSingleClass(findState);

        private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
            FindState findState = prepareFindState();
            findState.initForSubscriber(subscriberClass);
            while (findState.clazz != null) {
                findState.subscriberInfo = getSubscriberInfo(findState);
                if (findState.subscriberInfo != null) {
                    SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                    for (SubscriberMethod subscriberMethod : array) {
                        if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                            findState.subscriberMethods.add(subscriberMethod);
                        }
                    }
                } else {
                    findUsingReflectionInSingleClass(findState);
                }
                findState.moveToSuperclass();
            }
            return getMethodsAndRelease(findState);
        }
    
    

    此处findState.subscriberInfo为null,subscriberInfoIndexes在使用默认构造器DEFAULT_BUILDER进行初始化时进行了赋值,故会执行for循环中的遍历操作,EventBusBuilder中SubscriberInfoIndexs初始化时并无内容,所以最终返回null

        private SubscriberInfo getSubscriberInfo(FindState findState) {
            if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
                SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
                if (findState.clazz == superclassInfo.getSubscriberClass()) {
                    return superclassInfo;
                }
            }
            if (subscriberInfoIndexes != null) {
                for (SubscriberInfoIndex index : subscriberInfoIndexes) {
                    SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
                    if (info != null) {
                        return info;
                    }
                }
            }
            return null;
        }
    
    

    所以,最终解析类中所有的订阅方法,最终还是通过findUsingReflectionInSingleClass(findState)进行,具体的说明会穿插在代码中。其核心就是通过类的字节码文件,通过反射方法,获取方法的注解和参数,然后将符合条件的方法存储到findState中

        private void findUsingReflectionInSingleClass(FindState findState) {
            Method[] methods;
            // 1. 通过反射查找所有的方法,异常后查找所有的public方法
            try {
                // This is faster than getMethods, especially when subscribers are fat classes like Activities
                methods = findState.clazz.getDeclaredMethods();
            } catch (Throwable th) {
                // Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
                methods = findState.clazz.getMethods();
                findState.skipSuperClasses = true;
            }
            // 2. 遍历所有的方法
            for (Method method : methods) {
                // 3. 通过getModifiers()获取方法的修饰符,获取公开且非abstract、static的方法
                int modifiers = method.getModifiers();
                if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                    // 4. 筛选出只有一个参数,包含有Subscribe的注解方法
                    Class<?>[] parameterTypes = method.getParameterTypes();
                    if (parameterTypes.length == 1) {
                        Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                        if (subscribeAnnotation != null) {
                            // 5. 解析出方法的第一个参数,即我们定义的事件Event类,然后添加到findState中
                            Class<?> eventType = parameterTypes[0];
                            if (findState.checkAdd(method, eventType)) {
                                ThreadMode threadMode = subscribeAnnotation.threadMode();
                                findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                        subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                            }
                        }
                    } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                        String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                        throw new EventBusException("@Subscribe method " + methodName +
                                "must have exactly 1 parameter but has " + parameterTypes.length);
                    }
                } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                    String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                    throw new EventBusException(methodName +
                            " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
                }
            }
        }
    
    

    如何判断FindState内部是否包含了当前事件,主要是通过checkAdd方法的两级检查

    1. 如果添加到anyMethodByEventType这个map返回null,代表未包含过该方法和event,添加
    2. 如果返回不为空,检查方法的签名
     boolean checkAdd(Method method, Class<?> eventType) {
                // 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
                // Usually a subscriber doesn't have methods listening to the same event type.
                Object existing = anyMethodByEventType.put(eventType, method);
                if (existing == null) {
                    return true;
                } else {
                    if (existing instanceof Method) {
                        if (!checkAddWithMethodSignature((Method) existing, eventType)) {
                            // Paranoia check
                            throw new IllegalStateException();
                        }
                        // Put any non-Method object to "consume" the existing Method
                        anyMethodByEventType.put(eventType, this);
                    }
                    return checkAddWithMethodSignature(method, eventType);
                }
            }
    
    

    Java检查一个类中方法签名,主要是靠方法名和参数名,满足方法名相同、参数类型相同且被@Subscribe修饰的函数,在一个类中不可能存在两个;考虑类继承体系,若这样的两个函数分别来自父类和子类,则最终被加入的是子类的函数。

        private boolean checkAddWithMethodSignature(Method method, Class<?> eventType) {
            methodKeyBuilder.setLength(0);
            methodKeyBuilder.append(method.getName());
            methodKeyBuilder.append('>').append(eventType.getName());
            // 1. 拼接新方法的key, 方法名+参数名
            String methodKey = methodKeyBuilder.toString();
            // 2. 获取老方法的类字节码value
            Class<?> methodClass = method.getDeclaringClass();
            // 3. 将key和value放入map集合,并返回key对应的老的方法值
            Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass);
            // 4. 如果不存在key对应的原始方法,或者原始的方法是当前方法(或当前方法的超类),加入到Map中,否认将原始方法放入map
            if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
                // Only add if not already found in a sub class
                return true;
            } else {
                // Revert the put, old class is further down the class hierarchy
                subscriberClassByMethodKey.put(methodKey, methodClassOld);
                return false;
            }
        }
    

    完成所有的check操作后,我们会将所有的订阅方法封装到FindState方法中,并通过getMethodsAndRelease()方法,将所有的订阅方法组合成一个list集合并返回到register()方法

        private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
            List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
            findState.recycle();
            synchronized (FIND_STATE_POOL) {
                for (int i = 0; i < POOL_SIZE; i++) {
                    if (FIND_STATE_POOL[i] == null) {
                        FIND_STATE_POOL[i] = findState;
                        break;
                    }
                }
            }
            return subscriberMethods;
        }
    

    我们回到register()方法,然后遍历订阅方法数组,然后将订阅事件类型和订阅者关系,封装到subscriptionsByEventType中

        // Must be called in synchronized block
        private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
            // 1.获取当前订阅方法所关联的事件类
            Class<?> eventType = subscriberMethod.eventType;
            // 2.创建订阅对象,包含订阅类与订阅方法等成员变量
            Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
            // 3.从map中获取事件相关联的订阅对象集合
            CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
            if (subscriptions == null) {
              // 4.创建subscriptions的集合,并将事件类和订阅对象集合放入subscriptionsByEventType中
                subscriptions = new CopyOnWriteArrayList<>();
                subscriptionsByEventType.put(eventType, subscriptions);
            } else {
                if (subscriptions.contains(newSubscription)) {
                    throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                            + eventType);
                }
            }
            
             // 5.为subscriptions集合添加新的订阅对象newSubscription
            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;
                }
            }
    
            // 6.subscribedEvents存入新的事件
            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);
                }
            }
        }
    

    2. 发送事件

    我们发送事件时无需初始化,直接调用EventBus.getDefault().post(event)即可

        /** Posts the given event to the event bus. */
        public void post(Object event) {
        
            // 1. 获取当前线程绑定的PostingThreadState对象
            PostingThreadState postingState = currentPostingThreadState.get();
            // 2. 获取PostingThreadState中的event的list集合,并将当前需要发送的事件加入集合
            List<Object> eventQueue = postingState.eventQueue;
            eventQueue.add(event);
    
            // 3. 若当前的发送线程状态非正在发送
            if (!postingState.isPosting) {
                // 4. 更新成员变量isMainThread与isPosting值
                postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
                postingState.isPosting = true;
                if (postingState.canceled) {
                    throw new EventBusException("Internal error. Abort state was not reset");
                }
                // 5.依次发送事件,并从队列中移除移除
                try {
                    while (!eventQueue.isEmpty()) {
                        postSingleEvent(eventQueue.remove(0), postingState);
                    }
                } finally {
                    // 6.所有时间发送完毕后更新状态
                    postingState.isPosting = false;
                    postingState.isMainThread = false;
                }
            }
        }
    
    

    currentPostingThreadState是ThreadLocal的实例,每次调用时会返回当前线程绑定的PostingThreadState对象。如果为空,则会创建一个新的PostingThreadState对象,并将对象与当前线程进行绑定,Thread.threadLocals = new PostingThreadState(),具体的原理可以参考Handler中的ThreadLocal详解

        private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
            @Override
            protected PostingThreadState initialValue() {
                return new PostingThreadState();
            }
        };
    
    

    PostingThreadState类内部包含一个存储有事件的list集合,以及向前线程事件的一些状态

    /** For ThreadLocal, much faster to set (and get multiple values). */
        final static class PostingThreadState {
            final List<Object> eventQueue = new ArrayList<Object>();
            boolean isPosting;
            boolean isMainThread;
            Subscription subscription;
            Object event;
            boolean canceled;
        }
    

    postSingleEvent()发送单个事件

        private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
            // 1.获取当前发送对象字节码
            Class<?> eventClass = event.getClass();
            boolean subscriptionFound = false;
            // 2.默认eventInheritance为ture,执行if中的代码
            if (eventInheritance) {
                List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
                int countTypes = eventTypes.size();
                for (int h = 0; h < countTypes; h++) {
                    Class<?> clazz = eventTypes.get(h);
                    // 3.发送指定事件
                    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));
                }
            }
        }
    
    

    通过当前的事件类,查找到所有包含这个类的接口和类对象

        /** Looks up all Class objects including super classes and interfaces. Should also work for interfaces. */
        private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
            synchronized (eventTypesCache) {
                // 1.默认eventTypesCache内部为空
                List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
                if (eventTypes == null) {
                    eventTypes = new ArrayList<>();
                    Class<?> clazz = eventClass;
                    while (clazz != null) {
                        eventTypes.add(clazz);
                        addInterfaces(eventTypes, clazz.getInterfaces());
                        clazz = clazz.getSuperclass();
                    }
                    eventTypesCache.put(eventClass, eventTypes);
                }
                return eventTypes;
            }
        }
    

    发送当前事件,通知注册EventBus时解析出的订阅类,调用订阅该事件的方法

        private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
            CopyOnWriteArrayList<Subscription> subscriptions;
            synchronized (this) {
                // 1.获取在我们register操作时在subscriptionsByEventType中放入的事件类与相关连的订阅集合
                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;
        }
    
        private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
            switch (subscription.subscriberMethod.threadMode) {
                // 1.默认所有的@Subscribe注解方法线程为当前发送线程:POSTING
                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);
            }
        }
    

    通过反射调用订阅该事件的类方法

        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);
            }
        }
    
    如有代码或者解释错误,欢迎指正

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