EventBus源码解析02-订阅者注册

柔光的暖阳◎ 2023-10-17 15:22 95阅读 0赞

接着，我们来看EventBus的订阅过程

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

这里，首先获取订阅者的class，接着，通过`subscriberMethodFinder.findSubscriberMethods`来查找订阅者的方法。

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        //从缓存中获取
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
    
        //ignoreGenerateIndex为true会优先使用编译时Index来查找，为false将忽略使用Index来查找
        if (ignoreGeneratedIndex) {
            //通过反射来获取方法集合
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            //优先通过Index来获取方法集合
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            //放到缓存中
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

一般通过`getDefault()`获取的EventBus对象，builder构建是ignoreGenerateIndex为false，故会调用`findUsingInfo()`。

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        //从缓存数组中获取(如果存在的话，不存在的话将新new一个)
        FindState findState = prepareFindState();
        //将findstate的subscriberClass和clazz变量赋值为subscriberClass
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            //从Index中获取subscriberInfo
            findState.subscriberInfo = getSubscriberInfo(findState);
            如果subscriberInfo存在
            if (findState.subscriberInfo != null) {
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        //将符合条件的方法添加到findState的集合中
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
                //说明Index中不存在，通过反射来查找Method
                findUsingReflectionInSingleClass(findState);
            }
            //移动到父类继续查找
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

在`findUsingInfo`中，会先从Inex中查找Method，如果Index中不存在，则通过反射来查找，最终会添加到findState的subscriberMethods集合中并返回。

再来看下`findUsingReflectionInSingleClass`，可以看到，主要就是通过反射遍历所有的Method，找到有`Subscribe`注解的Method，将其加入到subscriberMethods列表中返回。

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        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;
        }
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        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");
            }
        }
    }

获得这个类(及其父类)符合的方法后，会将其放到缓存列表METHOD\_CACHE中。

接着，回到`register`方法，获取subscriberMethods列表后，会遍历这个列表，并调用subscribe()方法

for (SubscriberMethod subscriberMethod : subscriberMethods) {
        subscribe(subscriber, subscriberMethod);
    }

在subscribe()方法中，最终完成订阅

// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        //创建被观察者对象
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        //根据类名获取相应的Subscription列表
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            //subscriptions列表不存在，则创建并添加到subscriptionsByEventType
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                //如果列表中已包含该Subscription，则抛出异常
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }
    
        int size = subscriptions.size();
        //根据priority(优先级)添加Subscription到相应的位置
        for (int i = 0; i <= size; i++) {
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        //将eventType存储到subscriber的typesBySubscriber列表中
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
    
        //如果是sticky (粘性事件)
        if (subscriberMethod.sticky) {
            //是否考虑所有子类，为true，子类也将能接收到
            //从EventBusBuilder可知，eventInheritance默认为true.
            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)) { //判断类型是否相同和是其子类或接口
                        Object stickyEvent = entry.getValue();
                        //分发事件
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                Object stickyEvent = stickyEvents.get(eventType);
                //分发事件
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }
    
    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());
        }
    }

#### 总结 ####

EventBus的订阅过程主要就是通过Index或者反射遍历`@Subscribe`注解，找到该类(及其父类，这个默认为true，可配置)所有的订阅方法，然后逐个进行subscribe()，添加到相应的集合中，供后续使用。  
如果是sticky事件，则从sticky的Set集合中取出相应的事件，如果事件符合当前这个订阅方法，则立即执行。