Android事件总线框架设计：EventBus3.0源码详解与架构分析(中)

上半部分主要是EventBus3.0架构分析，接下来开始EventBus3.0的源码分析了。

我们从EventBus3.0使用方式开始源码分析，先来分析注册事件~

1.注册源码分析

注册事件方式：

EventBus.getDefault().register(this);

EventBus的getDefault()是一个单例，确保只有一个EventBus对象实例

    public static EventBus getDefault() {
        EventBus instance = defaultInstance;
        if (instance == null) {
            synchronized (EventBus.class) {
                instance = EventBus.defaultInstance;
                if (instance == null) {
                    instance = EventBus.defaultInstance = new EventBus();
                }
            }
        }
        return instance;
    }

EventBus构造方法做什么呢？

   private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
    public EventBus() {
        this(DEFAULT_BUILDER);
    }

在EventBus无参构造方法调用了有一个参数的构造方法，参数传入的是EventBusBuilder对象，该对象配置EventBus相关默认的属性。

    EventBus(EventBusBuilder builder) {
        logger = builder.getLogger();
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadSupport = builder.getMainThreadSupport();
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        throwSubscriberException = builder.throwSubscriberException;
        eventInheritance = builder.eventInheritance;
        executorService = builder.executorService;
    }

这些属性可以通过配置EventBusBuilder来更改

      EventBus.builder()
            .logNoSubscriberMessages(false)
            .sendNoSubscriberEvent(false)
            .installDefaultEventBus();

接下来分析EventBus注册重要的一个方法register，先看下它的源码

   public void register(Object subscriber) {
        // 获取当前注册类的Class对象
        Class subscriberClass = subscriber.getClass();
        // 查找当前注册类中被@Subscribe注解标记的所有方法
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            // 遍历所有的订阅方法，完成注册
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

从register方法源码可以看出，主要完成订阅方法查找和注册，订阅方法查找由findSubscriberMethods完成，注册由subscribe完成。
首先看下findSubscriberMethods方法源码实现

   // 缓存订阅方法，key：当前注册类的Class  value：订阅方法集合
   private static final Map, List> METHOD_CACHE = new ConcurrentHashMap<>();

    List findSubscriberMethods(Class subscriberClass) {
        // 先从缓存中获取当前注册类的订阅方法，如果找到，直接返回
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        // ignoreGeneratedIndex默认值为false
        // 主要作用：是否忽略由APT(注解处理器)生成的订阅索引
        if (ignoreGeneratedIndex) {
            // 如果忽略，则使用反射技术查找所有被@Subscribe注解标记的所有订阅方法
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            // 默认不忽略，但是如果没有使用APT(注解处理器)生成的订阅索引，则还是通过反射技术查找
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        // 如果当前注册类中没有订阅方法，则抛出异常
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            // 当前注册类中有订阅方法，添加到缓存中，以便再次注册直接使用
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

findSubscriberMethods方法主要干的事情很清晰，首先从缓存中获取当前注册类的订阅方法，如果找到，直接返回；如果没有找到，通过APT(注解处理器)或者反射技术查找当前注册类中的所有订阅方法，如果没有找到，抛出异常，否则缓存到内存中，并返回当前注册类中所有的订阅方法。

其中查找当前注册类中的所有订阅方法通过findUsingInfo方法实现

    private List findUsingInfo(Class subscriberClass) {
        // FindState
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            // 如果没有使用APT(注解处理器)生成订阅方法索引，返回null，则进入else语句中
            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 {
                // 使用反射技术查找当前注册类中所有的订阅方法
                findUsingReflectionInSingleClass(findState);
            }
            // 从父类中继续查找，直到父类为null
            findState.moveToSuperclass();
        }
        // 返回注册类中所有的订阅方法，并释放findState中状态，同时把findState对象放回缓存池中
        return getMethodsAndRelease(findState);
    }

findUsingInfo方法主要是从当前注册类及父类中查找所有的订阅方法，首先从通过APT(注解处理器)生成订阅方法索引中查找，如果没有使用APT(注解处理器)生成，则通过反射技术查找。

先来看下通过反射技术怎么去查找？关键方法findUsingReflectionInSingleClass源码如下：

    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();
            // 方法必须是public的，且不能是ABSTRACT或者STATIC
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class[] parameterTypes = method.getParameterTypes();
                // 方法的参数必须是有一个
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    // 方法必须被@Subscribe注解标记
                    if (subscribeAnnotation != null) {
                        // 方法参数类型
                        Class eventType = parameterTypes[0];
                        // 用来判断该方法是否已经添加过
                        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");
            }
        }
    }

订阅方法的查找已经分析完了，然后就是subscribe方法源码分析了

 private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        // 订阅事件类型，也就是订阅方法的参数类型
        Class eventType = subscriberMethod.eventType;
        // Subscription封装当前注册类的对象和订阅方法
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        // subscriptionsByEventType是一个Map，key为订阅事件类型，value为Subscription集合
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        // 将newSubscription添加到subscriptions集合中
        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;
            }
        }

        // typesBySubscriber也是一个Map，key为注册类的对象，value为当前注册类中所有订阅方法中的参数类型集合
        List> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        // 默认sticky为false，代表不是粘性事件，下面代码先不看，后面说到粘性事件再来分析它
        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).
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, 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);
            }
        }
    }

subscribe方法中的subscriptionsByEventType和typesBySubscriber两个Map。在事件发布时用到
subscriptionsByEventType来完成事件的处理，在取消注册时用到subscriptionsByEventType和typesBySubscriber这两个Map，后面会具体分析到～～

2.取消注册源码分析

取消注册方式：

EventBus.getDefault().unregister(this);

取消注册调用EventBus的unregister方法，下面是它的源码

    public synchronized void unregister(Object subscriber) {
        // typesBySubscriber是一个Map，注册的时候已经将key为当前注册类的对象，value为当前注册类所有订阅方法的参数类型放入当前Map中
        List> subscribedTypes = typesBySubscriber.get(subscriber);
        // 如果subscribedTypes不为null， 说明当前类有注册
        if (subscribedTypes != null) {
            for (Class eventType : subscribedTypes) {
                // 根据事件类型eventType，取消订阅
                unsubscribeByEventType(subscriber, eventType);
            }
            // 将注册的对象从Map中移除
            typesBySubscriber.remove(subscriber);
        } else {
            logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
        }
    }

从unregister方法源码可知，先根据当前取消注册类的对象从typesBySubscriber缓存中找到所有订阅方法的事件类型，然后根据事件类型，取消订阅。接下来看下unsubscribeByEventType源码：

    private void unsubscribeByEventType(Object subscriber, Class eventType) {
        // subscriptionsByEventType也是一个Map，注册的时候已经将key为订阅事件类型，value为Subscription对象集合放入当前Map中
        List subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions != null) {
            int size = subscriptions.size();
            for (int i = 0; i < size; i++) {
                Subscription subscription = subscriptions.get(i);
                if (subscription.subscriber == subscriber) {
                    subscription.active = false;
                    // 移除Subscription
                    subscriptions.remove(i);
                    i--;
                    size--;
                }
            }
        }
    }

从取消注册源码分析可知，主要是从typesBySubscriber和subscriptionsByEventType这两个Map中移除注册类对象和移除订阅方法。

3.发布普通事件

普通事件的发布，可以通过下面方式：

EventBus.getDefault().post("hello, eventbus!");

可以看到发布事件通过post方法完成

    public void post(Object event) {
        // currentPostingThreadState是一个PostingThreadState类型的ThreadLocal
        // PostingThreadState维护者事件队列和线程模型
        PostingThreadState postingState = currentPostingThreadState.get();
        List