EventBus的使用，以及源码分析

EventBus的使用

EventBus能够简化各组件间的通信，能够有效的分离事件的发送方和接收方，能避免复杂和容易出错的依赖性和生命周期的问题。

EventBus的三要素

• Event事件，可以是任意类型。
• Subscriber事件订阅者，需要进行注册。
• Publisher事件的发布者。

EventBus的用法

添加依赖

implementation 'org.greenrobot:eventbus:3.1.1'

注册事件

override fun onStart() {
    EventBus.getDefault().register(this)
    super.onStart()
}

当我们需要在Activity或Fragment里订阅事件的时候，我们需要注册EventBus。我们一般选择在onStart()里去注册。onStop()里去解除。

解除事件

override fun onStop() {
    EventBus.getDefault().unregister(this)
    super.onStop()
}

解除事件一定需要，为了防止内存泄漏。

发送事件

EventBus.getDefault().post("发送EventBus事件，任何类型都可以。")

处理事件

@Subscribe(threadMode = ThreadMode.MAIN)
fun MessageEventPost(message: String) {
    //todo
}

EventBus注解参数

注解代码

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Subscribe {
    ThreadMode threadMode() default ThreadMode.POSTING;
    boolean sticky() default false;
    int priority() default 0;
}

threadMode()：运行在那个线程，枚举类型。(POSTING,MAIN,MAIN_ORDERED,BACKGROUND,ASYNC)
sticky():是否是粘性事件。

所谓粘性事件，就是在发送事件之后再订阅该事件也能收到该事件。

priority():优先级（0-100）

EventBus源码分析

EventBus注册代码

  public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        //通过subscriberMethodFinder 来找到订阅者订阅了哪些事件.返回一个 SubscriberMethod 对象的 List, SubscriberMethod里包含了这个方法的 Method 对象,
        //以及将来响应订阅是在哪个线程的 ThreadMode ,以及订阅的事件类型 eventType ,以及订阅的优先级 priority ,
        //以及是否接收粘性 sticky 事件的 boolean 值，其实就是解析这个类上的所有 Subscriber 注解方法属性。
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }
       
    List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        //先在缓存中读取
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        //ignoreGeneratedIndex 默认值为false，findUsingInfo(Class<?> subscriberClass)
        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 {
            //订阅者Class存入缓存
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }
    
    
    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        //获取FindState()
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            //findState.subscriberInf == null is true
            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 {
                //核心代码
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }
    
     private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // 通过发射获取订阅类的所有方法
            methods = findState.clazz.getDeclaredMethods();
        } catch (Throwable th) {
            methods = findState.clazz.getMethods();
            findState.skipSuperClasses = true;
        }
        for (Method method : methods) {
            //获取类的修饰符
            int modifiers = method.getModifiers();
            //找到所有声明为public的方法
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                //获取参数的class
                Class<?>[] parameterTypes = method.getParameterTypes();
                //只可以包含一个参数
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        //获取事件的Class，也就是方法参数的Class
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            // 获取 ThreadMode
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            // 往集合里面添加 SubscriberMethod ，解析方法注解所有的属性
                            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");
            }
        }
    }

执行完subscriberMethodFinder.findSubscriberMethods(subscriberClass);会通过类对象的 class 去解析这个类中的所有 Subscribe 注解方法的所有属性值，一个注解方法对应一个 SubscriberMethod 对象，包括 threadMode，priority，sticky，eventType，method。效果如下图:

image.png

接下来看subscribe这个方法，该方法把subscriber，SubscriberMethod分别存好，存入如下两个集合：

//key 是 Event 参数的类 -> 上面的String.class
//value 是存放Subscription的集合 Subscription里面包含两个类 一个是subscriber订阅者（反射执行对象），一个是SubscriberMethod     注解方法的所有属性参数值
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;

// key 是所有的订阅者
// value 是所有订阅者里面方法的参数的 class，eventType
private final Map<Object, List<Class<?>>> typesBySubscriber;

subscribe方法如下：

    
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        //subscriber MainActivity eventType = String
        Class<?> eventType = subscriberMethod.eventType;
        //将MainActivity 和 SubscriberMethod 再次封装
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        //获取订阅了此事件类的所有订阅者列表
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            //线程安全的ArrayList
            subscriptions = new CopyOnWriteArrayList<>();
            //添加
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        int size = subscriptions.size();
        //处理优先级
        for (int i = 0; i <= size; i++) {
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        //通过subscriber 获取 List<Class<?>>
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        //将事件类加入 订阅者事件列表中
        subscribedEvents.add(eventType);
        //粘性处理事件
        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);
            }
        }
    }

subscriptionsByEventType 集合架构图如下：

image.png

EventBus post代码

遍历刚刚的map，拿到map的内容，通过反射进行方法的调用

    public void post(Object event) {
        //event："发送EventBus事件，任何类型都可以。"
        //currentPostingThreadState 是一个 ThreadLocal
        PostingThreadState postingState = currentPostingThreadState.get();
        //获取到线程独有的变量
        List<Object> eventQueue = postingState.eventQueue;
        //将事件加入队列
        eventQueue.add(event);
        
        if (!postingState.isPosting) {
            //判断是否是主线程
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
                //循环遍历队列，发送单个事件调用postSingleEvent
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }
    
    
      private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        //获取事件的Class String.class
        Class<?> eventClass = event.getClass();
        //是否找到订阅者
        boolean subscriptionFound = false;
        // 是否支持事件继承，默认true
        if (eventInheritance) {
            //查找eventClass 所有的父类和接口
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                //依次向eventClass的父类或接口的订阅方法发送事件
                Class<?> clazz = eventTypes.get(h);
                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
            }
        } else {
            subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
        }
        if (!subscriptionFound) {
            if (logNoSubscriberMessages) {
                logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }
    
    
        private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            //得到Subscription列表
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            //遍历 subscriptions
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    //发送事件
                    postToSubscription(subscription, event, postingState.isMainThread);
                    //是否被取消了
                    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) {
        // 根据不同的线程模式执行对应
        switch (subscription.subscriberMethod.threadMode) {
            //和发送事件处于同一线程
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            //主线程
            case MAIN:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            //子线程
            case MAIN_ORDERED:
                if (mainThreadPoster != null) {
                    mainThreadPoster.enqueue(subscription, event);
                } else {
                    // temporary: technically not correct as poster not decoupled from subscriber
                    invokeSubscriber(subscription, event);
                }
                break;
            //和发送事件处于不同线程
            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);
        }
    }