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EventBus 3.1.1 自己对源码的理解

EventBus 3.1.1 自己对源码的理解

作者: Serven_ | 来源:发表于2019-07-19 15:18 被阅读0次

    一:使用步骤

    1、eventbus3.1.1版本依赖:

       implementation 'org.greenrobot:eventbus:3.1.1'
    

    2、在 Activity 或 Fragment 中进行注册:

      EventBus.getDefault().register(this);
    

    3、在注册的 Activity 或 Fragment 中进行接收,需要写一个接收方法:

    3.1 、注解上使用默认参数接收方法

      @Subscribe
      public void receive(String data) {
        // TODO: Create by Ysw 2019/7/18 进行相关 data 的数据处理
      }
    

    3.2、注解上进行自定义参数值接收方法

      @Subscribe(threadMode = ThreadMode.MAIN, sticky = true, priority = 1)
      public void receive(Object data) {
         // TODO: Create by Ysw 2019/7/18 进行相关 data 的数据处理
      }
    

    4、在注册 EventBus 的 Activity 或 Fragment 的 onDestroy 生命周期方法中进行注销

      @Override
      protected void onDestroy() {
           super.onDestroy();
           EventBus.getDefault().unregister(this);
      }
    

    5、在需要发出通知的地方进行调用,可以是 Fragment Activity 或者别的工具类中,EventBus 目前不支持跨进程通讯,需保持在同一个进程中进行一个完整的操作。

      private void sendMessage(){
        EventBus.getDefault().post("这个是我发的测试消息");
        EventBus.getDefault().post(new Friend("Ysw",18));
      }
    

    二:我们从注册开始着手解析。

    我们进入 EventBus 的 register() 方法中,看看其做了什么事情

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

    register 方法中做了三件事,第一通过反射拿到 class 对象,二,通过这个 class 对象去遍历这个类里面的方法并返回一个呗 @Subscribe 所注释了的方法集合。三,对这个方法集合进行遍历并做相应的操作。

    我们来看他是怎么进行类里面方法遍历并返回相关方法集合的,我们进入到 subscriberMethodFinder 中的subscriberMethodFinder() 方法:

         // 这是一个 Map 容器,key 值是一个 class 类,value 是一个方法集合
        private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
    
        List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        // 第一次进来,容器肯定为空,找不到相关的 方法集合
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        // ignoreGeneratedIndex 默认值是 false
        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;
        }
    }
    

    进入 findUsingInfo(Class<?> subscriberClass) 方法:

       private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            // findState.subscriberInfo 没有进行赋值,进入 findUsingReflectionInSingleClass(findState) 方法
            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);
    }
    

    findUsingReflectionInSingleClass(findState) 方法

       private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            // 通过反射 获取这个类中的所有方法集合
            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;
        }
        // 遍历这个集合,获取被 Subscribe 所注释的方法并添加到 subscriberMethods 中
        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");
            }
        }
    }
    

    getMethodsAndRelease(FindState findState) 方法 直接将 findState.subscriberMethods 方法集合返回

      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;
    }
    

    最后将 subscriberMethods 添加到 METHOD_CACHE map容器中并返回

       List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
           //   代码省略
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
          //   代码省略
    }
    

    回到 EventBus 的 subscribe(subscriber, subscriberMethod) 方法

         private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
              Class<?> eventType = subscriberMethod.eventType;
              Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
              CopyOnWriteArrayList<Subscription> 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);
                     }
              }
    
              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;
                  }
              }
    
              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);
                  }
              }
          }
    

    在subscriptionsByEventType去查找一个CopyOnWriteArrayList<Subscription> ,如果没有则创建一个新的CopyOnWriteArrayList;
    然后将这个CopyOnWriteArrayList放入subscriptionsByEventType中,这里的subscriptionsByEventType是一个Map,key为eventType,value为CopyOnWriteArrayList<Subscription>,这个Map非常重要,后续还会用到它;
    接下来,就是添加newSubscription,它属于Subscription类,里面包含着subscriber和subscriberMethod等信息,同时这里有一个优先级的判断,说明它是按照优先级添加的。优先级越高,会插到在当前List靠前面的位置;
    typesBySubscriber这个类也是一个Map,key为subscriber,value为subscribedEvents,即所有的eventType列表,这个类我找了一下,发现在EventBus#isRegister()方法中有用到,应该是用来判断这个Subscriber是否已被注册过。然后将当前的eventType添加到subscribedEvents中;
    最后,判断是否是sticky。如果是sticky事件的话,到最后会调用checkPostStickyEventToSubscription()方法。

      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, isMainThread());
        }
    }
    

    接着看是如何让发送消息的,post() 方法

      public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);
    
        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }
    

    进入 postSingleEvent(eventQueue.remove(0), postingState)方法

       private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                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));
            }
        }
    }
    

    进入 postSingleEventForEventType()方法

       private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            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;
    }
    

    最后调用 postToSubscription() 方法

      private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            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;
            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);
        }
    }
    

    最后根据线程类别进行调用
    POSTING:执行invokeSubscriber()方法,就是直接反射调用
    MAIN:首先去判断当前是否在UI线程,如果是的话则直接反射调用,否则调用mainThreadPoster#enqueue(),即把当前的方法加入到队列之中,然后通过handler去发送一个消息,在handler的handleMessage中去执行方法。具体逻辑在HandlerPoster.java中;
    MAIN_ORDERED:与上面逻辑类似,顺序执行我们的方法。
    BACKGROUND:判断当前是否在UI线程,如果不是的话直接反射调用,是的话通过。backgroundPoster.enqueue()将方法加入到后台的一个队列,最后通过线程池去执行。
    ASYNC:与BACKGROUND的逻辑类似,将任务加入到后台的一个队列,最终由Eventbus中的一个线程池去调用,这里的线程池与BACKGROUND逻辑中的线程池用的是同一个
    有参考这位大神的分析并结合自己的理解,给自己做个笔记,感谢大神的分享 EventBus 3.1.1 源码解析

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