基於版本：Guava 22.0

Wiki：EventBus

0. EventBus簡介

提供了發布-訂閱模型，可以方便的在EventBus上註冊訂閱者，發布者可以簡單的將事件傳遞給EventBus，EventBus會自動將事件傳遞給相關聯的訂閱者。

支持同步/異步模式。

只能用於線程間通信。

1. EventBus類圖

EventBus是同步實現

AsyncEventBus是異步實現

2. 代碼實例

        EventBus eventBus = new EventBus();
        
        eventBus.register(new Object() {
            @Subscribe
            
 
public void listen(Object subReport) throws Exception {
                System.out.println("receive object event!");
            }

            @Subscribe
            public void listen(Integer subReport) throws Exception {
                System.out.println("receive integer event!");
            }
        });

        eventBus.post(Integer.valueOf(
 
1));

這段代碼的輸出如下：

receive integer event!
receive object event!

3. EventBus.register()

EventBus.register
  /**
   * Registers all subscriber methods on {@code object} to receive events.
   *
   * @param object object whose subscriber methods should be registered.
   */
  public void register(Object object) {
    subscribers.register(object);
  }


SubscriberRegistry.register
  
 
/**
   * Registers all subscriber methods on the given listener object.
   */
  void register(Object listener) {
    Multimap<Class<?>, Subscriber> listenerMethods = findAllSubscribers(listener);//獲取傳入的listener中含有的所有的監聽者

    for (Map.Entry<Class<?>, Collection<Subscriber>> entry : listenerMethods.asMap().entrySet()) {
      Class<?> eventType = entry.getKey();//監聽的事件類型
      Collection<Subscriber> eventMethodsInListener = entry.getValue();//對應的監聽器本身

      CopyOnWriteArraySet<Subscriber> eventSubscribers = subscribers.get(eventType);

      if (eventSubscribers == null) {
        CopyOnWriteArraySet<Subscriber> newSet = new CopyOnWriteArraySet<Subscriber>();
        eventSubscribers =
            MoreObjects.firstNonNull(subscribers.putIfAbsent(eventType, newSet), newSet);
      }

      eventSubscribers.addAll(eventMethodsInListener);
    }
  }


  /**
   * Returns all subscribers for the given listener grouped by the type of event they subscribe to.
    分析傳入的對象，遍歷其中所有含有Subscribe註解而且只含有一個參數的方法，然後將其包裝成監聽者並返回。
   */
  private Multimap<Class<?>, Subscriber> findAllSubscribers(Object listener) {
    Multimap<Class<?>, Subscriber> methodsInListener = HashMultimap.create();
    Class<?> clazz = listener.getClass();
    for (Method method : getAnnotatedMethods(clazz)) {//getAnnotatedMethods方法最終會調用到下面的SubscriberRegistry.getAnnotatedMethodsNotCached方法，這個方法會用反射處理傳入的clazz及其所有的父類，提取出含有Subscribe註解並且有且只有一個參數的方法
      Class<?>[] parameterTypes = method.getParameterTypes();
      Class<?> eventType = parameterTypes[0];//獲取這個方法的唯一參數的Class
      methodsInListener.put(eventType, Subscriber.create(bus, listener, method));//將EventBus，監聽者對象，監聽者方法包裝一下並放入map，後續觸發事件的時候會用到
    }
    return methodsInListener;
  }

SubscriberRegistry.getAnnotatedMethodsNotCached
  private static ImmutableList<Method> getAnnotatedMethodsNotCached(Class<?> clazz) {
    Set<? extends Class<?>> supertypes = TypeToken.of(clazz).getTypes().rawTypes();//連clazz的父類也會處理
    Map<MethodIdentifier, Method> identifiers = Maps.newHashMap();
    for (Class<?> supertype : supertypes) {
      for (Method method : supertype.getDeclaredMethods()) {
        if (method.isAnnotationPresent(Subscribe.class) && !method.isSynthetic()) {//含有Subscribe註解，而且不是合成的方法
          // TODO(cgdecker): Should check for a generic parameter type and error out
          Class<?>[] parameterTypes = method.getParameterTypes();
          checkArgument(
              parameterTypes.length == 1,
              "Method %s has @Subscribe annotation but has %s parameters."
                  + "Subscriber methods must have exactly 1 parameter.",
              method,
              parameterTypes.length);//方法必須有且只有一個參數

          MethodIdentifier ident = new MethodIdentifier(method);
          if (!identifiers.containsKey(ident)) {
            identifiers.put(ident, method);
          }
        }
      }
    }
    return ImmutableList.copyOf(identifiers.values());
  }

Guava源碼學習（五）EventBus