EventBus 源码学习

Posted 2021-01-19 it-worker365

打开一看，原来相关代码并不多，下面看下细节

主要方法也就是注册，取消注册和发送事件，可以看到两个主要的变量就是subscribers和dispatcher

  public void register(Object object) {
    subscribers.register(object);
  }

首先看下注册类中做了什么，在下面的注册类中，可以看到首先调用了这个方法，对传入的listener类型，利用反射拿到信息

  /**
   * Returns all subscribers for the given listener grouped by the type of event they subscribe to.
   */
  private Multimap<Class<?>, Subscriber> findAllSubscribers(Object listener) {
    Multimap<Class<?>, Subscriber> methodsInListener = HashMultimap.create();
    Class<?> clazz = listener.getClass();
    for (Method method : getAnnotatedMethods(clazz)) {
      Class<?>[] parameterTypes = method.getParameterTypes();
      Class<?> eventType = parameterTypes[0];
      methodsInListener.put(eventType, Subscriber.create(bus, listener, method));
    }
    return methodsInListener;
  }

debug下就清楚了

  /** Registers all subscriber methods on the given listener object. */
  void register(Object listener) {
    Multimap<Class<?>, Subscriber> listenerMethods = findAllSubscribers(listener);

    for (Entry<Class<?>, Collection<Subscriber>> entry : listenerMethods.asMap().entrySet()) {
      Class<?> eventType = entry.getKey();
      Collection<Subscriber> eventMethodsInListener = entry.getValue();

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

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

      eventSubscribers.addAll(eventMethodsInListener);
    }
  }

 

注册后这里的订阅者会保存如下信息。

其次是分发

  public void post(Object event) {
    Iterator<Subscriber> eventSubscribers = subscribers.getSubscribers(event);
    if (eventSubscribers.hasNext()) {
      dispatcher.dispatch(event, eventSubscribers);
    } else if (!(event instanceof DeadEvent)) {
      // the event had no subscribers and was not itself a DeadEvent
      post(new DeadEvent(this, event));
    }
  }

可见是先拿注册信息，再分发

    @Override
    void dispatch(Object event, Iterator<Subscriber> subscribers) {
      checkNotNull(event);
      checkNotNull(subscribers);
      Queue<Event> queueForThread = queue.get();
      queueForThread.offer(new Event(event, subscribers));

      if (!dispatching.get()) {
        dispatching.set(true);
        try {
          Event nextEvent;
          while ((nextEvent = queueForThread.poll()) != null) {
            while (nextEvent.subscribers.hasNext()) {
              nextEvent.subscribers.next().dispatchEvent(nextEvent.event);
            }
          }
        } finally {
          dispatching.remove();
          queue.remove();
        }
      }
    }

 这里的dispatch通过反射执行

  /** Dispatches {@code event} to this subscriber using the proper executor. */
  final void dispatchEvent(final Object event) {
    executor.execute(
        new Runnable() {
          @Override
          public void run() {
            try {
              invokeSubscriberMethod(event);
            } catch (InvocationTargetException e) {
              bus.handleSubscriberException(e.getCause(), context(event));
            }
          }
        });
  }

再下级

  /**
   * Invokes the subscriber method. This method can be overridden to make the invocation
   * synchronized.
   */
  @VisibleForTesting
  void invokeSubscriberMethod(Object event) throws InvocationTargetException {
    try {
      method.invoke(target, checkNotNull(event));
    } catch (IllegalArgumentException e) {
      throw new Error("Method rejected target/argument: " + event, e);
    } catch (IllegalAccessException e) {
      throw new Error("Method became inaccessible: " + event, e);
    } catch (InvocationTargetException e) {
      if (e.getCause() instanceof Error) {
        throw (Error) e.getCause();
      }
      throw e;
    }
  }

至此结束调用，这种设计还蛮特别的，值得学习