code: func (bus *EventBus) HasCallback(topic string) bool {
  	bus.lock.Lock()
  	defer bus.lock.Unlock()
  	_, ok := bus.handlers[topic]
  	if ok {
  		return len(bus.handlers[topic]) > 0
  	}
  	return false
  }
docs: // HasCallback returns true if exists any callback subscribed to the topic.