event_bus.go
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
}
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
}
// HasCallback returns true if exists any callback subscribed to the topic.