code: func (a *Adaptor) On(event string, f func(msg Message)) bool {
  	if a.client == nil {
  		return false
  	}
  	a.client.Subscribe(event, 0, func(client paho.Client, msg paho.Message) {
  		f(msg)
  	})
  	return true
  }
docs: // On subscribes to a topic, and then calls the message handler function when data is received