code: func (el *ChannelEventListener) RegPreparingTransfer() *ChannelEventListener {
  	el.PreparingTransfers = make(chan struct{})
  	el.ids = append(el.ids, el.em.RegisterListener(func(data interface{}) {
  		el.PreparingTransfers <- struct{}{}
  	}, event.EventPreparingTransfer))
  	return el
  }
docs: // RegPreparingTransfer registers this listener to listen to when a transfer is being prepared.