1-event_subscribers_initializer_or_global_config.rb

bus = On::Bus.new.
  subscribe(DonationReceipt).
  subscribe(NotifyAccountingSystem).
  subscribe(UpdatePublicStats).
  subscribe(LogInternalMetrics).
  subscribe(UpdateAnalyticsDashboard).
  subscribe(RealtimeViewerUpdate)

2-a-controller-like-thing.rb

class DonationsController # < ActionController::Base
  def create
    listener = bus.
      on(:donation_created) {|donation| send_user_to_payment_gateway(donation) }.
      on(:donation_invalid) {|donation| render donation.errors, status: :bad_request }

    # or

    listener = bus.on(
      donation_created: ->(donation) { send_user_to_payment_gateway(donation) },
      donation_invalid: ->(donation) { render donation.errors, status: :bad_request }
    )

    DonationService.new.add_donation(current_user, params[:donation], listener)
  end

  private
  def send_user_to_payment_gateway(donation)
    #...
  end
end

3-give_donation.rb

class DonationService
  def initialize(options={})
    @donation_class = options.fetch(:donation_class) { Donation }    
  end

  def add_donation(donor, donation_attributes, listener)
    donation = @donation_class.new(donation_attributes.merge(donor: donor))
    if donation.valid?
      donation.save!
      listener.donation_created(donation)
    else
      listener.donation_invalid(donation)
    end
  end
end

4-donation_receipt.rb

class DonationReceipt
  def donation_created(donation)
    Mailer.delay.send_donation_receipt(donation)
  end
end

5-notify_accounting_system.rb

class NotifyAccountingSystem
  def donation_created(donation)
    #..
  end
end