wisper_listener_framework.rb

def create
  checkout = CheckoutOrder.new current_user, params
  checkout.subscribe(AnalyticsListener.new)
  checkout.on(:checkout_success) do |order|
    respond_with ...
  end
  checkout.on(:checkout_failed_for_one_reason) do |order|
    do_a_thing...
  end
  checkout.on(:checkout_failed_for_another_reason) do |order|
    do_a_thing...
  end

  checkout.do_checkout
end

class AnalyticsListener
  def checkout_success
    # notify google analytics / kissmetrics
  end
end