context.rb

class Context
  def initialize(*listeners)
    @listeners = listeners
  end  
                                                                                                                                                                                  
  def signal(method, *args)
    @listeners.each do |listener|
      listener.send method, *args
    end  
  end  
end

registration.rb

class Registration < Context                                                                                                                                                    
  def register!(username)
    user = User.new username: username
    if user.save
      signal :registration_success, user 
    else 
      signal :registration_failure, user 
    end
    user
  end  
end

registrations_controller.rb

class Api::V1::RegistrationsController < ApplicationController
  def create
    Registration.new(self).register!(params[:username])
  end  

  def registration_success(user)                                                                                                                                                  
    render json: user 
  end  

  def registration_failure(user)
    render json: {errors: user.errors}
  end  
end