user_creator.rb

class UserCreator
  def initialize(listener)
    @listener = listener
  end
  
  def create(attributes)
    user = User.new(attributes)
    
    if user.save
      # send email
      # do things
      @listener.creation_successful
    else
      @listener.creation_unsuccessful(user)
    end
  end
end

users_controller.rb

class UsersController < ApplicationController
  def create
    user_creater = UserCreator.new(self)
    user_creater.create(user_params)
  end
  
  def creation_successful
    redirect_to users_url, notice: 'Yay.'
  end
  
  def creation_unsuccessful(user)
    @user = user
    render :new
  end
  
  hide_action :creation_successful, :creation_unsuccessful
  
  private
  
  def user_params
    params.require(:user).permit(:foo, :bar)
  end
end