Service locator with Rails Initializer for rails service loader that loads the services from a config file.

listings_controller.rb

class ListingsController < ApplicationController

  def index

    movie_listing_service = ServiceLocator.get_service_instance(:MovieListing)

    @available_movies = movie_listing_service.available_movies
    
    render nothing: true #this is just an example don't get hung up on it :)
  end
end

listings_controller_spec.rb

#An example rails controller spec using this. Note that in your spec helper you could just have a before(:each) to reset the locator.

require "spec_helper"

describe ListingsController do

  describe "GET #index" do
    it "lists all the movies." do
      gold_finger = mock("Goldfinger: an Awesome bond movie")
      ferris_buellers_day_off = mock("Ferris Bueller's Day off")
      fake_movie_listing_provider_service = mock("Fake Movie Listing Provider Service", available_movies: [gold_finger, ferris_buellers_day_off])

       locator = ServiceLocator.instance
       locator.register(:MovieListingProvider, fake_movie_listing_provider_service)
       ServiceLocator.load_locator(locator)

      get :index

      assigns(:available_movies).should == [gold_finger, ferris_buellers_day_off]
    end
  end
end

service_locator.rb

class ServiceLocator
  include Singleton

  def self.reset_instance
    self.instance_variable_set("@singleton__instance__", self.send(:new))
  end

  def initialize
    @services = {}
  end

  #Do this in your setup file
  def register(service_name, instance)
    @services[service_name] = instance
  end

  def get_service_instance(service_name)
    service_instance = @services[service_name]
    raise NoSuchServiceInstanceException if service_instance.nil?

    service_instance
  end

  class NoSuchServiceInstanceException < Exception; end
end

service_locator_initializer.rb

require "active_support/inflector"
#Require any gems that contain services that you'd like to use here
#eg: require "spacebook_api_services" 
require "metflix"

require_relative File.join(Rails.root,"app/services/metflix_movie_listing_service") #Normally this would probably live in a gem!
services = YAML.load_file(File.join(File.expand_path( "..", File.dirname(__FILE__)), "services.yml"))

services.each do |service_name, service_info|
  service_instance = service_info["class"].constantize.new(*service_args)

  ServiceLocator.register(service_name.to_sym, service_instance)
endrequire "active_support/inflector"

require_relative File.join(Rails.root,"app/services/metflix_movie_listing_service") #Normally this would probably live in a gem!
services = YAML.load_file(File.join(File.expand_path( "..", File.dirname(__FILE__)), "services.yml"))

services.each do |service_name, service_info|
  service_instance = service_info["class"].constantize.new(*service_args)

  ServiceLocator.register(service_name.to_sym, service_instance)
end

service_locator_spec.rb

require "spec_helper" #Could easily live in its own gem and be run/developed outside of rails but I was lazy ;)

describe ServiceLocator do

  describe ".instance" do
    it "returns the only locator instance for the Service Locator Singleton" do
        ServiceLocator.instance.should == ServiceLocator.instance
    end
  end

  describe ".reset_instance" do
    it "sets the locator singleton's only instance to a new locator" do
      original_instance = ServiceLocator.instance

      ServiceLocator.reset_instance

      ServiceLocator.instance.should_not == original_instance
    end
  end

  describe ".get_service_instance" do
    it "should return a properly constructed instance for the service named by the given symbol" do
      locator = ServiceLocator.instance #Service locator instance from singleton ;)
      charlie_horse_service = mock("A properly constructed charlie horse service instance")
      locator.register(:CharlieHorseService, charlie_horse_service)

      ServiceLocator.instance.get_service_instance(:CharlieHorseService).should == charlie_horse_service
    end


    it "should throw a NoSuchServiceInstance exception if there is no such instance registered with the locator" do
      expect { ServiceLocator.instance.get_service_instance(:CrazyDataService) }.to raise_exception ServiceLocator::NoSuchServiceInstanceException
    end
  end

end

services.yml

MovieListing:
  class: MetFlixMovieListingService
  args:
    - abc_123 #api_key
    - false #sandbox_mode