Class to allow the creation of Rails route helpers for external applications without interfering with the application routes

shareable_route_set.rb

class ShareableRouteSet
  include ActionController::UrlWriter

  def initialize
    @route_set = ActionController::Routing::RouteSet.new
    @map = ActionController::Routing::RouteSet::Mapper.new(@route_set)

    self.draw(@map)
    self.install_public_helpers
  end

  # Sub-classes should override this to create their route mappings.
  # Ex.
  # map.resources :cats
  # map.resources :dogs
  def draw(map)
    raise RuntimeError.new('Create a sub-class of ShareableRouteSet and define the draw(map) method.')
  end

  # Loads the routes into the Rails environment. Defining your routes once in
  # a ShareableRouteSet, then loading them into your environment allows you to
  # easily package the ShareableRouteSet into a gem to be used elsewhere.
  def load_into_environment
    load_into_route_set(ActionController::Routing::Routes)
  end

  # Loads the routes into the supplied route set.
  def load_into_route_set(route_set)
    route_set.draw { |map| self.draw(map) }
  end

  # Returns the URL for the given options. This delegates to Rails' UrlWriter.
  def url_for_with_our_routes(options)
    # UrlRewriter is hard-coded to use ActionController::Routing::Routes,
    # so we're temporarily replacing the instance with our route set.
    # This is a bit of a crappy hack, but it's arguably better than copying
    # the code in UrlRewriter.

    original_routes = ActionController::Routing::Routes
    silence_warnings { ActionController::Routing.const_set("Routes", @route_set) }

    return url_for_without_our_routes(options)
  ensure
    silence_warnings { ActionController::Routing.const_set("Routes", original_routes) }
  end
  alias_method_chain :url_for, :our_routes

  protected

  def install_public_helpers
    # Install all the generated helpers
    @route_set.install_helpers(self.class)

    # Rails makes the helpers protected, but we want them public
    # in this context.

    # Get the list of protected methods
    protected_methods = self.protected_methods.collect(&:to_sym)

    # Remove :install_public_helpers since it should stay protected
    protected_methods.reject! { |sym| sym == :install_public_helpers }

    # Make it so.
    self.class.send(:public, *protected_methods)
  end

  class << self
    # Override default_url_options to be specific to this set of routes.
    def default_url_options(options = nil)
      @default_url_options = options unless options.nil?
      @default_url_options
    end
  end
end

# class CatRoutes < ShareableRouteSet
#   default_url_options(:host => 'cats.example.com')
# 
#   def draw(map)
#     map.resources :cats
#   end
# end
# 
# class DogRoutes < ShareableRouteSet
#   default_url_options(:host => 'dogs.example.com')
# 
#   def draw(map)
#     map.resources :dogs
#   end
# end; nil
# 
# CatRoutes.new.cats_url
# DogRoutes.new.dogs_url