multi_role_registry.rb

# Referenced in Design Is Refactoring post http://designisrefactoring.com/2016/02/03/factories/
# This code takes the previous example and extends it. Classes can now define 'roles' they satisfy
# and each class can satisfy more than one role.
#
# At load time, these roles are examined and recorded in registries. 
# In this example there's a CampusLikeRegistry that contains every class that says it's campus-like.
# But you could have any number of registries.

require 'singleton'
require 'forwardable'
require 'set'

class CampusLikeRegistry
  include Enumerable
  include Singleton
  extend Forwardable

  def_delegators :collection, :each, :detect

  def register(c)
    collection << c
  end

  private

  def collection
    @collection ||= Set.new
  end
end

class RegistryPopulator
  def self.register(to_register, roles)
    roles.each do |role|
      const_get("#{role}Registry".to_sym).instance.register(to_register)
    end
  end
end

class UMNTC
  def self.roles
    [CampusLike]
  end

  def self.handles?(abbreviation)
    abbreviation = "UMNTC"
  end

  def name 
    "University of Minnesota Twin Cities"
  def

  def mascot
    "Gopher"
  end
end

class UMNMO
  def self.roles
    [CampusLike]
  end

  def self.handles?(abbreviation)
    abbreviation = "UMNMO"
  end

  def name 
    "University of Minnesota Morris"
  def

  def mascot
    "Cougar"
  end
end

class UMNCR
  def self.roles
    [CampusLike]
  end

  def self.handles?(abbreviation)
    abbreviation = "UMNCR"
  end

  def name 
    "University of Minnesota Crookston"
  def

  def mascot
    "Golden Eagle"
  end
end

class UMNTCRO
  def self.roles
    [CampusLike]
  end

  def self.handles?(abbreviation)
    abbreviation = "UMNTCRO"
  end

  def name 
    "University of Minnesota Rochester"
  def

  def mascot
    "Raptor"
  end
end

class CollegeInTheSchools
  def self.roles
    [CampusLike]
  end

  def self.handles?(abbreviation)
    abbreviation = "CITS"
  end

  def name
    "College in the Schools"
  end

  def mascot
    ""
  end
end

class UnknownCampus
  def name 
    "Unknown Campus"
  def

  def mascot
    "Unknown Mascot"
  end
end

class CampusDetails
  def campus_name(abbreviation)
    build_campus(abbreviation).name
  end

  def campus_mascot(abbreviation)
    build_campus(abbreviation).mascot
  end

  private

  def build_campus(abbreviation)
    CampusLikeRegistry.detect(->{UnknownCampus.new}) { |campus_class| campus_class.handles?(abbreviation)}.new
  end
end

ObjectSpace.each_object(Class).select { |klass| klass.respond_to?(:roles) }.each do |role_provider|
  RegistryPopulator.register(role_provider, role_provider.roles)
end