class-registry.rb

module RoadForest
  module Utility
    class ClassRegistry
      #Extend a module with this in order to make it the registrar for a
      #particular purpose.
      #The top of a class heirarchy will make "register" immediately available
      #to subclasses. Otherwise, classes can say Module::registry.add(name,
      #self)
      #
      module Registrar
        def registry
          @registry ||= ClassRegistry.new(self)
        end

        def register(name)
          registrar.registry.add(name, self)
        end

        def get(name)
          registrar.registry.get(name)
        end
        alias [] get

        def all_names
          registrar.registry.names
        end

        def map_classes(&block)
          registrar.map_classes(&block)
        end

        def self.extended(mod)
          (
            class << mod; self; end
          ).instance_exec(mod) do |mod|
            define_method :registrar do
              mod
            end
          end
        end
      end

      def initialize(registrar, purpose=nil)
        if purpose.nil?
          if registrar.respond_to?(:registry_purpose)
            @purpose = registrar.registry_purpose
          else
            @purpose = registrar.name
          end
        else
          @purpose = purpose
        end
        @classes = {}
      end

      def map_classes
        names.map do |name|
          yield get(name)
        end
      end

      def add(name, klass)
        @classes[name.to_sym] = klass
        @classes[name.to_s] = klass
      end

      def names
        @classes.keys.select{|key| key.is_a? Symbol}
      end

      def get(name)
        @classes.fetch(name)
      rescue KeyError
        raise "No #@purpose class registered as name: #{name.inspect} (there are: #{names.inspect})"
      end
    end
  end
end