registerable.rb

#Keeps a list of registered names associated with classes at a "base" class
#Example:
#
#  class Base
#    extend Registerable
#  end
#
#  class Foo < Base
#    register :foo
#  end
#
#  class Bar < Base
#    register :bar
#  end
#
#  Base.registered == {'foo' => Foo, 'bar' => Bar}
#  Base.class_for(:foo) == Base::Foo
#  Base::Foo.identifier == :foo
module Registerable


  class NotRegisteredError < StandardError
  end


  #Create a registry on the class to which this module has been extended to contain
  #a map of names to classes inheriting from it.
  def self.extended(klass)
    #All registered classes inheriting from klass are stores in a class
    #instance variable on klass.
    class << klass; def registered; @registered ||= {}; end; end
    #Save the class using a closure, defining a singleton method on klass.
    #User define_method to get the closure (block), but define_method defines
    #instance methods on the receiver, so we need to get klass's metaclass first.
    (class << klass;self;end).send(:define_method, :base){ klass }
  end


  def register(identifier)
    base.registered[identifier.to_s] = self
  end


  def class_for(identifier)
    base.registered[identifier.to_s] || raise(NotRegisteredError, "Couldn't find class registered with \"#{identifier}\"")
  end


  def create(identifier, *a, &b)
    class_for(identifier).new(*a, &b)
  end


end