DCI via SimpleDelegator and a hint of metaprogramming

gazebo.rb

get '/give/me/gazebo/not/gazelle/:min_size' do

  pricing = PriceCalculator.new

  ParamsExtractor.new(pricing).parse(params)
  GazelleFinder.new(pricing).find_all_gazelles
  GazelleFilter.new(pricing).remove_sick_gazelles       # pricing has access to #gazelles

  GazeboBuilder.new(pricing).calculate_size_of_gazebo
  GazeboPricer.new(pricing).calculate_final_price       # pricing no longer has #gazelles

  pricing.response.to_xml
end

class PricingCalculator

  attr_accessor :minimum_size     # sure, not a great domain example

end


class ParamsExtractor < SimpleDelegator

  def initialize(obj)
    super
  end

  # sometimes you just want to work with your data model
  def parse(params)
    self.minimum_size = params[:min_size]
  end

end
 
class GazelleFinder < SimpleDelegator

  def initialize(obj)
    super
    @obj = obj
  end

  # sometimes you want to generate "temporary" data for next steps
  def find_all_gazelles(params)
    (class << @obj; self; end).instance_eval do
      define_method(:gazelles) do
        [1,2,3]
      end
    end
  end

end

# But by the time we call pricing.response, we want to undef #gazelles,
# because it was just a temporary mutation of our PricingTransaction
# state, and it should not be available after we calculated the price.

forgive my metaprogramming fu if I'm missing something, but isn't

(class << @obj; self; end).instance_eval do
  define_method(:gazelles) do
    [1,2,3]
  end
end

the same as

def @obj.gazelles do
  [1,2,3]
end