mixin_with_args.rb

class Object
  def self.include_with_args(base, *args)
    base.included_with_args(self, *args) if base.respond_to?(:included_with_args)
    include(base)
  end
end

describe 'mixins with args' do
  it 'calls hook method with args' do
    module AMixin end

    class AClassUsingMixinWithArgs; end # make class available

    AMixin.should_receive(:included_with_args).with(AClassUsingMixinWithArgs, 1, 2, 3)

    class AClassUsingMixinWithArgs
      include_with_args AMixin, 1, 2, 3
    end
  end

  it 'adds methods from mixin' do
    module MixinWithMethods
      def a_method
      end
    end

    class AClassWithMixedInBehavior
      include_with_args MixinWithMethods
    end

    AClassWithMixedInBehavior.new.should respond_to(:a_method)
  end
end

value_object_example.rb

require 'core_ext/mixin_with_args'

module DDD
  module ValueObject
    def self.included_with_args(base, *equality_attributes)
      # convert to strings now to prevent excessive casting on equality comparison
      names_as_strings = equality_attributes.collect { |n| "@#{n}" }
      base.instance_variable_set(:@equality_attributes, names_as_strings)
    end

    def ==(other)
      return false unless other.is_a?(self.class)

      self.class.instance_variable_get(:@equality_attributes).each do |attribute|
        return false if instance_variable_get(attribute) != other.instance_variable_get(attribute)
      end

      true
    end
  end
end

describe DDD::ValueObject do
  it 'adds equality method generated from attributes' do
    class ExampleRange
      include_with_args DDD::ValueObject, :from, :to

      def initialize(from, to)
        @from, @to = from, to
      end
    end

    ExampleRange.new(1, 2).should eq(ExampleRange.new(1, 2))
    ExampleRange.new(1, 2).should_not eq(ExampleRange.new(1, 3))
    ExampleRange.new(1, 2).should_not eq(ExampleRange.new(3, 1))
    ExampleRange.new(1, 2).should_not eq(1)
  end
end

without_args_mixin.rb

# not directly comparable since this is slower.
# just to show the pure ruby implementation of DDD::ValueObject using a "abstract" method equality_attribute enforcement.

module DDD
  module ValueObject
    def ==(other)
      return false unless other.is_a?(self.class)

      equality_attributes.each do |attribute|
        return false if instance_variable_get("@#{attribute}") != other.instance_variable_get("@#{attribute}")
      end

      true
    end

    def equality_attributes
      raise NotImplementedError, "#{self.class} must implement #equality_attributes"
    end
  end
end

describe DDD::ValueObject do
  it 'adds equality method generated from attributes' do
    class ExampleRange
      include DDD::ValueObject

      def initialize(from, to)
        @from, @to = from, to
      end

      def equality_attributes
        [:from, :to]
      end
    end

    ExampleRange.new(1, 2).should eq(ExampleRange.new(1, 2))
    ExampleRange.new(1, 2).should_not eq(ExampleRange.new(1, 3))
    ExampleRange.new(1, 2).should_not eq(ExampleRange.new(3, 1))
    ExampleRange.new(1, 2).should_not eq(1)
  end
end