Overriding def_delegator

In the project I work on, I make heavy use of the def_delegator method (yeah composition). One of the intrinsic problems of the delegation, is that my tests can get rather convoluted (and tightly coupled to implementation). I'm sure I could look into mock methods, but I needed an immediate solution.

Enter the ConditionallyOverrideDelegation module. This is a rails specific solution as it makes use of alias_method_chain.

module ConditionallyOverrideDelegation
  def self.included(base)
    base.extend(Macro)
  end
  module Macro

    # class Foo
    #   extend Forwardable
    #   def_delegator :bar, :name
    #   conditionally_override_delegation :name
    # end
    # class Bar
    #   def name
    #     'Bar'
    #   end
    # end
    # 
    # @foo = Foo.new
    # @foo.bar.name == @foo.name
    # => true
    #
    # @foo.name_with_delegation_override = @foo.bar.name * 2
    # @foo.bar.name == @foo.name
    # => false
    #
    # Take a look at the tests to see how this truly behaves
    #
    # Really you probably only want to use this method in test
    #
    def conditionally_override_delegation(*targets)
      targets.flatten.each do |target|
        aliased_target, punctuation = target.to_s.sub(/([?!=])$/, ''), $1
        src = <<-EOV
        def #{aliased_target}_with_delegation_override#{punctuation}(*args)
          if @#{aliased_target}_with_delegation_override.nil?
            #{aliased_target}_without_delegation_override#{punctuation}(*args)
          else
            @#{aliased_target}_with_delegation_override
          end
        end
        EOV
        module_eval src, __FILE__, __LINE__
        alias_method_chain target, :delegation_override
        attr_writer "#{aliased_target}_with_delegation_override"
      end
    end

  end
end

And the obligatory tests

require File.dirname(__FILE__) + '/../test_helper'
require 'conditionally_override_delegation'
class ConditionallyOverrideDelegationTest < Test::Unit::TestCase
  class AbstractUnit
    extend Forwardable
    include ConditionallyOverrideDelegation
    
    def_delegators :related_object, :to_s, :string=, :string, :string?
    
    conditionally_override_delegation :to_s, :string
    
    def related_object
      @related_object ||= RelatedObject.new
    end
  end
  
  class RelatedObject
    def to_s
      'Hello World'
    end
    attr_accessor :string
    def string?
      !!string
    end
  end
  
  def setup
    @unit = AbstractUnit.new
  end
  
  def test_delegation_is_working
    assert_equal @unit.related_object.to_s, @unit.to_s
  end

  def test_overridding_delegation_is_working
    @unit.to_s_with_delegation_override = 'Turkey Sandwich'
    assert_equal 'Turkey Sandwich', @unit.to_s
  end

  def test_overridding_delegation_is_working_when_override_is_false
    @unit.to_s_with_delegation_override = false
    assert_equal false, @unit.to_s
  end

  def test_overridding_delegation_is_working_when_override_is_nil
    @unit.to_s_with_delegation_override = nil
    assert_equal @unit.related_object.to_s, @unit.to_s
  end

  def test_overridding_delegation_is_working_with_punctuation
    @unit.related_object.string = 'Troy Mclure'
    assert_equal 'Troy Mclure', @unit.string
    
    @unit.string_with_delegation_override = 'Pancake'
    assert_equal 'Pancake', @unit.string
  end
end