Specifying default values with Ruby's attr_reader and attr_accessor

njonsson (Nils Jonsson) — Thu, 02 Aug 2007 14:03:03 GMT

Dependency injection in Ruby is as easy as falling off a log. As Jamis Buck has pointed out, DI is a good thing, but DI frameworks for Ruby are overkill. The language makes them unnecessary.

Here's how to enhance attr_reader and attr_accessor so that they can receive an options hash for specifying the default value of an attribute.



module AttrWithDefaultExtension

  

  module ClassMethods

    

    def attr_accessor(*args)

      attrs, attrs_with_defaults = split_for_last_hash(args)

      attrs_with_defaults.each do |name, default|

        attr_reader_with_default name, default

        attr_writer              name

      end

      

      super(*attrs)

    end

    

    def attr_reader(*args)

      attrs, attrs_with_defaults = split_for_last_hash(args)

      attrs_with_defaults.each do |name, default|

        attr_reader_with_default name, default

      end

      

      super(*attrs)

    end

    

  private

    

    def attr_reader_with_default(name, default)

      define_method(name) do

        unless instance_variable_defined?("@#{name}")

          default = default.call(self) if default.kind_of?(Proc)

          instance_variable_set "@#{name}", default

        end

        instance_variable_get "@#{name}"

      end

    end

    

    def split_for_last_hash(args)

      if args.last.kind_of?(Hash)

        [args[0...-1], args.last]

      else

        [args, {}]

      end

    end

    

  end

  

  def self.included(other_module)

    other_module.extend ClassMethods

  end

  

end



class Object; include AttrWithDefaultExtension; end

Here are the unit tests. They demonstrate not only the enhanced behavior of attr_reader and attr_accessor, but also that the standard behavior remains unbroken.



require 'test/unit'



module AttrWithDefaultExtensionTest

  

  class TwoStandardAttrReaders < Test::Unit::TestCase

    

    attr_reader :foo, :baz

    

    def test_should_not_define_first_instance_variable

      assert_equal false, instance_variable_defined?(:@foo)

    end

    

    def test_should_return_first_instance_variable_when_sent_first_attr_reader

      @foo = 'bar'

      assert_equal 'bar', foo

    end

    

    def test_should_not_define_second_instance_variable

      assert_equal false, instance_variable_defined?(:@baz)

    end

    

    def test_should_return_second_instance_variable_when_sent_second_attr_reader

      @baz = 'bat'

      assert_equal 'bat', baz

    end

    

  end

  

  class TwoStandardAttrAccessors < Test::Unit::TestCase

    

    attr_accessor :foo, :baz

    

    def test_should_not_define_first_instance_variable

      assert_equal false, instance_variable_defined?(:@foo)

    end

    

    def test_should_return_first_instance_variable_when_sent_first_attr_reader

      @foo = 'bar'

      assert_equal 'bar', foo

    end

    

    def test_should_set_first_instance_variable_when_sent_first_attr_writer

      self.foo = 'bar'

      assert_equal 'bar', @foo

    end

    

    def test_should_not_define_second_instance_variable

      assert_equal false, instance_variable_defined?(:@baz)

    end

    

    def test_should_return_second_instance_variable_when_sent_second_attr_reader

      @baz = 'bat'

      assert_equal 'bat', baz

    end

    

    def test_should_set_second_instance_variable_when_sent_second_attr_writer

      self.baz = 'bat'

      assert_equal 'bat', @baz

    end

    

  end

  

  class TwoStandardAttrReadersAndADefault < Test::Unit::TestCase

    

    attr_reader :foo, :baz, :blit => 'blat'

    

    def test_should_not_define_first_instance_variable

      assert_equal false, instance_variable_defined?(:@foo)

    end

    

    def test_should_return_first_instance_variable_when_sent_first_attr_reader

      @foo = 'bar'

      assert_equal 'bar', foo

    end

    

    def test_should_not_define_second_instance_variable

      assert_equal false, instance_variable_defined?(:@baz)

    end

    

    def test_should_return_second_instance_variable_when_sent_second_attr_reader

      @baz = 'bat'

      assert_equal 'bat', baz

    end

    

    def test_should_not_define_third_instance_variable

      assert_equal false, instance_variable_defined?(:@blit)

    end

    

    def test_should_set_third_instance_variable_when_sent_third_attr_reader

      blit

      assert_equal 'blat', @blit

    end

    

    def test_should_not_set_third_instance_variable_if_already_set_when_sent_third_attr_reader

      @blit = 'splat'

      assert_equal 'splat', blit

    end

    

  end

  

  class TwoStandardAttrAccessorsAndADefault < Test::Unit::TestCase

    

    attr_accessor :foo, :baz, :blit => 'blat'

    

    def test_should_not_define_first_instance_variable

      assert_equal false, instance_variable_defined?(:@foo)

    end

    

    def test_should_return_first_instance_variable_when_sent_first_attr_reader

      @foo = 'bar'

      assert_equal 'bar', foo

    end

    

    def test_should_set_first_instance_variable_when_sent_first_attr_writer

      self.foo = 'bar'

      assert_equal 'bar', @foo

    end

    

    def test_should_not_define_second_instance_variable

      assert_equal false, instance_variable_defined?(:@baz)

    end

    

    def test_should_return_second_instance_variable_when_sent_second_attr_reader

      @baz = 'bat'

      assert_equal 'bat', baz

    end

    

    def test_should_set_second_instance_variable_when_sent_second_attr_writer

      self.baz = 'bat'

      assert_equal 'bat', @baz

    end

    

    def test_should_not_define_third_instance_variable

      assert_equal false, instance_variable_defined?(:@blit)

    end

    

    def test_should_set_third_instance_variable_when_sent_third_attr_reader

      blit

      assert_equal 'blat', @blit

    end

    

    def test_should_not_set_third_instance_variable_if_already_set_before_sent_third_attr_reader

      @blit = 'splat'

      assert_equal 'splat', blit

    end

    

    def test_should_set_third_instance_variable_when_sent_third_attr_writer

      self.blit = 'splat'

      assert_equal 'splat', @blit

    end

    

  end

  

  class AProcDefault < Test::Unit::TestCase

    

    attr_accessor :my_object_id => Proc.new { |obj| obj.object_id }

    

    def test_should_not_define_instance_variable

      assert_equal false, instance_variable_defined?(:@my_object_id)

    end

    

    def test_should_set_instance_variable_to_result_of_proc_when_sent_attr_reader

      my_object_id

      assert_equal object_id, @my_object_id

    end

    

    def test_should_not_set_instance_variable_if_already_set_before_sent_attr_reader

      @my_object_id = 'foo'

      assert_equal 'foo', my_object_id

    end

    

    def test_should_set_instance_variable_when_sent_attr_writer

      self.my_object_id = 'foo'

      assert_equal 'foo', @my_object_id

    end

    

  end

  

  class AProcWithinAProcDefault < Test::Unit::TestCase

    

    attr_accessor :my_proc => Proc.new { Proc.new { } }

    

    def test_should_not_define_instance_variable

      assert_equal false, instance_variable_defined?(:@my_proc)

    end

    

    def test_should_set_instance_variable_to_result_of_outer_proc_when_sent_attr_reader

      my_proc

      assert_kind_of Proc, @my_proc

    end

    

    def test_should_not_set_instance_variable_if_already_set_before_sent_attr_reader

      @my_proc = 'foo'

      assert_equal 'foo', my_proc

    end

    

    def test_should_set_instance_variable_when_sent_attr_writer

      self.my_proc = 'foo'

      assert_equal 'foo', @my_proc

    end

    

  end

  

end

DZone Snippets: injection code

Specifying default values with Ruby's attr_reader and attr_accessor