Gain class-level accessor methods by including module in your class

class_accessors.rb

#! /usr/bin/env ruby

module ClassAccessors

  def self.included(klass)

    class << klass

      def class_attr_reader(*args)
        args.each do |attr|
          define_singleton_method attr do
            instance_variable_get "@#{attr}"
          end
        end
      end

      def class_attr_writer(*args)
        args.each do |attr|
          define_singleton_method "#{attr}=" do |val|
            instance_variable_set "@#{attr}", val
          end
        end
      end

      def class_attr_accessor(*args)
        class_attr_reader(*args)
        class_attr_writer(*args)
      end

    end
  end
end

test_class_accessors.rb

#! /usr/bin/env ruby
require_relative 'class_accessors'

# ensure using gem version, not the one built into YARV
# require 'rubygems'; gem 'minitest'

require 'minitest/autorun'

describe ClassAccessors do

  before do
    @klass = Class.new do
      include ClassAccessors
      class_attr_reader :a, :bar
      class_attr_writer :b, :baz
      class_attr_accessor :c
    end
  end

  after do
    @klass = nil
  end

  it 'should have readers' do
    @klass.must_respond_to :a
    @klass.must_respond_to :c
    @klass.must_respond_to :bar
  end

  it 'should have writers' do
    @klass.must_respond_to :'b='
    @klass.must_respond_to :'c='
    @klass.must_respond_to :'baz='
  end

  it 'should not have too much readers and writers' do
    @klass.wont_respond_to :'a='
    @klass.wont_respond_to :'bar='
    @klass.wont_respond_to :b
    @klass.wont_respond_to :baz
  end

  it 'should get values' do
    @klass.a.must_be_nil
    lambda { @klass.instance_eval { @a = 85 } }.call
    @klass.a.must_equal 85
    lambda { @klass.instance_eval { @a = 82 } }.call
    @klass.a.must_equal 82
    @klall.bar.must_be_nil
    lambda { @klass.instance_eval { @bar = 5 } }.call
    @klass.bar.must_equal 5
    lambda { @klass.instance_eval { @bar = 6 } }.call
    @klass.bar.must_equal 6
  end

  it 'should set values' do
    lambda { @klass.instance_eval { @b } }.call.must_be_nil
    @klass.b = 21
    lambda { @klass.instance_eval { @b } }.call.must_equal 21
    @klass.b = 5
    lambda { @klass.instance_eval { @b } }.call.must_equal 5
    lambda { @klass.instance_eval { @baz } }.call.must_be_nil
    @klass.baz = "aa"
    lambda { @klass.instance_eval { @baz } }.call.must_equal "aa"
    @klass.baz = "foo"
    lambda { @klass.instance_eval { @baz } }.call.must_equal "foo"
  end

  it 'should get and set values' do
    @klass.c.must_be_nil
    @klass.c = 4
    @klass.c.must_equal 4
    @klass.c = 33
    @klass.c.must_equal 33
  end

end