class Controller
  attr_reader :tag

  def initialize
    @tag = "Controller"
    @window = Window.new(self)
    @window.show
    hello
    hello_again
  end
end

class Window
  attr_reader :tag

  def initialize(controller)
    @controller = controller
  end

  def show
    @tag = "World"

    @controller.instance_eval do
      def hello
        puts "Hello #{tag}!"   # => Hello Controller!
      end
    end

    # Find controller's instance variable that is current self.
    window = @controller.instance_variables.find { |var| @controller.instance_variable_get(var) == self }

    # Now we can use the outer self within the instance_eval.
    @controller.instance_eval(%Q/
      def hello_again
        tag = #{window}.tag
        puts "Hello #{tag}!"   # => Hello World!
      end
    /)
  end
end

Controller.new