railsのコントローラーとビューの連携はこんな感じ

controller_and_view.rb

require 'erb'
class ERB
  def self.call(source)
    new(source, nil, nil, '@output_buffer').src 
  end
end
class Template
  def initialize(source, path)
    @path = path
    @source = source
  end
  def render(view)
    compile(view)
    view.send(method_name)
  end
  def compile(view)
    mod = view.singleton_class
    code = ERB.call(@source)
    mod.module_eval(<<-RUBY
      def #{method_name}
        #{code}
        @output_buffer
      end
    RUBY
    )
  end
  def method_name
    "_#{@path.gsub(/[^a-z_]/, '_')}"
  end
end
class Controller
  attr_accessor :response_body
  def view_context_class
    helpers = _helpers if respond_to?(:_helpers)
    @view_context_class ||= Class.new(Object) do
      include helpers
    end
  end
  def view_context
    view_context_class.new
  end
  def render(path)
    template = Template.new(File.binread(path), path)
    @response_body = template.render(view_context)
  end
end

each.txt.erb

<% (1..3).each do|i| %>
<%= i%>
<% end %>

hello.txt.erb

<%= say_hello("world") %>

test.rb

#!/usr/bin/ruby -I.
require 'test/unit'
require 'controller_and_view'
module HelloHelper
  def say_hello name
    "hello, #{name}"
  end
end
class HelloController < Controller
  def _helpers
    HelloHelper
  end
end
class ActionViewTests < Test::Unit::TestCase
   def test_controller_render
    controller = HelloController.new
    controller.render("hello.txt.erb")
    assert_equal "hello, world\n", controller.response_body
  end
  def test_template_render
    controller = HelloController.new
    view = controller.view_context
    path = "./hello.txt.erb"
    Template.new(File.binread(path), path).compile(view)
    path = "./each.txt.erb"
    Template.new(File.binread(path), path).compile(view)
    assert_equal "hello, world\n", view.send("___hello_txt_erb")
    assert_equal "\n1\n\n2\n\n3\n\n", view.send("___each_txt_erb")
  end
  def test_template_method_name
    template = Template.new("", "./hello.txt.erb")
    assert_equal "___hello_txt_erb", template.method_name
  end

end