hello_world_controller.rb

class HelloWorldController < AbstractController::Base
  include AbstractController::Rendering
  include AbstractController::Layouts
  include AbstractController::Helpers
  include AbstractController::Translation
  include AbstractController::AssetPaths
  include ActionController::UrlWriter

  # Uncomment if you want to use helpers defined in ApplicationHelper in your views
  # helper ApplicationHelper

  # Make sure your controller can find views
  self.view_paths = "app/views"

  # You can define custom helper methods to be used in views here
  # helper_method :current_admin
  # def current_admin; nil; end

  def show
    render template: "hello_world/show"
    # or, for partials:
    # render partial: "hello_world/show"
  end
end

show.html.erb

<h1>Hello, World!</h1>

usage.rb

# From your model, observer or library, call:

HelloWorldController.new.show
# => "<h1>Hello, World!</h1>\n"