Exposure Pattern

simple_exposure.rb

# Exposure pattern to use declarative interfaces in controller (which is the basic feature of the library decent_exposure).
#
# Using this pattern you can dramatically reduce or remove completely the need for instance variables in controllers and views.
#
# Example :
#
# BEFORE (traditional way in Ruby on Rails)
#
# class ArticlesController < ApplicationController
#   before_action :load_current_blog
#   before_action :load_articles
#
#   ...
#
#   private
#
#   def load_current_blog
#     @current_blog = Blog.find(params[:id])
#   end
#
#   def load_articles
#     @articles_sorted = @current_blog.articles.order(:created_at, :desc)
#   end
#
#  ...
#  then in views you access data using the instance variables (@foo for example). If for any reason you don't have some variables defined, and call @foo.each for example on one of them you get a NoMethodError: undefined method `each' for nil:NilClass and you don't know which variable is not defined if you don't look at the view.
#  In controller tests it's difficult to test instance variables, you have to use helpers like "assigns"...
#
#
#  AFTER
#
# class ArticlesController < ApplicationController
#   expose(:current_blog) { Blog.find(params[:id]) }
#   expose(:articles_sorted) { current_blog.articles.order(:created_at, :desc) }
#
#  ...
#  then in views you access data using current_blog and articles_sorted which are methods. If this time you call foo.each and the foo method is not defined you simply get a 'undefined local variable or method foo', you get the name of the undefined variable in the exception (foo).
#  It's also way more easy to test those methods in controller tests because you can call them direcly with 'controller.foo' to get the result. You don't need to rely on helpers like assigns.
#
module SimpleExposure
  extend ActiveSupport::Concern

  class_methods do
    def expose(exposure_name, &block)
      class_variable_set(:"@@#{exposure_name}_proc", Proc.new)

      class_eval <<-EVAL
        helper_method :#{exposure_name}

        def #{exposure_name}
          @#{exposure_name} ||= @@#{exposure_name}_proc.call
        end
      EVAL
    end
  end
end