Controller Observer: It's very similar like Sweeper but the observer should be manually added into config/application.rb. The benefit is that the observe still can be callable in the non-request environment such as console or rake.

controller_example.rb

# In Controller such as comments_controller.rb

observer      :comment_observer, only: [:create, :destroy]

controller_observer.rb

module ControllerObserver
  extend ActiveSupport::Concern

  module ClassMethods #:nodoc:
    def observer(*observers)
      configuration = observers.extract_options!

      observers.each do |observer|
        observer_instance = (observer.is_a?(Symbol) ? Object.const_get(observer.to_s.classify) : observer).instance
        around_filter(observer_instance, :only => configuration[:only])
      end
    end
  end
end

observer_example.rb

class CommentObserver < ActionController::Caching::Sweeper
  def current_user
    controller ? controller.send(:current_user) : nil
  end
end

Usage:

# In Controller such as comments_controller.rb

observer      :comment_observer, only: [:create, :destroy]