tracks/vendor/rails/actionpack/lib/action_controller/components.rb

module ActionController #:nodoc:
  # Components allow you to call other actions for their rendered response while executing another action. You can either delegate
  # the entire response rendering or you can mix a partial response in with your other content.
  #
  #   class WeblogController < ActionController::Base
  #     # Performs a method and then lets hello_world output its render
  #     def delegate_action
  #       do_other_stuff_before_hello_world
  #       render_component :controller => "greeter",  :action => "hello_world", :params => { :person => "david" }
  #     end
  #   end
  #
  #   class GreeterController < ActionController::Base
  #     def hello_world
  #       render :text => "#{params[:person]} says, Hello World!"
  #     end
  #   end
  #
  # The same can be done in a view to do a partial rendering:
  #
  #   Let's see a greeting:
  #   <%= render_component :controller => "greeter", :action => "hello_world" %>
  #
  # It is also possible to specify the controller as a class constant, bypassing the inflector
  # code to compute the controller class at runtime:
  #
  # <%= render_component :controller => GreeterController, :action => "hello_world" %>
  #
  # == When to use components
  #
  # Components should be used with care. They're significantly slower than simply splitting reusable parts into partials and
  # conceptually more complicated. Don't use components as a way of separating concerns inside a single application. Instead,
  # reserve components to those rare cases where you truly have reusable view and controller elements that can be employed
  # across many applications at once.
  #
  # So to repeat: Components are a special-purpose approach that can often be replaced with better use of partials and filters.
  module Components
    def self.included(base) #:nodoc:
      base.class_eval do
        include InstanceMethods
        extend ClassMethods
        helper HelperMethods

        # If this controller was instantiated to process a component request,
        # +parent_controller+ points to the instantiator of this controller.
        attr_accessor :parent_controller

        alias_method_chain :process_cleanup, :components
        alias_method_chain :set_session_options, :components
        alias_method_chain :flash, :components

        alias_method :component_request?, :parent_controller
      end
    end

    module ClassMethods
      # Track parent controller to identify component requests
      def process_with_components(request, response, parent_controller = nil) #:nodoc:
        controller = new
        controller.parent_controller = parent_controller
        controller.process(request, response)
      end
    end

    module HelperMethods
      def render_component(options)
        @controller.send!(:render_component_as_string, options)
      end
    end

    module InstanceMethods
      # Extracts the action_name from the request parameters and performs that action.
      def process_with_components(request, response, method = :perform_action, *arguments) #:nodoc:
        flash.discard if component_request?
        process_without_components(request, response, method, *arguments)
      end

      protected
        # Renders the component specified as the response for the current method
        def render_component(options) #:doc:
          component_logging(options) do
            render_for_text(component_response(options, true).body, response.headers["Status"])
          end
        end

        # Returns the component response as a string
        def render_component_as_string(options) #:doc:
          component_logging(options) do
            response = component_response(options, false)

            if redirected = response.redirected_to
              render_component_as_string(redirected)
            else
              response.body
            end
          end
        end

        def flash_with_components(refresh = false) #:nodoc:
          if !defined?(@_flash) || refresh
            @_flash =
              if defined?(@parent_controller)
                @parent_controller.flash
              else
                flash_without_components
              end
          end
          @_flash
        end

      private
        def component_response(options, reuse_response)
          klass    = component_class(options)
          request  = request_for_component(klass.controller_name, options)
          new_response = reuse_response ? response : response.dup

          klass.process_with_components(request, new_response, self)
        end

        # determine the controller class for the component request
        def component_class(options)
          if controller = options[:controller]
            controller.is_a?(Class) ? controller : "#{controller.camelize}Controller".constantize
          else
            self.class
          end
        end

        # Create a new request object based on the current request.
        # The new request inherits the session from the current request,
        # bypassing any session options set for the component controller's class
        def request_for_component(controller_name, options)
          new_request         = request.dup
          new_request.session = request.session

          new_request.instance_variable_set(
            :@parameters,
            (options[:params] || {}).with_indifferent_access.update(
              "controller" => controller_name, "action" => options[:action], "id" => options[:id]
            )
          )

          new_request
        end

        def component_logging(options)
          if logger
            logger.info "Start rendering component (#{options.inspect}): "
            result = yield
            logger.info "\n\nEnd of component rendering"
            result
          else
            yield
          end
        end

        def set_session_options_with_components(request)
          set_session_options_without_components(request) unless component_request?
        end

        def process_cleanup_with_components
          process_cleanup_without_components unless component_request?
        end
    end
  end
end