Hexagonal extraction of a controller action from reverb.com, showing reuse between controller and Grape/Roar based API. This is sample code stripped down to the essentials and is not guaranteed to work as-is :)

watch_listing.rb

class Reverb::Actions::WatchListing
  def self.watch(user, product, listener)
    if product.owner?(user)
      listener.failure(I18n.t('flash.watchlist.error_own'))
    else
      Reverb::Analytics.track(user, :watch_product) # FIXME, this doesn't belong here
      user.user_watch_products.create(:product_id => product.id)
      listener.success
    end
  end
end

watch_listing_api.rb

module Reverb
  class Wants < Grape::API
    post '/wants' do
      class WatchListingResponder < SimpleDelegator
        def success
          {"success" => true}
        end

        def failure(message)
          error!({ "error" => message}, 412)
        end
      end

      Reverb::Actions::WatchListing.watch(current_user, Product.find(params[:id]), WatchListingResponder.new(self))
    end
  end
end

watched_products_controller.rb

class Dashboard::Buying::WatchedProductsController < Dashboard::BaseController
  
  def create
    product = Product.find(params[:id])
    Reverb::Actions::WatchListing.watch(current_user, product, WatchListingResponder.new(self))
    redirect_to product_url(product)
  end

  private

  class WatchListingResponder < SimpleDelegator
    def success
      flash[:success] = "Listing has been added to #{link_to_watchlist}."
    end

    def failure(message)
      flash[:error] = message
    end

    def link_to_watchlist
      view_context.link_to 'your watchlist', dashboard_buying_watched_products_url
    end
  end
end

watched_products_controller_spec.rb

describe Dashboard::Buying::WatchedProductsController do
  stub_user

  let(:product) { mock('product', :id => 1) }

  before { Product.stub(:find).with("1") { product } }

  describe "#create" do
    let(:responder) { Dashboard::Buying::WatchedProductsController::WatchListingResponder.new(subject) }

    it "watches the listing" do
      Reverb::Actions::WatchListing.should_receive(:watch).with(user, product, anything)
      post :create, :id => product.id
      response.should redirect_to product_url(product)
    end

    it "handles error display" do
      responder.failure("foo")
      flash[:error].should == "foo"
    end

    it "handles success display" do
      responder.success
      flash[:success].should =~ /Listing has been added to.*your watchlist.*/
    end
  end
end

What about that approach: https://github.com/krisleech/wisper ?