Use same format, prefix, formatter and error handler for multiple grape api's

api-namespace-api.rb

module Namespace
  module API
    extend ActiveSupport::Concern

    included do
      format :xml
      prefix :api

      formatter :xml, xml_formatter
      rescue_from :all do |e|
        error!(
            {
                error: "#{e.class} error",
                message: e.message
            },
            e.try( :status ) || 500
        )
      end

      helpers do
        def current_user
          #...
        end

        def authenticate!
          error!( '401 Unauthorized', 401 ) unless current_user
        end
      end
    end

    class_methods do
      def xml_formatter
        lambda { |object,env|
          if object.is_a? Hash
            # removes hashie from collection
            object[object.keys.first].as_json.to_xml :root => object.keys.first, :skip_types => true
          elsif object.is_a? String
            # what. no.
            object
          else
            # removes hash from single
            object.to_xml
          end
        }
      end
    end
  end
end

api-route-api.rb

module Route
  class API < Grape::API
    include Univercity::API
  end
end