Mixin with namespaced macro to inherit view prefixes from several modules.

sums_prefix.rb

# Mixin with namespaced macro to inherit view prefixes from several modules.
#
# @example Imagine legacy controllers…
#   module V1
#     class BaseController < ActionController::Metal; end
#     # => _prefixes: ['api/v1/base']
#
#     class PostsController < BaseController; end
#     # => _prefixes: ['api/v1/base', 'api/v1/posts']
#   end
#
# @example …and inherited actual version
#   module V2
#     class PostsController < V1::PostsController; end
#     # => _prefixes: ['v1/base', 'v1/posts', 'v2/posts']
#   end
#
# @example Extend namespace with SumsPrefix, if it's wanted to include 'v2/base' in the view prefixes of V2::PostsController!
#   module V2
#     extend SumsPrefix
#
#     class BaseController < V1::BaseController; end
#     # => _prefixes: ['v1/base', 'v2/base']
#
#     class PostsController < Sum(V1::PostsController, BaseController); end
#     # => _prefixes: ['v1/base', 'v1/posts', 'v2/base', 'v2/posts']
#   end
module SumsPrefix
  NAME_TEMPLATE = 'Sum_%sController'

  private

  # @!visibility public

  # @overload Sum(parent, *rest)
  #   Hooks the given class with view prefixes from parents
  #   @param parent [Class] class to inherit
  #   @param rest [Array<Class>] classes to include prefixes from
  #   @return [Class]
  #     a middleware controller inherited from a +parent+ class and
  #     defined in the caller's namespace with name template {NAME_TEMPLATE}
  def Sum(*parents)
    @sumed_classes = Concurrent::Map.new unless defined?(@sumed_classes)
    @sumed_classes.fetch_or_store(parents.hash) do
      parent, *rest = parents
      const_name = (NAME_TEMPLATE % parents.map(&:controller_name).join(?_).classify).to_sym
      sumed = const_set(const_name, Class.new(parent))
      prefixes = rest.inject(parent._prefixes) { |a, e| (e._prefixes - a) + a }
      sumed.instance_variable_set(:@_prefixes, prefixes)
      sumed
    end
  end
end