module Shamu
  module Events

    # Add event dispatching support to a {Services::Service}
    module Support
      extend ActiveSupport::Concern

      included do

        # ============================================================================
        # @!group Dependencies
        #

        # @!attribute
        # @return [Events::EventsService] the events service to publish messages
        #     to.
        attr_dependency :events_service, Events::EventsService

        #
        # @!endgroup Dependencies

      end

      # (see Support.event_channel)
      def event_channel
        self.class.event_channel
      end

      private

        # @!visibility public
        #
        # Publish the given `message` to the {#events_service}.
        #
        # @param [Events::Message, Symbol] message the custom event specific message to
        #     publish.
        # @param [String] channel to publish to.
        # @param [Hash] message_attrs arguments to use when creating an
        # instance of `message`.
        #
        # If `message` is a symbol, looks for a {Message} class in
        # {ServiceNamespace}::{OptionalServiceDomain}Events::{name.caemlize}.
        # @return [void]
        def event!( message, channel: event_channel, **message_attrs )
          if message.is_a?( Symbol )
            message = self.class
                          .event_message_namespace
                          .const_get( message.to_s.camelize )
                          .new( message_attrs )
          end
          events_service.publish channel, message
        end

      class_methods do
        # @!visibility public
        #
        # The channel to {#publish_event publish events} to. Defaults to the
        # transformed name of the service class.
        #
        #     Users::UsersService              => users
        #     Users::ProfileService            => users/profile
        #     Users::Profiles::ProfilesService => users/profiles
        #
        # @return [String] the name of the channel.
        def event_channel
          @event_channel ||= begin
            base_name = name || "Events"
            parts     = base_name.split( "::" )
            parts[-1].sub!( /Service$/, "" )
            parts.pop if parts[-1] == parts[-2] || ( parts.length > 1 && parts[-1].blank? )
            parts.join( "/" ).underscore
          end
        end

        # The module that holds the per-message event classes for the service.
        # @return [Module]
        def event_message_namespace
          @event_message_namespace ||=
            begin
              namespace = name.deconstantize
              return unless namespace.present?

              namespace = namespace.constantize
              domain    = name.demodulize.sub( /Service/, "" ).singularize

              # Must use exceptions instead of const_defined? so that rails has
              # a change to autoload the constant.
              begin
                namespace.const_get( "#{ domain }Events" )
              rescue NameError
                namespace.const_get( "Events" )
              end
            end
        end
      end

    end
  end
end