some_controller.rb

class CommentsController < ApplicationController

  def create
    app.create_comment(params)
  end

  private

  def app
    MyRailsApp.new
  end
  
end

# this is a facade over the domain which wires up the right adapters and the right domain objects
class MyRailsApp
  def create_comment(params)
    comment = Comment.new(params)
    comment_processor.new_comment(comment)
  end

  private

  def comment_processor
    listener = FanoutAnnouncer.new(twitter_poster, facebook_poster, comment_persister)
    CommentProcessor.new(listener)
  end

  def twitter_poster
    TwitterPoster.new
  end

  def facebook_poster
    FacebookPoster.new
  end

  def comment_persister
    SQLCommentPersister.new
  end
end

# example class
class CommentProcessor < Struct.new(:listener)
  def new_comment(comment)
    new_comment = comment.with_language(language_for(comment.body))
                         .with_spam(is_spam(comment.body))
    listener.announce('new_comment', new_comment)
  end
end

# immutable value object for carrying data about a comment
class Comment
  include Virtus::ValueObject

  attribute :body, String

  def with_language
    # ...
  end
end

Yes, I certainly agree that it makes the comment processor simpler, although I wonder if it lessens its cohesion. In order to send a message to the process I need one instance, and in order to subscribe to its updates I need another. I guess so long as you didn't need to be doing dynamic subscription/unsubscription outside of factories it wouldn't matter, which would probably be the case in all server side apps.

I think we're largely on the same page.

On 12 Jul 2012, at 21:19, Gregory Moeck wrote: in order to subscribe to its updates I need another object

Well, only if you want to have more than one listener. The FanoutAnnouncer is just a proxy that forwards messages on to multiple objects. If you only had on listener you could just plug that one in directly.

I think we're largely on the same page.

Indeed. :)

Just a small detail, but was interested if you would pass in the entire params hash into the facade or filter on the comment attributes. The example never specifies the comment attributes; i.e. params[:comment].