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

You have to admit that the code in the CommentProcessor is simpler though? I'm not suggesting you use that instance for other objects, just that it owns the job of managing who is listening to events from the processor and fanning them out. I built something like this in Cucumber a long, long time ago.[1]

I don't test the facade, no. I'm still playing about with this idea, but I personally like it better than a bunch of use-case objects as my 'route in'. It keeps the controllers so simple there's almost nothing left.

[1] https://github.com/cucumber/cucumber/blob/master/lib/cucumber/broadcaster.rb

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].