some_controller.rb

class CommentsController < ApplicationController

  def create
    translator.translate params
  end

  private
  
  def translator
    CommentRequestTranslator.new(processor)
  end

  def processor
    comment_processor = CommentProcessor.new
    comment_processor.add_listener twitter_poster
    comment_processor.add_listener facebook_poster
    comment_processor.add_listener comment_persister
    comment_processor
  end

  def twitter_poster
    TwitterPoster.new
  end

  def facebook_poster
    FacebookPoster.new
  end

  def comment_persister
    SQLCommentPersister.new
  end
end

#example class
class CommentProcessor
  def initialize
    @announcer = Announcer.new
  end

  def add_listener(listener)
    @announcer.add_listener listener
  end

  def new_comment(comment)
    new_comment = comment.with_language(language_for(comment.body))
                         .with_spam(is_spam(comment.body))
    @announcer.announce('new_comment', new_comment)
  end
end

Right now this is the controller calling the processor. After the change, proposed above by Gregory, controller would call the validator which would call the processor, so at the end, the controller would:

• create a processor (as it is now)
• create a validator, passing in the processor (so validator would delegate when everything is OK)
• pass a comment to validator.

But that would introduce a processing logic into controller (or would it not?). What if there are two more controllers using a processor right now? They would still be passing a comment directly to processor instead of validator... :-/

Is that not a problem that the controller is actually creating the processor instance? Making the validator implementing a processor contract won't help here. Shouldn't controllers ask someone (a comment processor factory?) for an instance of processor, so your solution with adding a validator into the processing chain would "kick in" everywhere?

I would agree that it can be somewhat of an issue to have the controller creating the instances of the objects. I'd generally move the creation of the objects into an infrastructure layer and then just have the controllers call into the domain. I would often however still create a "translator" or "adapter" within the controller, and pass the domain into its constructor when translation from the network request into several messages to the domain is necessary.