Rails Observer Alternatives for 4.0

Question

With Observers officially removed from Rails 4.0, I\'m curious what other developers are using in their place. (Other than using the extracted gem.) While Observers were cer

Answer 1

My alternative to Rails 3 Observers is a manual implementation which utilizes a callback defined within the model yet manages to (as agmin states in his answer above) "flip the dependency...coupling".

My objects inherit from a base class which provides for registering observers:

class Party411BaseModel

  self.abstract_class = true
  class_attribute :observers

  def self.add_observer(observer)
    observers << observer
    logger.debug("Observer #{observer.name} added to #{self.name}")
  end

  def notify_observers(obj, event_name, *args)
    observers && observers.each do |observer|
    if observer.respond_to?(event_name)
        begin
          observer.public_send(event_name, obj, *args)
        rescue Exception => e
          logger.error("Error notifying observer #{observer.name}")
          logger.error e.message
          logger.error e.backtrace.join("\n")
        end
    end
  end

end

(Granted, in the spirit of composition over inheritance, the above code could be placed in a module and mixed in each model.)

An initializer registers observers:

User.add_observer(NotificationSender)
User.add_observer(ProfilePictureCreator)

Each model can then define its own observable events, beyond the basic ActiveRecord callbacks. For instance, my User model exposes 2 events:

class User < Party411BaseModel

  self.observers ||= []

  after_commit :notify_observers, :on => :create

  def signed_up_via_lunchwalla
    self.account_source == ACCOUNT_SOURCES['LunchWalla']
  end

  def notify_observers
    notify_observers(self, :new_user_created)
    notify_observers(self, :new_lunchwalla_user_created) if self.signed_up_via_lunchwalla
  end
end

Any observer that wishes to receive notifications for those events merely needs to (1) register with the model that exposes the event and (2) have a method whose name matches the event. As one might expect, multiple observers can register for the same event, and (in reference to the 2nd paragraph of the original question) an observer can watch for events across several models.

The NotificationSender and ProfilePictureCreator observer classes below define methods for the events exposed by various models:

NotificationSender
  def new_user_created(user_id)
    ...
  end

  def new_invitation_created(invitation_id)
    ...
  end

  def new_event_created(event_id)
    ...
  end
end

class ProfilePictureCreator
  def new_lunchwalla_user_created(user_id)
    ...
  end

  def new_twitter_user_created(user_id)
    ...
  end
end

One caveat is that the names of all events exposed across all the models must be unique.