require 'singleton'

module ActiveRecord
  # Observer classes respond to lifecycle callbacks to implement trigger-like
  # behavior outside the original class. This is a great way to reduce the
  # clutter that normally comes when the model class is burdened with
  # functionality that doesn't pertain to the core responsibility of the
  # class. Example:
  #
  #   class CommentObserver < ActiveRecord::Observer
  #     def after_save(comment)
  #       Notifications.deliver_comment("admin@do.com", "New comment was posted", comment)
  #     end
  #   end
  #
  # This Observer sends an email when a Comment#save is finished.
  #
  # == Observing a class that can't be inferred
  #
  # Observers will by default be mapped to the class with which they share a name. So CommentObserver will
  # be tied to observing Comment, ProductManagerObserver to ProductManager, and so on. If you want to name your observer
  # differently than the class you're interested in observing, you can use the Observer.observe class method:
  #
  #   class AuditObserver < ActiveRecord::Observer
  #     observe Account
  #
  #     def after_update(account)
  #       AuditTrail.new(account, "UPDATED")
  #     end
  #   end
  #
  # If the audit observer needs to watch more than one kind of object, this can be specified with multiple arguments:
  #
  #   class AuditObserver < ActiveRecord::Observer
  #     observe Account, Balance
  #
  #     def after_update(record)
  #       AuditTrail.new(record, "UPDATED")
  #     end
  #   end
  #
  # The AuditObserver will now act on both updates to Account and Balance by treating them both as records.
  #
  # == Available callback methods
  #
  # The observer can implement callback methods for each of the methods described in the Callbacks module.
  #
  # == Triggering Observers
  # 
  # In order to activate an observer, you need to call Observer.instance. In Rails, this can be done in controllers
  # using the short-hand of for example observer :comment_observer.
  class Observer
    include Singleton

    # Attaches the observer to the supplied model classes.
    def self.observe(*models)
      define_method(:observed_class) { models }
    end

    def initialize
      [ observed_class ].flatten.each do |klass| 
        klass.add_observer(self)
        klass.send(:define_method, :after_find) unless klass.respond_to?(:after_find)
      end
    end
  
    def update(callback_method, object) #:nodoc:
      send(callback_method, object) if respond_to?(callback_method)
    end
    
    private
      def observed_class
        if self.class.respond_to? "observed_class"
          self.class.observed_class
        else
          Object.const_get(infer_observed_class_name)
        end
      end
      
      def infer_observed_class_name
        self.class.name.scan(/(.*)Observer/)[0][0]
      end
  end
end