module ActiveModel::Observing::ClassMethods

def add_observer(observer)
  unless observer.respond_to? :update
    raise ArgumentError, "observer needs to respond to `update'"
  end
  @observer_instances ||= []
  @observer_instances << observer
end

def count_observers
  @observer_instances.size
end

def inherited(subclass)
  super
  notify_observers :observed_class_inherited, subclass
end

def instantiate_observer(observer) #:nodoc:
  # string/symbol
  if observer.respond_to?(:to_sym)
    observer = observer.to_s.camelize.constantize.instance
  elsif observer.respond_to?(:instance)
    observer.instance
  else
    raise ArgumentError, "#{observer} must be a lowercase, underscored class name (or an instance of the class itself) responding to the instance method. Example: Person.observers = :big_brother # calls BigBrother.instance"
  end
end

def instantiate_observers
  observers.each { |o| instantiate_observer(o) }
end

def notify_observers(*arg)
  if defined? @observer_instances
    for observer in @observer_instances
      observer.update(*arg)
    end
  end
end

def observers
  @observers ||= []
end

def observers=(*values)
  @observers = values.flatten
end