Observer Pattern example for http://rubysource.com/design-patterns-in-ruby-observer-singleton/

car.rb

class Car
  attr_reader :mileage, :service
  def initialize(mileage = 0, service = 3000)
    @mileage, @service = mileage, service
    @observers = [Notifier.new]
  end
  def log(miles)
    @mileage += miles
    notify_observers(self, miles)
  end
  def notify_observers(*args)
    @observers.each { |o| o.update(*args) }
  end
end

notifier.rb

class Notifier
  def update(car, miles)
    puts "The car has logged #{miles} miles, totaling #{car.mileage} miles traveled."
    puts "The car needs to be taken in for a service!" if car.service <= car.mileage
  end
end