test.rb

# Observer
# ----------------

module Observable

  def add_observer(observer)
    observers.push(observer)
  end

  def remove_observer(observer)
    observers.delete(observer)
  end

  def notify_observers(event, options)
    observers.each do |observer|
      observer.notify(event, options)
    end
  end

  def observers
    @observers ||= []
  end
end

class Foo
  include Observable

  def sup
    self.notify_observers('sup', { :message => 'Wassup' })
  end
end

class Bar
  def notify(event, options)
    puts "Bar.notify: #{event}, #{options}"
  end
end

f = Foo.new
b = Bar.new
f.add_observer(b)
f.sup