react.rb

require 'observer'
class ReactiveValue
  include Observable

  attr_reader :value

  def set(value)
    @value = value
    changed
    notify_observers(self)
  end

  def +(other)
    ReactivePlus.new(self, other)
  end
end

class ReactivePlus
  def initialize(a, b)
    @a_value = a.value
    a.add_observer(self, :updated_a)

    @b_value = b.value
    a.add_observer(self, :updated_b)
  end

  def value
    @a_value + @b_value
  end

  def updated_a(a)
    @a_value = a.value
  end

  def updated_b(b)
    @b_value = b.value
  end
end


a = ReactiveValue.new
a.set(10)
b = ReactiveValue.new
b.set(10)

c = a + b

p c.value # => 20

a.set(20)
p c.value # => 30