observer_pattern.rb

require "observer"

class Currency
   include Observable

   def initialize(symbol,rate)
     @rate = rate
     @symbol = symbol
   end

   def rate=(new_rate)
     changed  if new_rate != @rate
     @rate = new_rate
     notify_observers(@symbol,@rate)
   end
end

class Graph
   def initialize(currencies)
      currencies.each do |currency|
        currency.add_observer(self)
      end
   end

   def update(symbol, rate)
      puts "new rate for #{symbol} : #{rate}"
      #.. update the graph
   end
end

eur = Currency.new('EUR',1.2768)
usd = Currency.new('USD',1)

graph = Graph.new [eur,usd]

eur.rate = 1.2767 #=> new rate for EUR : 1.2767