gistfile1.rb

class Model
  
  def initialize
    @observers ||= {}
  end
  
  def observe(key, callback = nil, &blk)
    @observers[key] ||= []
    if block_given?
      @observers[key] << blk
    end
    return if callback.nil?
    @observers[key] << callback
  end
  
  def remove_observer(key, callback)
    @observers[key].delete(callback)
  end
  
  def self.observable(*observables)
    
    observables.each do |obs|
      self.class_eval do
        alias_method "no_callback_#{obs.to_s}=", "#{obs.to_s}="
        define_method "#{obs.to_s}=" do |value|
          send "no_callback_#{obs.to_s}=", value
          notify(obs.to_sym, value)
        end
      end
    end
    return nil
  end
  
  def notify(key, value)
    @observers[key].each { |callback| callback.call(value) }
  end  
end

class Foo < Model
  attr_accessor :x, :y
  observable :x, :y
end

m = Foo.new


m.observe(:x) do |update|
  puts "updated from block (update is: #{update})"
end

callback = lambda { |update| puts 
  "updated from lambda (update is :#{update})" }
m.observe(:x, callback)

m.x = 2


m.observe(:y, callback)

m.y = 1

m.remove_observer(:x, callback)

m.x = 1
m.y = 2