gistfile1.rb

class Model
  attr_accessor :x
  
  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 x=(x)
    @x = x
    notify(:x, @x)
  end
  
  private def notify(key, value)
    @observers[key].each { |callback| callback.call(value) }
  end
end

m = Model.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.remove_observer(:x, callback)

m.x = 1