Monitor an object with delegation.

object_monitor.rb

# BasicObject from 
# simonecarletti.com/blog/2010/05/understanding-ruby-and-rails-proxy-patter-delegation-and-basicobject/
class BasicObject
  instance_methods.each do |m|
    undef_method(m) if m.to_s !~ /^__|^nil\?$|^send$|^object_id$/
  end
end

class ObjectMonitor < BasicObject
  attr_reader :object
  def initialize(object)
    @object = object
  end
  
  def do_something_on_change
    Kernel.puts "changed: #{@old} => #{object.inspect}"
  end
  
  def method_missing(symbol, *args, &block)
    @old = object.inspect
    object.send(symbol, *args, &block)
    do_something_on_change
  end

  def self.[](obj)
    new(obj)
  end
end

s = ObjectMonitor["something"]

s << " goes here"
s << "oh"