Attribute Observers

attribute_observers.rb

module ObservableAttributes
  
  module Observers
    def observers
      # A little hack to prevent the need for an @observers instance var
      class << self
        @observers ||= Hash.new
      end
    end
    
    def bind event, property, &proc
      event, property = event.to_sym, property.to_sym
      observers[event] ||= Hash.new
      observers[event][property] ||= Array.new
      observers[event][property] << proc
    end
  end
  
  module ClassMethods
    include Observers
    
    def attr_reader property
      define_method property do
        value = instance_variable_get :"@#{property}"
        trigger :read, property.to_sym, value
        value
      end
    end

    def attr_writer property
      define_method :"#{property}=" do |value|
        instance_variable_set :"@#{property}", value
        trigger :write, property.to_sym, value
      end
    end

    def attr_accessor property
      attr_reader property
      attr_writer property
    end
    
    def testing
    end
    
  end # ClassMethods
  
  module InstanceMethods
    include Observers

    def trigger method, property, value
      all_available_observers = [observers, self.class.observers]
      all_available_observers.each do |observer_hash|
        if observer_hash.include? method
          if observer_hash[method].include? property
            observer_hash[method][property].each { |proc| proc.call(value) }
          end
        end
      end
    end
    
  end # InstanceMethods
  
  def self.included base
    base.extend ClassMethods
    base.send :include, InstanceMethods
  end
  
  
end

class Person
  include ObservableAttributes
  
  attr_accessor :age
  attr_accessor :name
end

Person.bind :write, :age do |value|
  puts "Change observed by class"
end

david = Person.new

david.bind :write, :age do |value|
  puts "Change observed by instance"
end

david.name = "David"
david.age = 25

p david

I think this does more than I need; I can't figure out what parts I need, exactly. Basically, I just want to bind attr_* to a database backend.