validatable.rb

validatable.rb

module Validatable
  def self.included(base)
    base.extend ClassMethods
  end
  module ClassMethods
    def validate(attribute, &block)
      attr_accessor attribute unless defined?(attribute)
      alias_method "#{attribute}_orig=", "#{attribute}="
      define_method("#{attribute}=") do |val|
        self.__send__("#{attribute}_orig=", val) if block.(val)
      end
    end
  end
end

class X
  attr_accessor :val
  include Validatable
  validate :val do |val|
    /\d+/.match val.to_s
  end
end

p X.new.tap {|x| x.val = 'hoge' }.val
p X.new.tap {|x| x.val = 5 }.val