Meta Programming in Ruby Suraj Shirvankar 

Ruby Objects • Every thing in ruby is an object except for blocks; • Methods are called by passing messages to objects 

Monkey patching Allows you to refine a class / Add new methods Not recommended for Classes that you didn't build yourself class String def censor “######” end end “VPN”.sensor => “#####” 

MethodNotFound class Car ….. ….. end @car = Car.new @car.autodrive NoMethodError: undefined method `autodrive' for Car:Class 

What happens in the background class Car ….. ….. end The message ‘autodrive’ is sent to the car instance Ruby looks for the method ‘autodrive' and if it doesn't find the ‘autodrive’ method it calls ‘method_missing’ method_missing then throws the error NoMethodError 

Naive way of implementing dynamic methods def method_missing(method_name, *args, &block) if method_name == “autodrive” “Awaiting google’s implementation” else super end end @car.autodrive => “Awaiting google’s implementation” 

But Wait ! @car.autodrive => “Awaiting google’s implementation” @car.respond_to? :autodrive => false class Car def respond_to_missing?(method_name, include_private = false) method_name == “autodrive” || super end end 

Better approach Dynamic method definition 

class Car define_method(:break) do “Need to repair” end end @car.break => “Need to repair” @car.respond_to?(:break) => true 

Module_eval Instance_eval class_eval 

Demo 

Re-implementing FactoryGirl 

FactoryBoy 

Tests first require 'test/unit' require_relative '../lib/factory_boy' class TestFactoryBoy < Test::Unit::TestCase def setup FactoryBoy.define do factory :user do name "test" cat "meow" end end @user = FactoryBoy.build :user end def test_creates_user assert_equal @user.class, User end def test_has_name assert_equal @user.name, "test" end def test_meow assert_equal @user.cat, "meow" end end 

Implementation module FactoryBoy @factories = {} def self.define(&block) module_eval(&block) end def self.build(klass) instance = const_get(@factories[klass].class_name.capitalize).new @factories[klass].our_methods.each do |key, value| instance.class.class_eval do define_method(key.to_sym) { value } end end instance end def self.factory(klass, &block) instance = @factories[klass] = BaseObject.new(klass.to_s.capitalize) instance.instance_eval(&block) end class BaseObject < BasicObject attr_accessor :our_methods, :class_name def initialize(klass) @our_methods = {} @class_name = klass.to_s end def method_missing(method, *args, &block) @our_methods[method] = args.first end end end 

https://github.com/h0lyalg0rithm/meta_programming_meetup