Presented at the November '07 ThoughtWorks UK Away Day, the talk was centered around the various techniques used to create and mold your objects and domain model using declaritive, self-inspecting, and self-modifying code.

1. Metaprogramming with Ruby Julie Yaunches Farooq Ali

4. Exploring the plain define_method alias_method foo.send :bar instance_variable_get class_eval instance_eval module_eval instance_variable_set eval block.call Class.new method_missing class << self block.binding Foo.instance_methods method_added method_removed

11. Object Introspection: Instance Variables > @event = 'Away Day' > instance_variable_get '@event' => &quot;Away Day&quot; > instance_variable_set '@region', 'UK' > @region => &quot;UK&quot; > instance_variables => [&quot;@event&quot;, &quot;@region&quot;]

12. Object Introspection: Methods > 'Away Day’. methods => [&quot;capitalize&quot;, &quot;split&quot;, &quot;strip&quot;, …] > 5. public_methods => [&quot;+&quot;, &quot;/&quot;, &quot;integer?&quot;, …]

13. Object Introspection: Methods > 'abc'.method(:capitalize) => #<Method: String#capitalize> class method strip capitalize String gsub split class method unbind arity Method to_proc call

15. Introspection in Action class Person def initialize(name, age, sex) @name = name @age = age @sex = sex end end Person.new('Farooq', 23, :male) Did we just think in a for-loop?!

16. Introspection in Action class Person def initialize(attributes) attributes.each do |attr, val| instance_variable_set(&quot;@#{attr}&quot;, val) end end end Person.new :name => 'Farooq', :age => '23', :sex => :male

32. You can define methods on the fly too! define_method :say_hello do puts &quot;Hello World!&quot; end

35. Dynamic Method Definition in Action class Person attr_reader :name, sex end class Person { string name, sex; public string Name { get {return name;} } public string Sex { get {return sex;} } }

36. Dynamic Method Definition in Action class Object def Object.attr_reader(*attrs) attrs.each do |attr| define_method(attr) do instance_variable_get(&quot;@#{attr}&quot;) end end end end

37. Dynamic Method Definition in Action class Object def Object.attr_reader(*attrs) attrs.each do |attr| define_method(attr) do instance_variable_get(&quot;@#{attr}&quot;) end end end end

38. Another example: Lazy Loading class Person List<Person> friends; public List<Person> Friends { get { friends = friends ?? LoadFriends(); return friends; } }

39. Lazy Loading class Person lazy_loaded_attr :friends end

40. Lazy Loading class Object def self.lazy_loaded_attr(*attrs) attrs.each do |attr| define_method(attr) do eval &quot;@#{attr} ||= load_#{attr}&quot; end end end end

41. Lazy Loading class Person lazy_loaded_attr :friends, :children, :parents end class Person List<Person> friends, children, parents; public List<Person> Friends { get { friends = friends ?? LoadFriends(); return friends; } } public List<Person> Children { get { children = children ?? LoadChildren(); return children; } } public List<Person> Parents { get { parents = parents ?? LoadParents(); return parents; } } }

42. Declarative code class Album < ActiveRecord::Base has_many :tracks belongs_to :artist acts_as_taggable has_many :lyrics, :through => :tracks end

43. Declarative code with define_method class Album < ActiveRecord::Base has_many :tracks end class Album < ActiveRecord::Base def tracks Track.find :all, :conditions => &quot;album_id=#{id}&quot; end end define_method :tracks …

44. Declarative code with define_method class ActiveRecord::Base def self.has_many(records) define_method(records) do foreign_key = &quot;#{self.name.underscore}_id&quot; klass = records.to_s.classify.constantize klass.find :all, :conditions => &quot;#{foreign_key}=#{id}&quot; end end end

45. Person Person walk() define_method :walk define_method :everyone ? Person everyone() Define instance method: Define class method:

46. Person Person walk() define_method :walk define_method :everyone Person' Person' everyone() Define instance method: Define class method:

47. Person Person walk() define_method :walk define_method :everyone Person' Person' everyone() Define instance method: Define class method: Person everyone()

53. Decorator Pattern with Aliasing class LoginService alias_method :original_login, :login def login(user,pass) log_event &quot;#{user} logging in&quot; original_login(user,pass) end end

54. Decorator Pattern with Aliasing class LoginService alias_method_chain :login, :logging def login_with_logging(user,pass) log_event &quot;#{user} logging in&quot; login_without_logging(user,pass) end end

64. Questions?