Prezentácia z Rubyslavy #4 (@tkramar, @jsuchal)

1. Postobjektové
programovanie v Ruby
@tkramar, @jsuchal

2. Paradigma
model, súbor princípov, spôsob uchopenia
problému

3. Aké paradigmy poznáte?
procedurálna (C, Basic, COBOL, ..)
zdieľané dáta, procedúry operujúce nad dátami
objektová (Ruby, Java, Python, ..)
zapuzdrené dáta, metódy operujúce nad stavom objektu
funkcionálna (Lisp, Haskell, Scheme, ..)
dáta = program, čisté výrazy, bez vedľajších efektov
logická (PROLOG)
deklaratívne programovanie - fakty a predikáty, odvodzovanie

4. Objekt
identita
stav
správanie

5. class Person
@name = ''
def initialize(name)
@name = name
end
def say
"Ahoj, ja som #{@name}"
end
end

6. Person
Stav
@name
Správanie
def say
"Ahoj, ja som #{@name}"
end
Identita
p = Person.new('Janko')
o.object_id

7. Identita
sú dva objekty rovnaké?
==
sú dva objekty rovnaké a rovnakého typu?
eql?
sú dva objekty totožné?
equals?

8. Identita
p1 = Person.new('Janko')
#=> #<Person:0x00000001b5c100 @name="Janko">
p2 = Person.new('Janko')
#=> #<Person:0x00000001b556e8 @name="Janko">
family = []
family << p1
family << p2
family.uniq
#=> [#<Person:0x00000001b5c100 @name="Janko">,
#<Person:0x00000001b556e8 @name="Janko">]

9. class Person
def ==(other)
other.name == @name
end
def eql?(other)
self == other
end
end

10. Identita
p1 = Person.new('Janko')
#=> #<Person:0x00000001b5c100 @name="Janko">
p2 = Person.new('Janko')
#=> #<Person:0x00000001b556e8 @name="Janko">
family = []
family << p1
family << p2
family.uniq
#=> [#<Person:0x00000001b5c100 @name="Janko">,
#<Person:0x00000001b556e8 @name="Janko">]

11. class Person
def hash
@name.hash
end
end

12. Identita
p1 = Person.new('Janko')
#=> #<Person:0x00000001b5c100 @name="Janko">
p2 = Person.new('Janko')
#=> #<Person:0x00000001b556e8 @name="Janko">
family = []
family << p1
family << p2
family.uniq
#=> [#<Person:0x00000001b5c100 @name="Janko">]

13. Základné koncepty
inheritance (dedenie)
method overloading (preťažovanie metód)
method overriding (prekonávanie metód)

14. Dedenie
class Instrument
def tune
...
end
end
class Violin < Instrument
def play
tune
"Taa daa"
end
end

15. Overloading v ruby nefunguje
class Instrument
def play
"Ta da"
end
def play(octave)
"Ta da #{octave}"
end
end
i = Instrument.new
i.play #=> ArgumentError: wrong number of arguments (0 for 1)
i.play('c dur') #=> "Ta da c dur"

16. Overriding
class Instrument
def play
"Ta da"
end
end
class Violin < Instrument
def play
"Tiii diii tiii"
end
end
v = Violin.new
v.play #=> "Tiii diii tiii"

17. Polymorfia
class Instrument
def play
end
end
class Violin < Instrument
def play
"Taa daa"
end
end
class Drum < Instrument
def play
"Dum dum"
end
end
orchestra = [Violin.new, Violin.new, Drum.new]
orchestra.each { |i| i.play }
#=> "Taa daa", "Taa daa", "Dum dum"

18. Všetko v Ruby je Objekt

19. Aj nil
Object.class #=> Class
nil.class #=> NilClass

20. ActiveSupport try
p = Person.find(params[:id])
p.address.downcase
#=> NoMethodError: undefined method `downcase'
# for nil:NilClass
p.address.try :downcase #=> nil

21. Aj metóda
class Person
def greet(other_person)
"Ahoj #{other_person}"
end
end
m = Person.instance_method(:greet)
m.class #=> UnboundMethod
m.arity #=> 1
m.name #=> :greet
m.parameters #=> [[:req, :other_person]]

22. Aj trieda
Class.class #=> Class
.. o metaprogramovaní inokedy

23. Moduly
def Class < Module

24. Modul ako namespace
module Rubyslava
class OOP
end
end
module Pyvo
class OOP
end
end
Rubyslava::OOP == Pyvo::OOP #=> false

25. Modul ako nositeľ kódu
module Rubyslava
def hello
"Rubyslava"
end
end

26. Mixiny
class RubyslavaAPyvo
include Rubyslava
end
rp = RubyslavaAPyvo.new
rp.hello #=> "Rubyslava"
class RubyslavaAPyvo
extend Rubyslava
end
RubyslavaAPyvo.hello #=> "Rubyslava"

27. Viacnásobné dedenie
class RubyslavaAPyvo < Array
include Rubyslava, Pyvo
end
RubyslavaAPyvo.ancestors
#=> [RubyslavaAPyvo, Rubyslava, Pyvo
# Array, Object, Kernel, BasicObject]

28. Viditeľnosť v moduloch
moduly zdieľajú všetko
inštančné premenné (@)
metódy

29. module AdditionalBehaviour
def show
greet
end
end
class Test
include AdditionalBehaviour
def greet
"hello"
end
end
Test.new.show #=> "hello"

30. module GreetingBehavior
def greet
"hello"
end
end
module RelyOnGreetingBehavior
def show
greet
end
end
class Test
include GreetingBehavior, RelyOnGreetingBehavior
end
Test.new.show #=> "hello"

31. Zo života
Enumerable
acts_as_..
ActiveSupport::Concern

32. class Tree
include Enumerable
def each(&block)
leafs.each do { |n| yield n }
end
end
t = Tree.new
t.map
t.inject
t.sum
...

33. class Post
acts_as_taggable
end
p = Post.find(1)
p.tag_list #=> ["Ruby", "OOP"]

34. ActiveSupport::Concern
module Annotatable
extend ActiveSupport::Concern
included do
has_many :notes, :as => :annotatable,
:order => "created_at DESC"
end
def most_annotated
joins(:notes).group(:id).order("COUNT(*) DESC").first
end
end
class Project < ActiveRecord::Base
include Annotatable, Importable, Versionable
end

35. Idiómy
jazykovo špecifické vzory

36. Ruby idiómy
bloky
a, b = b,a
a, b, c = array
(cachovanie) @var ||= ..

37. Bloky
transaction do |transaction|
transaction.rollback if error?
end
Dir.chdir('/') do
end
with_disabled_keys do
end
with_nested_loop_plan do
end

38. Vzory
Všeobecné riešenie často sa opakujúceho
problému. Nie je to knižnica, ani kód, skôr
šablóna.

39. "When I see patterns in
my programs, I consider it
a sign of trouble."
(Paul Graham)

40. Vzory, ktoré v Ruby
netreba programovať

41. Singleton
Problém: Zabezpečiť jednotný prístup k zdrojom
(napr. databáza)
Zlo, česť výnimkám (ActiveSupport::Inflections)

42. Singleton
class Logger
def initialize
@log = File.open("log.txt", "a")
end
@@instance = Logger.new
def self.instance
@@instance
end
def log(msg)
@log.puts(msg)
end
private_class_method :new
end
Logger.instance.log('message 1')

43. Singleton (lepšie)
require 'singleton'
class Logger
include Singleton
def initialize
@log = File.open("log.txt", "a")
end
def log(msg)
@log.puts(msg)
end
end
Logger.instance.log('message 2')

44. Iterator
Problém: Umožniť manipuláciu s kompozitným
objektom bez odhalenia jeho internej štruktúry

45. Iterator
class Tree
include Enumerable
def each(&block)
leafs.each { |e| yield e }
end
end

46. Decorator
Problém: pridávanie funkcionality do triedy
počas behu programu
Dekorátor obalí pôvodnú triedu a kde treba
rozšíri správanie

47. class StringDecorator < String
def starts_with? substr
# ...
end
end
title = StringDecorator.new("Rubyslava")

48. Decorator = open classes
class String
def starts_with? substr
# ...
end
end
"aye aye captain".starts_with? "pirate"
#=> false

49. Delegator
Problém: Skrytie vnútorných závislostí

50. Delegator
include 'forwardable'
class Car
extend Forwardable
def_delegators :@car_computer, :velocity,
:distance
def initialize
@car_computer = CarComputer.new
end
end
c = Car.new
c.velocity
c.distance

51. Proxy
Problém: Skrytie implementačných detailov
načítavania zdroja

52. Proxy
require 'delegate'
class Future < SimpleDelegator
def initialize(&block)
@_thread = Thread.start(&block)
end
def __getobj__
__setobj__(@_thread.value) if @_thread.alive?
super
end
end
google = Future.new do
Net::HTTP.get_response(URI('http://www.google.com')).body
end
yahoo = Future.new do
Net::HTTP.get_response(URI('http://www.yahoo.com')).body
end
puts google
puts yahoo

53. Ďalšie užitočné vzory

54. Template method
Problém: Často sa opakujúci boilerplate kód.
Kostra programu, algoritmu.

55. class Game
def play
prepare_board
initialize_score
while not end_of_game?
make_move
end
end
end
class Carcassonne < Game
def make_move
tile = tiles.pick_random
...
end
end
class Agricola < Game
def make_move
peasant = select_peasant
...
end
end

56. Composite
Problém: V stromových štruktúrach
zamaskovať implementačný detail:
uzly a listy.

57. class Person
def say
"#{@name}"
end
end
class Family
attr_accessor :members
def initialize(members)
@members = members
end
def say
@members.each { |m| m.say }
end
end
f = Family.new(Person.new('Janko'), Person.new('Tomas'))
t = Family.new(Person.new('Ferko'), f)
t.say #=> "Ferko", "Janko", "Tomas"

58. Strategy
Problém: Výber vhodného algoritmu
za behu programu

59. class FileLogger < Logger
def log(message)
File.open('a') { |f| f << message }
end
end
class DatabaseLogger < Logger
def log(message)
Log.create(:message => message)
end
end

60. State
Problém: Zmena správania objektu
v rôznych stavoch

61. class TrafficLight
include AlterEgo
state :proceed, :default => true do
handle :color do
"green"
end
transition :to => :caution, :on => :cycle!
end
state :caution do
handle :color do
"yellow"
end
transition :to => :stop, :on => :cycle!
end
state :stop do
handle :color do
"red"
end
transition :to => :proceed, :on => :cycle!
end
end

62. light = TrafficLight.new
light.color # => "green"
light.cycle!
light.color # => "yellow"
light.cycle!
light.color # => "red"
light.cycle!
light.color # => "green"

63. Observer
Problém: Oddelenie core funkcionality
a špecifického správania v určitých
stavoch objektu.

64. class Comment < ActiveRecord::Base
end
class CommentObserver < ActiveRecord::Observer
def after_save(comment)
Notifications.deliver_comment("admin@do.com",
"New comment was posted", comment)
end
end

65. Zápachy v kóde
(Bad smells)

66. Vytváranie vlastného
typového systému
is_a?, kind_of?
lepšie je respond_to?

67. class Container
def add(node)
if node.kind_of? Array
node.each { |n| @nodes << n }
else
@nodes << node
end
end
end
container.add(Tree.new('a', 'b')) #=> ???

68. class Container
def add(node)
unless node.respond_to? :each
node = [node]
end
node.each { |n| @nodes << n }
end
end
container.add(Tree.new('a', 'b')) #=> ['a', 'b']

69. Protipríklad
ActiveRecord::sanitize_sql_for_assignment
def sanitize_sql_for_assignment(assignments)
case assignments
when Array
sanitize_sql_array(assignments)
when Hash
sanitize_sql_hash_for_assignment(assignments)
else assignments
end
end

70. Visitor + typy
def visit object
method = "visit_#{object.class.name}"
send method, object
end
module Arel
module Visitors
class WhereSql < Arel::Visitors::ToSql
def visit_Arel_Nodes_SelectCore o
"WHERE #{o.wheres.map { |x| visit x }.join ' AND ' }"
end
end
end
end

71. Shotgun surgery
ak niečo mením tak musím na viacerých
miestach
crosscutting concerns
AOP (AspectJ)

72. AspectJ
pointcut writableHeaderMethods() :
execution(* (WritableRequestHeader|| WritableResponseHeader) .*(..));
before(HeaderWrapper header) :
writableHeaderMethods() && this(header) {
...
}

73. ActiveRecord::Observer
ActiveController::Caching::Sweeper
before/after/around filtre

74. Caching
class DocumentController < ApplicationController
caches_page :show
caches_action :index
def show; end
def index; end
end

75. Autorizácia
class DocumentController < ApplicationController
before_filter :find_document
def show
# render @document
end
private
def find_document
@document = Document.find_by_id(params[:id])
end
end

76. class DocumentController < ApplicationController
before_filter :find_document
around_filer :authorize
def show
# render @document
end
private
def find_document
@document = Document.find_by_id(params[:id])
end
def authorize
if current_user.can_view?(@document)
yield
else
render :not_authorized
end
end
end

77. Ťažko testovateľná trieda
skúsiť si najprv napísať test
rcov

78. Duplicitný kód
reek
metric_fu
rubymine
refaktoring
vzor Template

79. Často sa meniaci kód
churn

80. Dlhá metóda

81. Dlhý zoznam parametrov
*args
nahradenie objektom
fluent interfaces

82. def params(*args)
puts args.class
puts args.inspect
end
params('a', {:hello => 2}, 42)
#=> Array
#=> ["a", {:hello=>2}, 2]

83. activerecord 2.x
vs
activerecord 3.x
Visit.first(:select => "happened_at",
:order => "happened_at ASC").happened_at
Visit.select(:happened_at)
.order("happened_at ASC").first.happened_at

84. Primitívna obsesia
class Debt < ActiveRecord::Base
composed_of :debit,
:class_name => "Money",
:allow_nil => true,
:mapping => [%w(debit_amount amount),
%w(debit_currency currency)]
end
debt.debit # => Money

85. switch/case
Template polymorfia
<% subject.infos.each do |info| %>
<%= render :partial => "#{info.class.to_s}_detail",
:subject => subject %>
<% end %>

86. Dátová trieda
Skinny controller, fat model

87. Dátová trieda
class SubjectController < ApplicationController
def show
if @subject.updated_at < Time.now - 2.weeks.ago
@subject.refresh
end
end
end
class Subject < ActiveRecord::Base
end

88. Dátová trieda 2
class SubjectController < ApplicationController
def show
@subject.update_if_stale
end
end
class Subject < ActiveRecord::Base
def update_if_stale
refresh if updated_at < Time.now - 2.weeks.ago
end
end
class Person < Subject
def update_if_stale
# update vsetky firmy kde je subjekt
end
end

89. Komentáre
# download new version of article
def perform(x)
# load article from the url 'x'
...
end
def download_article(url)
...
end

90. SOLID Principles

91. Single responsibility
principle
Objekt by mal mať iba jednu zodpovednosť
observers
cache sweepers
moduly (include, extend)

92. Open/closed principle
Triedy by mali byť otvorené pre rozširovanie,
ale uzavreté pre modifikáciu
v ruby tomu ťažko zabrániť

93. Liskov substitution principle
Každý nadtyp by mal byť nahraditeľný
podtypom
kruh/kružnica
čo s polomerom?

94. Interface segregation
principle
Hierarchia v rámci rozhraní - radšej viac
špecifických rozhraní ako jedno obrovské
v ruby nemá zmysel

95. Dependency inversion
principle
Závislosť musí byť na rozhraní, nie na
konkrétnej implementácii
dependency injection

96. Domain driven design
ambiguous language
Array#first, Array#second, Array#forty_two

97. class Subject < ActiveRecord::Base
has_many :debts
def is_suspicious?
debts.any? or in_liquidation?
end
end

98. Záver
With great power comes great responsibility
(Spidermanov otec)