Opening up to change

The video for this talk is here: http://andypike.com/blog/conferences/arrrrcamp-2015/

The one constant in software is change.

Your code will need to change and adapt over time. We should try to write code that is easy to change in the future. If possible, change without having to edit existing code.

This talk is all about the O in SOLID: the Open-Closed principle. I'll explain what the Open-Closed principle is, why this is important and how to structure and refactor code to make it "open for extension but closed for modification".

In this talk I'll show you, by example, how to make your code more changeable. In fact, so changeable that you will be able to extend what your program does and how it behaves without modifying a single line of existing code. Starting with a real world example that is painful to extend, I’ll refactor it over many iterations until it truly is Open-Closed. I'll show techniques, trade-offs and some gotchas from real world experience which will help you write more flexible programs in the future.

If you’ve never heard of the Open-Closed principle or are unsure how to put it into practice then this talk is for you. I would love to help open the door of this technique and give you the ability to alter your system in a painless way by opening it up to change.

This talk was given at Arrrrcamp 2015 http://2015.arrrrcamp.be.

Transcript

1. Opening'up'to'change Andy%Pike @andypike
2. Hello @andypike @andypike
3. Say$hi @andypike
4. @andypike
5. @andypike
6. @andypike
7. @andypike
8. @andypike
9. @andypike
10. @andypike
11. @andypike
12. @andypike
13. Changing'so*ware @andypike
14. O.C.P. @andypike
15. @andypike
16. @andypike
17. Open/closed+principle @andypike
18. What%is%it? @andypike
19. @andypike
20. Open%for%extension, but%closed%for%modifica6on @andypike
21. Classes&can&only&be&modified&to&correct& errors.&New&or&changed&features&would& require&that&a&different&class&be& created. @andypike
22. S.O.L.I.D. @andypike
23. Robert'Mar*n (Uncle'Bob) @andypike
24. Why$is$OCP$important? @andypike
25. Risk%&%Cost @andypike
26. Simple'vs'Easy @andypike
27. A"good"design"is"easier"to"change" than"a"bad"design —"Dave"Thomas @andypike
28. Embrace(change @andypike
29. Stop%your%whining! @andypike
30. Slight'Sidetrack @andypike
31. What%is%the%number%one%thing%that% makes%our%programs%harder%to% change?* (*#a%er#extensive#research#and#polling#of#exactly#one#person#7#me) @andypike
32. if @andypike
33. if book_loan.due_on < Date.today && book_loan.returned == false # Do something end # If you can't avoid it, at least name the concept: if book_loan.overdue? # Do something end @andypike
34. Using&OCP @andypike
35. Inheritance @andypike
36. class Car def accelerate end def change_gear end end @andypike
37. class Convertible < Car def open_roof end end # Then we can do this: my_car = Convertible.new my_car.open_roof my_car.accelerate @andypike
38. Basis%of%original open/closed%thinking @andypike
39. Dependency(Injec+on @andypike
40. class Library def loan(book) # Do something interesting log("The book #{book.title} was loaned out") end def log(message) # Write line to file... end end @andypike
41. class FileLogger def write(message) # Write line to file... end end @andypike
42. class Library def initialize(logger = FileLogger.new) @logger = logger end def loan(book) log("The book #{book.title} was loaned out") end def log(message) @logger.write(message) end end @andypike
43. Library.new.loan(book) Library.new(FileLogger.new).loan(book) Library.new(DatabaseLogger.new).loan(book) Library.new(SlackLogger.new).loan(book) @andypike
44. class LogEverywhere def write(message) FileLogger.new.write(message) DatabaseLogger.new.write(message) SlackLogger.new.write(message) end end @andypike
45. When%a%role%needs to%change%just%create a%class%and%pass%it%in @andypike
46. Decorator(Pa*ern @andypike
47. def import_products(file_path) CSV.read(file_path, :headers => true).each do |row| if row.field("name").present? # do something interesting end end end @andypike
48. class ProductRow def initialize(row) @row = row end def valid? @row.field("name").present? end end @andypike
49. require "delegate" class ProductRow < SimpleDelegator def valid? field("name").present? end end @andypike
50. def products_in(file_path) CSV.read(file_path, :headers => true).map do |r| ProductRow.new(r) end end def import_products(file_path) products_in(file_path).each do |row| if row.valid? # do something interesting end end end @andypike
51. def valid_products_in(file_path) CSV.read(file_path, :headers => true) .map { |p| ProductRow.new(p) } .select(&:valid?) end def import_products(file_path) valid_products_in(file_path).each do |row| # do something interesting end end @andypike
52. Useful'when'separa.ng concerns,'or'when'not'in control'of'object'crea.on @andypike
53. Command'Pa)ern @andypike
54. class Image def open(file_path) # Load image file end def blur # Blur the image, return new version end def greyscale # Greyscale the image, return new version end end @andypike
55. # Blur and greyscale an image image = Image.new.open(file_path) image.blur.greyscale @andypike
56. class Blur def apply_to(image) # Blur the image, return new version end end class Greyscale def apply_to(image) # Greyscale the image, return new version end end @andypike
57. class Image def open(file_path) # Load image file end def apply(*commands) commands.reduce(self) do |image, command| command.apply_to(image) end end end @andypike
58. image = Image.new.open(file_path) # instead of this: image.blur.greyscale # you can use this: image.apply(Blur.new, Greyscale.new) @andypike
59. Encapsulate+behaviour+in a+class+and+pass+it+around @andypike
60. Service'Locator'Pa-ern @andypike
61. class NotificationCentre def send_welcome_to(user) send_email(user.email, :welcome) end def send_email(email_address, content) # Uses some service to send emails end end @andypike
62. class NotificationCentre def send_welcome_to(user) if user.notification_method == :sms send_sms(user.mobile, :welcome) else send_email(user.email, :welcome) end end def send_sms(mobile_number, content) # Uses some service to send sms end def send_email(email_address, content) # ... @andypike
63. class NotificationCentre def send_welcome_to(user) if user.notificaton_method == :webhook send_webhook(user.webhook_url, :welcome) elsif user.notificaton_method == :sms send_sms(user.mobile, :welcome) else send_email(user.email, :welcome) end end def send_webhook(url, content) # Make HTTP call to url end @andypike
64. @andypike
65. class EmailNotifier def deliver(user, content) # Send an email to `user.email`... end end @andypike
66. class NotificationCentre def send_welcome_to(user) if user.notificaton_method == :webhook WebhookNotifier.new.deliver(user, :welcome) elsif user.notificaton_method == :sms SmsNotifier.new.deliver(user, :welcome) else EmailNotifier.new.deliver(user, :welcome) end end end @andypike
67. class NotificationCentre def send_welcome_to(user) case user.notificaton_method when :webhook WebhookNotifier.new.deliver(user, :welcome) when :sms SmsNotifier.new.deliver(user, :welcome) else EmailNotifier.new.deliver(user, :welcome) end end end @andypike
68. class NotificationCentre def send_welcome_to(user) notifier_for(user).new.deliver(user, :welcome) end def notifier_for(user) case user.notification_method when :webhook WebhookNotifier when :sms SmsNotifier else EmailNotifier end end end @andypike
69. class NotificationCentre def send_welcome_to(user) notifier_for(user).new.deliver(user, :welcome) end def notifier_for(user) NotifierRegistry.notifier_for(user.notification_method) end end @andypike
70. class NotifierRegistry def self.notifier_for(notification_method) case notification_method when :webhook WebhookNotifier when :sms SmsNotifier else EmailNotifier end end end @andypike
71. class NotifierRegistry def self.notifier_for(notification_method) registry.fetch(notification_method) { EmailNotifier } end def self.registry { :webhook => WebhookNotifier, :sms => SmsNotifier, :email => EmailNotifier } end end @andypike
72. method_options = NotifierRegistry.methods.map { |m| [m.titleize, m] } select :user, :notification_method, method_options @andypike
73. Popula'ng*the*registry @andypike
74. Configura)on @andypike
75. NotifierRegistry.registry = { :webhook => WebhookNotifier, :sms => SmsNotifier, :email => EmailNotifier } @andypike
76. Auto%Register: By%Conven2on @andypike
77. class NotifierRegistry @registry = {} def self.register(notification_method, notifier) @registry[notification_method] = notifier end def self.notifier_for(notification_method) @registry.fetch(notification_method.to_sym) { EmailNotifier } end end @andypike
78. def self.load Dir["./notifiers/*.rb"].each { |file| require file } Module.constants .map(&:to_s) .select { |name| name.end_with?("Notifier") } .each do |name| method = name.chomp("Notifier").downcase.to_sym notifier = Object.const_get(name) register(method, notifier) # => :email, EmailNotifier end end @andypike
79. Auto%Register: Self%Registra1on @andypike
80. class NotifierRegistry @registry = {} def self.register(notifier) @registry[notifier.notification_method] = notifier end def self.notifier_for(notification_method) @registry.fetch(notification_method.to_sym) { EmailNotifier } end end @andypike
81. module SelfRegisteringNotifier def self.included(base) base.send(:extend, ClassMethods) end module ClassMethods attr_accessor :notification_method def send_notifications_via(notification_method) self.notification_method = notification_method NotifierRegistry.register(self) end end end @andypike
82. class EmailNotifier include SelfRegisteringNotifier send_notifications_via :email def deliver(user, content) # Sends an email... end end @andypike
83. Rails&Autoload&Gotcha @andypike
84. # Do not eager load code on boot. config.eager_load = false @andypike
85. def self.notifier_for(notification_method) ensure_loaded @registry.fetch(notification_method.to_sym) { EmailNotifier } end def ensure_loaded return if already_loaded? Dir["./notifiers/*.rb"].each { |file| require_dependency(file) } loaded! end def self.already_loaded? loaded? || Rails.configuration.eager_load end @andypike
86. Demo @andypike
87. @andypike
88. @andypike
89. We#extended#behaviour without#a#single#modifica7on! @andypike
90. Popula'ng*the*Registry @andypike
91. Look$up$behaviour$at$run0me. When$change$comes,$register a$new$class. @andypike
92. Balance @andypike
93. It#may#seem#like#overkill#in#the# beginning,#but#as#the#codebase#ages# you#will#reap#the#rewards#of#good# object#design. —"Andy"Pike @andypike
94. Yes,%I%just%quoted%myself @andypike
95. More%than%just%design%pa1erns @andypike
96. Code%that%is%easier%to understand%is%easier%to%change @andypike
97. Code%is%read%many%more -mes%than%it%is%wri1en @andypike
98. We#follow#exis+ng#code#styles @andypike
99. Did$you$spot$a$theme? @andypike
100. Rails:'Err,'where'do'I'put'them? @andypike
101. Summary @andypike
102. Things'to'consider • Create'Classes • SOLID • Simple'vs'Easy • Balance @andypike
103. My#Guidelines • Name&things&well • Make&things&small • Avoid&condi6onals • Avoid&nils @andypike
104. We're%hiring! @andypike
105. @andypike @andypike
106. Credits • Bertrand_Meyer,Photo:,By,David.Monniaux,(Own,work),[CC,BY? SA,3.0,(hDp:/ /creaHvecommons.org/licenses/by?sa/3.0),or,GFDL, (hDp:/ /www.gnu.org/copyleQ/fdl.html)],,via,Wikimedia, Commons • Simple,vs,Easy,graph:,hDps:/ /blog.wearewizards.io/some?a? priori?good?qualiHes?of?soQware?development • Robert,MarHn,photo:,By,Tim?bezhashvyly,(Own,work),[CC,BY?SA, 4.0,(hDp:/ /creaHvecommons.org/licenses/by?sa/4.0)],,via, @andypike