SOLID Principles Through Tests

We care about writing quality code, we have read the definition of SOLID principles several times and we know how important they are for writing good OO code, but are we really following those principles? Is there a pragmatic way of following them in our day to day jobs or are they just some principles a few computer scientists wrote?

Fortunately there is, SOLID principles are not just good ideas , they are intended to help us write better code, enjoy our jobs more and be happy programmers. So, where should we start? We should start where we always do. By writing tests, yes, for real.

As Kent Beck says "TDD doesn't drive good design. TDD gives you immediate feedback about what is likely to be bad design", so we need to go a step further. In this talk we will see how writing tests is not just doing TDD is about having good test coverage, it's also about driving our code towards good design, one that follows SOLID principles.

Presented at: LA RubyConf 2014

Transcript

1. SOLID Principles Through Tests
2. Sebastian Sogamoso @sebasoga
4. Are you doing TDD?
5. Does that mean your code is easy to maintain?
6. Not necessarily
7. “TDD doesn't drive good design. TDD gives you immediate feedback about what is likely to be bad design” Kent Beck
8. Listen to your tests
9. Tests are the first client of the system
10. If testing is painful, there’s a problem with the design
11. Making code testable doesn’t necessarily makes the code better
12. “Good design makes a class more testable” Michael Feathers
13. Solving your design problems also solves your testing problems
14. That is what SOLID principles are good for
15. Set of 5 object design level principles
17. Single responsibility Open closed Liskov Substitution Interface segregation Dependency inversion
18. Avoid having code that’s rigid, fragile, immobile and viscous
19. =
20. Write code that will help you save time and money
21. Requirement
22. We need to model a shipping method for an e-commerce system
23. Which should calculate the cost of shipping a given order
24. Based on: - The amount of items to ship, excluding digital items - A fixed rate that depends on the amount of items - The distance between origin and destination
25. Rspec code incoming
26. describe Shipping::StandardMethod do describe "#cost" do context "when all items in the order are digital" do…end context "when only one item in the order is not digital" do…end context "when more than one item in the order are not digital" do…end end ... end
27. describe Shipping::StandardMethod do describe "#cost" do context "when all items in the order are digital" do…end context "when only one item in the order is not digital" do context "when the origin and destination are in the same state" do…end context "when the origin and destination are in the same country" do…end context "when the origin and destination are in different countries" do…end end context "when more than one item in the order are not digital" do context "when the origin and destination are in the same state" do…end ! context "when the origin and destination are in the same country" do…end ! context "when the origin and destination are in different countries" do…end end end ... end
28. describe Shipping::StandardMethod do describe "#cost" do let(:items) { double(:items) } let(:order) { double(:order) } let(:origin) { double(:origin).as_null_object } let(:destination) { double(:destination) } subject { Shipping::StandardMethod.new(order, origin, destination) } context "when all items in the order are digital" do it "returns zero" do items.stub(:reject).with(:digital?).and_return([]) expect(subject.cost).to be_zero end end context "when only one item in the order is not digital" do before { items.stub(:reject).with(:digital?).and_return([1]) } context "when the origin and destination are in the same state" do it "returns the correspondent shipping cost" do origin.stub(same_state?: true) expect(subject.cost).to eq (1 * 9.99 * 1) end end context "when the origin and destination are in the same country" do it "returns the correspondent shipping cost" do origin.stub(same_country?: true) expect(subject.cost).to eq (1 * 9.99 * 2) end end context "when the origin and destination are in different countries" do it "returns the correspondent shipping cost" do expect(subject.cost).to eq (1 * 9.99 * 3) end end end context "when more than one item in the order are not digital" do before { items.stub(:reject).with(:digital?).and_return([1]) } context "when the origin and destination are in the same state" do it "returns the correspondent shipping cost" do origin.stub(same_state?: true) expect(subject.cost).to eq (1 * 5.99 * 1) end end ! context "when the origin and destination are in the same country" do it "returns the correspondent shipping cost" do origin.stub(same_country?: true) expect(subject.cost).to eq (1 * 5.99 * 2) end end ! context "when the origin and destination are in different countries" do it "returns the correspondent shipping cost" do expect(subject.cost).to eq (1 * 5.99 * 3) end end end end ... end
29. module Shipping class StandardMethod attr_reader :order, :origin, :destination def cost items_quantity * rate * distance end ... private def items_quantity order.items.reject(:digital?).length end def rate if items_quantity > 1 5.99 else 9.99 end end def distance return 1 if origin.same_state? destination return 2 if origin.same_country? destination 3 end end end
30. def cost items_quantity * rate * distance end
31. def items_quantity order.items.reject(:digital?).length end
32. def rate if items_quantity > 1 5.99 else 9.99 end end
33. def distance return 1 if origin.same_state? destination return 2 if origin.same_country? destination 3 end
34. module Shipping class StandardMethod attr_reader :order, :origin, :destination def cost items_quantity * rate * distance end ... private def items_quantity order.items.reject(:digital?).length end def rate if items_quantity > 1 5.99 else 9.99 end end def distance return 1 if origin.same_state? destination return 2 if origin.same_country? destination 3 end end end
35. Iceberg Class
36. Public API
37. Public API Private methods
40. Single Responsibility
41. A class should have one, and only one, reason to change
42. Extract class?
43. Probably
44. Let the tests show you the answer
45. Wishful testing
46. describe Shipping::StandardMethod do describe "#cost" do let(:order) { double(:order) } let(:origin) { double(:origin) } let(:destination) { double(:destination) } subject { Shipping::StandardMethod.new(order, origin, destination) } context "when all items in the order are digital" do it "returs zero" do order.stub(non_digital_items: 0) expect(subject.cost).to be_zero end end ! context "when at least one item in the order is not digital" do it "calculates the cost of shipping" do Shipping::Rate.stub(:new).with(1) { double(calculate: 10) } Shipping::Distance.stub(:new).with(origin, destination) { double(calculate: 2) } order.stub(non_digital_items: 1) expect(subject.cost).to eq (1 * 10 * 2) end end end ... end
47. context "when all items in the order are digital" do it "returns zero" do order.stub(non_digital_items: 0) expect(subject.cost).to be_zero end end
48. order.stub(non_digital_items: 0)
49. context "when at least one item in the order is not digital" do it "calculates the cost of shipping" do Shipping::Rate.stub(:new).with(1) { double(calculate: 10) } Shipping::Distance.stub(:new).with(origin, destination) { double(calculate: 2) } order.stub(non_digital_items: 1) expect(subject.cost).to eq (1 * 10 * 2) end end
50. Shipping::Rate.stub(:new). with(1) { double(calculate: 10) }
51. Shipping::Distance.stub(:new). with(origin, destination) { double(calculate: 2) }
52. order.stub(non_digital_items: 1)
53. module Shipping class StandardMethod attr_reader :order, :origin, :destination def cost items_quantity * rate * distance end ... private def items_quantity order.non_digital_items end def rate Shipping::Rate.new(items_quantity).calculate end ! def distance Shipping::Distance.new(origin, destination).calculate end end end
54. def cost items_quantity * rate * distance end
55. def items_quantity order.non_digital_items end
56. def rate Shipping::Rate.new(items_quantity).calculate end
57. def distance Shipping::Distance.new(origin, destination).calculate end
58. Liskov Substitution
59. Derived classes must be substitutable for their base classes
60. In Ruby that means that…
61. If it looks like a duck…
63. swims like a duck…
65. then it probably is a duck
67. Duck typing
68. describe Shipping::StandardMethod do describe "#cost" do ... end describe "#packing_days" do ... end describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end describe Shipping::ExpressMethod do describe "#cost" do ... end ! describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end
69. describe Shipping::StandardMethod do describe "#cost" do ... end describe "#packing_days" do ... end describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end describe Shipping::ExpressMethod do describe "#cost" do ... end ! ! ! ! describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end
70. describe Shipping::StandardMethod do describe "#cost" do ... end describe "#packing_days" do ... end describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end describe Shipping::ExpressMethod do describe "#cost" do ... end ! ! ! ! describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end
72. Contract tests
73. Verify whether an implementation respect the contract of the interface that implements
74. shared_examples "a shipping method" do it { should respond_to(:cost) } it { should respond_to(:packing_days) } it { should respond_to(:shipping_estimated_date) } it { should respond_to(:delivery_estimated_date) } end
75. describe Shipping::StandardMethod do ! it_should_behave_like "a shipping method" describe "#cost" do ... end describe "#packing_days" do ... end describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end describe Shipping::ExpressMethod do ! it_should_behave_like "a shipping method" describe "#cost" do ... end ! ! ! ! ! describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end
76. describe Shipping::StandardMethod do ! it_should_behave_like "a shipping method" describe "#cost" do ... end describe "#packing_days" do ... end describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end describe Shipping::ExpressMethod do ! it_should_behave_like "a shipping method" describe "#cost" do ... end ! describe "#packing_days" do ... end ! describe "#shipping_estimated_date" do ... end describe "#delivery_estimated_date" do ... end end
77. Requirement
78. We need to be able to ship an Order using any of the existing shipping methods
79. Existing shipping methods: - Standard - Express - Same day
80. describe Order do describe "#ship" do context "when shipping method is a Shipping::StandardMethod" do … end context "when shipping method is a Shipping::ExpressMethod" do … end context "when shipping method is a Shipping::SameDayMethod" do … end end end
81. class Order attr_reader :seller, :buyer, :items, :method ! def ship case method.class when Shipping::RegularMethod origin = seller.address destination = buyer.shipping_address method.ship(self, origin, destination) when Shipping::ExpressMethod origin = seller.address destination = "#{buyer.shipping_address}, #{buyer.zip}" method.send_items(items, origin, destination) when Shipping::SameDayMethod method.ship(items: items, origin: seller, destination: buyer) end end ... end
82. case method.class when Shipping::RegularMethod origin = seller.address destination = buyer.shipping_address method.ship(self, origin, destination) when Shipping::ExpressMethod origin = seller.address destination = "#{buyer.shipping_address}, #{buyer.zip}" method.send_items(items, origin, destination) when Shipping::SameDayMethod method.ship(items: items, origin: seller, destination: buyer) end
83. when Shipping::RegularMethod origin = @seller.address destination = @buyer.shipping_address @method.ship(self, origin, destination)
84. when Shipping::ExpressMethod origin = @seller.address destination = "#{@buyer.shipping_address}, #{@buyer.zip}" @method.send_items(@items, origin, destination)
85. when Shipping::SameDayMethod @method.ship(items: @items, origin: @seller, destination: @buyer)
86. If a new shipping method was introduced? Would Order class need to change?
87. class Order attr_reader :seller, :buyer, :items, :method ! def ship case method.class when Shipping::RegularMethod origin = seller.address destination = buyer.shipping_address method.ship(self, origin, destination) when Shipping::ExpressMethod origin = seller.address destination = "#{buyer.shipping_address}, #{buyer.zip}" method.send_items(items, origin, destination) when Shipping::SameDayMethod method.ship(items: items, origin: seller, destination: buyer) end end ... end
88. class Order attr_reader :seller, :buyer, :items, :method ! def ship case method.class when Shipping::RegularMethod origin = seller.address destination = buyer.shipping_address method.ship(self, origin, destination) when Shipping::ExpressMethod origin = seller.address destination = "#{buyer.shipping_address}, #{buyer.zip}" method.send_items(items, origin, destination) when Shipping::SameDayMethod method.ship(items: items, origin: seller, destination: buyer) when Shipping::NextDayMethod method.ship(items, seller, buyer) end end ... end
89. when Shipping::NextDayMethod method.ship(items, seller, buyer) end
90. Dependency Inversion
91. Depend on abstractions, not on concretions
92. Open Closed
93. You should be able to extend a classes behavior, without modifying it
94. class Order attr_reader :seller, :buyer, :items, :method ! def ship case method.class when Shipping::RegularMethod origin = seller.address destination = buyer.shipping_address method.ship(self, origin, destination) when Shipping::ExpressMethod origin = seller.address destination = "#{buyer.shipping_address}, #{buyer.zip}" method.send_items(items, origin, destination) when Shipping::SameDayMethod method.ship(items: items, origin: seller, destination: buyer) when Shipping::NextDayMethod method.ship(items, seller, buyer) end end ... end
95. class Order attr_reader :seller, :buyer, :items, :method ! def ship case method.class when Shipping::RegularMethod origin = seller.address destination = buyer.shipping_address method.ship(self, origin, destination) when Shipping::ExpressMethod origin = seller.address destination = "#{buyer.shipping_address}, #{buyer.zip}" method.send_items(items, origin, destination) when Shipping::SameDayMethod method.ship(items: items, origin: seller, destination: buyer) when Shipping::NextDayMethod method.ship(items, seller, buyer) end end ... end
96. describe Order do describe "#ship" do it "ships the order using the shipping method" do items = double(:items) seller = double(:seller) buyer = double(:buyer) method = double(:shipping_method) order = Order.new(items, seller, buyer) order.ship(method) ! expect(method). to have_received(:ship). with(items, seller, buyer) end end end
97. describe Order do describe "#ship" do it "ships the order using the shipping method" do items = double(:items) seller = double(:seller) buyer = double(:buyer) method = double(:shipping_method) order = Order.new(items, seller, buyer) order.ship(method) ! expect(method). to have_received(:ship). with(items, seller, buyer) end end end
98. describe Order do describe "#ship" do it "ships the order using the shipping method" do items = double(:items) seller = double(:seller) buyer = double(:buyer) method = double(:shipping_method) order = Order.new(items, seller, buyer) order.ship(method) ! expect(method). to have_received(:ship). with(items, seller, buyer) end end end
99. class Order attr_reader :items, :seller, :buyer ! def initialize(args) … end def ship(method) method.ship(items, buyer, items) end ... end
100. def ship(method) method.ship(items, buyer, items) end
102. Interface Segregation
103. Make fine grained interfaces that are client specific
104. > ActiveRecord::Base.instance_methods(false).sort => [:<=>, :==, :[], :[]=, :_accessible_attributes, :_accessible_attributes=, :_accessible_attributes?, :_active_authorizer, :_active_authorizer=, :_active_authorizer?, :_attr_readonly, :_attr_readonly?, :_commit_callbacks, :_commit_callbacks=, :_commit_callbacks?, :_create_callbacks, :_create_callbacks=, :_create_callbacks?, :_destroy_callbacks, ...] > ActiveRecord::Base.instance_methods(false).count => 123
105. > ActiveRecord::Base.instance_methods(false).sort => [:<=>, :==, :[], :[]=, :_accessible_attributes, :_accessible_attributes=, :_accessible_attributes?, :_active_authorizer, :_active_authorizer=, :_active_authorizer?, :_attr_readonly, :_attr_readonly?, :_commit_callbacks, :_commit_callbacks=, :_commit_callbacks?, :_create_callbacks, :_create_callbacks=, :_create_callbacks?, :_destroy_callbacks, ...] > ActiveRecord::Base.instance_methods(false).count => 123
106. > ActiveRecord::Base.instance_methods(false).count => 123 huge interface
107. shared_examples "an ActiveRecord::Base instance" do it { should respond_to(:<=>) } it { should respond_to(:==) } it { should respond_to(:[]) } it { should respond_to(:[]=) } it { should respond_to(:_accessible_attributes) } ... end
108. shared_examples "an ActiveRecord::Base instance" do it { should respond_to(:<=>) } it { should respond_to(:==) } it { should respond_to(:[]) } it { should respond_to(:[]=) } it { should respond_to(:_accessible_attributes) } ... end 118 to go
109. shared_examples "an ActiveRecord::Base instance" do it { should respond_to(:<=>) } it { should respond_to(:==) } it { should respond_to(:[]) } it { should respond_to(:[]=) } it { should respond_to(:_accessible_attributes) } ... end
110. > Shipping::StandardMethod.instance_methods(false).sort => [:cost, :packing_days, :shipping_estimated_date, :delivery_estimated_date] ! > Shipping::StandardMethod.instance_methods(false).count => 4
111. > Shipping::StandardMethod.instance_methods(false).sort => [:cost, :packing_days, :shipping_estimated_date, :delivery_estimated_date] ! > Shipping::StandardMethod.instance_methods(false).count => 4
112. > Shipping::StandardMethod.instance_methods(false).count => 4 narrow interface
113. shared_examples "a shipping method" do it { should respond_to(:cost) } it { should respond_to(:packing_days) } it { should respond_to(:shipping_estimated_date) } it { should respond_to(:delivery_estimated_date) } end
114. shared_examples "a shipping method" do it { should respond_to(:cost) } it { should respond_to(:packing_days) } it { should respond_to(:shipping_estimated_date) } it { should respond_to(:delivery_estimated_date) } end
115. Why is this important?
116. Manage dependencies
117. - coupled + cohesive
118. Code that’s easier to maintain
119. =
120. Happier developers
121. SOLID principles are not the holy grail
122. Apply the principles when it makes sense
123. “I'm not a great programmer; I'm just a good programmer with great habits” Kent Beck
124. SOLID principles help you acquire great object design habits
125. @sebasoga Thank you
126. Credits • http://www.flickr.com/photos/pulpolux/192250352/ - Title • http://www.flickr.com/photos/pulpolux/3318002502/ - Single responsibility • http://www.flickr.com/photos/pulpolux/1784671520/ - Liskov substitution • http://www.flickr.com/photos/pulpolux/757214357/ - Open closed • http://www.flickr.com/photos/zdenadel/2422697310/ - Dependency inversion • http://www.flickr.com/photos/pan-o/4987143590/ - Interface segregation • http://www.socwall.com/desktop-wallpaper/19991/underwater-iceberg-by-fanois/ - Iceberg class • http://little--vampire.tumblr.com/post/57019786320 - Titanic movie pic • http://www.dailymail.co.uk/news/article-2114836/Hilarious-DIY-car-repair-bodge-jobs-carried-amateur-mechanics.html - Car with trolley wheel • http://www.etsy.com/listing/172435308/double-sided-open-closed-funny-retail?ref=market - “Closed but still awesome” • http://www.flickr.com/photos/jydesign/2086002622/in/photostream/ - Smileys • http://memegenerator.net/Uncle-Bob - Uncle Bob’s meme • http://www.flickr.com/photos/rivalee/3234174656/ - 3D glasses • http://www.quickmeme.com/Grandma-finds-the-Internet/page/205/ - Store closed meme • http://picyou.com/UFGfN4 - Rose? meme