Why You Don't Get Mock Objects

Although the Ruby community has embraced TDD like no other community ever has, we have always looked at mock objects with disdain, and perhaps even a little hatred. I've heard conference speakers call them "Wastes of time", "Scams", and even "Testing Heresies". Why would anyone have ever developed these pieces of junk? In reality though many in the agile community have embraced mock objects within their testing cycles because they have found using these fake objects improves the design of their systems. But why not Rubyists? Most Rubyists don't get mock objects because they don't understand their history or their creators intent. They try to fit them into their current workflow without understanding them, and find them unhelpful and stupid. After all, almost all of the good literature on the subject is written in Java, and we know how frustrating it is to read Java when your used to Ruby. As such, this talk will attempt to demonstrate in Ruby the usefulness of mock objects, and how to use them to improve the design of your system. Specifically the following will be covered:
* Why do we need mock objects (Following the 'Tell, Don't Ask Principle')
* Why you should only mock objects you own
* Why you shouldn't use mocks to test boundary objects like external API calls
* Why you should mock roles, not Objects
* Why you should only mock your immediate neighbors
* Why listening to your unit tests will tell you about design problems in your code

Transcript

1. Gregory Moeck Why You Don’t Get Mock Objects @gregmoeck Thursday, September 29, 11
2. Introduction Thursday, September 29, 11
3. Me Thursday, September 29, 11
4. I Am Not A Primary Teacher Thursday, September 29, 11
5. I Am A Secondary Teacher Thursday, September 29, 11
6. “[The secondary teacher] should regard himself as learning from the masters along with his [students]. He should not act as if he were a primary teacher, using a great book as if it were just another textbook of the sort one of his colleagues might write. He should not masquerade as one who knows and can teach by virtue of his original discoveries...The primary sources of his own knowledge should be the primary sources of learning for his students, and such a teacher functions honestly only if he does not aggrandize himself by coming between the great books and their ... readers. He should not “come between” as a nonconductor, but he should come between as a mediator-as one who helps the less competent make more effective contacts with the best minds. Mortimer Adler, How to Read a Book, p. 60 Thursday, September 29, 11
7. Thursday, September 29, 11
8. Mock Objects Thursday, September 29, 11
9. The Ruby Community + Mock Objects = ? Thursday, September 29, 11
10. Love? Thursday, September 29, 11
11. Love? X Thursday, September 29, 11
12. Mock Objects Have Been Called: Thursday, September 29, 11
13. “Wastes Of Time Thursday, September 29, 11
14. “ Scams Thursday, September 29, 11
15. “ Testing Heresies Thursday, September 29, 11
16. Why? Thursday, September 29, 11
17. 1. Duplicate Implementation Thursday, September 29, 11
18. describe ShowController, "index" do context "when a TV show has no public videos" do it “should not display any shows” do Show.should_receive(:all) .with(:select => “id, name, video_count”, :conditions => “shows.video_count > 0”) .and_return([]) get ‘index’ response.body.should_not(match(/ #{@show.name}) end end end Francis Hwang’s 2008 RubyConf talk “Testing Heresies”. Thursday, September 29, 11
19. class ShowController def index @shows = Show.all( :select => “id, name, video_count”, :conditions =>“shows.video_count > 0” ) end end Thursday, September 29, 11
20. describe ShowController, "index" do context "when a TV show has no public videos" do it “should not display any shows” do Show.should_receive(:all) .with(:select => “id, name, video_count”, :conditions => “shows.video_count > 0”) .and_return([]) get ‘index’ response.body.should_not(match(/ #{@show.name}) end end end Thursday, September 29, 11
21. class ShowController def index @shows = Show.all( :select => “id, name, video_count”, :conditions =>“shows.video_count > 0” ) end end Thursday, September 29, 11
22. 2. Lead To Brittle Tests That Do A Poor Job Thursday, September 29, 11
23. module Codebreaker describe Game do describe "#start" do it "sends a welcome message" do output = double('output') game = Game.new(output) output.should_receive(:puts) .with('Welcome to Codebreaker!') game.start end end end end Nick Gauthier in his blog post “Everything that is wrong with Mock Objects” Thursday, September 29, 11
24. module Codebreaker class Game def initialize(output) @output = output end def start @output.puts(“Welcome to Codebreaker”) end end end Thursday, September 29, 11
25. Refactor To The Following: Thursday, September 29, 11
26. module Codebreaker class Game def initialize(output) @output = output end def start @output.writes(“Welcome to Codebreaker”) end end end Thursday, September 29, 11
27. The Tests Fail Thursday, September 29, 11
28. The Output Does Not Change Thursday, September 29, 11
29. QED Thursday, September 29, 11
30. Mocks Suck! Thursday, September 29, 11
31. Maybe Thursday, September 29, 11
32. Or Maybe You Just Don’t Understand Them Tell story about how I was going to give a presentation on testing, and though about slamming Mock Objects. Thursday, September 29, 11
33. Why Mocks? Thursday, September 29, 11
34. Mock Objects + Procedural Programming = Bad Idea Thursday, September 29, 11
35. Mocks Are Not Stubs Thursday, September 29, 11
36. Mocks Assert On Messages Thursday, September 29, 11
37. Stubs Return Values Thursday, September 29, 11
38. describe ShowController, "index" do context "when a TV show has no public videos" do it “should not display any shows” do Show.should_receive(:all) .with(:select => “id, name, video_count”, :conditions => “shows.video_count > 0”) .and_return([]) get ‘index’ response.body.should_not(match(/ #{@show.name}) end end end Thursday, September 29, 11
39. describe ShowController, "index" do context "when a TV show has no public videos" do it “should not display any shows” do Show.should_receive(:all) .with(:select => “id, name, video_count”, :conditions => “shows.video_count > 0”) .and_return([]) get ‘index’ response.body.should_not(match(/ #{@show.name}) end end end Thursday, September 29, 11
40. describe ShowController, "index" do context "when a TV show has no public videos" do it “should not display any shows” do Show.should_receive(:all) .with(:select => “id, name, video_count”, :conditions => “shows.video_count > 0”) .and_return([]) get ‘index’ response.body.should_not(match(/ #{@show.name}) end end end Thursday, September 29, 11
41. describe ShowController, "index" do context "when a TV show has no public videos" do it “should not display any shows” do Show.should_receive(:all) .with(:select => “id, name, video_count”, :conditions => “shows.video_count > 0”) .and_return([]) get ‘index’ response.body.should_not(match(/ #{@show.name}) end end end Thursday, September 29, 11
42. Mock Objects + OOP = Good Idea Thursday, September 29, 11
43. Key Idea In OOP: Thursday, September 29, 11
44. Objects Tell Objects To Do Things Thursday, September 29, 11
45. “The big idea is “messaging” -- that is what the kernal of Smalltalk/Squeak is all about...The key in making great and growable systems is much more to design how its modules communicate rather than what their internal behaviors should be. Alan Kay, Email Message Sent to the Squeak Mailing List. http://lists.squeakfoundation.org/pipermail/squeak-dev/1998-October/017019.html Thursday, September 29, 11
46. Thursday, September 29, 11
47. Thursday, September 29, 11
48. Change The Behavior Of The System By Composing Objects Thursday, September 29, 11
49. (Contrived) Example: Thursday, September 29, 11
50. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Thursday, September 29, 11
51. Imagine You Want To Save The Requests Before Sending Them Thursday, September 29, 11
52. Procedural Programming: Add Code To The Method Thursday, September 29, 11
53. OO Programming: Change Object Composition Thursday, September 29, 11
54. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Persist Ticket Requests Reserve Tickets Thursday, September 29, 11
55. What Would The Code For The Interface Look Like? Thursday, September 29, 11
56. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
57. Notice Two Things: Thursday, September 29, 11
58. 1. Follows Tell, Don’t Ask Principle Thursday, September 29, 11
59. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
60. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
61. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
62. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
63. 2. Hides It’s Internal State Thursday, September 29, 11
64. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
65. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
66. How Can You Assert On State? Thursday, September 29, 11
67. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = "" end def number_pressed(number) @current_display += number.to_s end def delete_pressed @current_display.chop! end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
68. You Would Have To Add Getters Just For Tests Thursday, September 29, 11
69. Bad Idea! Thursday, September 29, 11
70. How Then Do You Test? Thursday, September 29, 11
71. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Remember what Alan Kay said, it’s all about the messages Thursday, September 29, 11
72. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Thursday, September 29, 11
73. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Assert On The Message Thursday, September 29, 11
74. How? Thursday, September 29, 11
75. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Thursday, September 29, 11
76. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Fake Object Reserve Tickets Instead of talking to a real object, talk to a fake object whose job it is to record all of the messages that it receives, then you can assert on that. Thursday, September 29, 11
77. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Mock Object Reserve Tickets Thursday, September 29, 11
78. describe TicketMachineInterface do it "reserves the number of tickets inputted when the user submits a request" do request_handler = double('request_handler') request_handler.should_receive(:reserve).with(55) machine = TicketMachineInterface.new(request_handler) machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end Thursday, September 29, 11
79. describe TicketMachineInterface do it "reserves the number of tickets inputted when the user submits a request" do request_handler = double('request_handler') request_handler.should_receive(:reserve).with(55) machine = TicketMachineInterface.new(request_handler) machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end Thursday, September 29, 11
80. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Mock Object Reserve Tickets Thursday, September 29, 11
81. describe TicketMachineInterface do it "reserves the number of tickets inputted when the user submits a request" do request_handler = double('request_handler') request_handler.should_receive(:reserve).with(55) machine = TicketMachineInterface.new(request_handler) machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end Thursday, September 29, 11
82. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Mock Object Reserve Tickets Thursday, September 29, 11
83. describe TicketMachineInterface do it "reserves the number of tickets inputted when the user submits a request" do request_handler = double('request_handler') request_handler.should_receive(:reserve).with(55) machine = TicketMachineInterface.new(request_handler) machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end Thursday, September 29, 11
84. describe TicketMachineInterface do it "reserves the number of tickets inputted when the user submits a request" do request_handler = double('request_handler') request_handler.should_receive(:reserve).with(55) machine = TicketMachineInterface.new(request_handler) machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end Thursday, September 29, 11
85. Key Idea Thursday, September 29, 11
86. In OOP, Behavior Is Found In Messages Thursday, September 29, 11
87. Mocks Assert On Messages Talk about how tools are made to be used a certain way. A screwdriver might work as a hammer if you have no other tool, but it works better as a screwdriver. Thursday, September 29, 11
88. Key Mocking Rules Thursday, September 29, 11
89. Mock Roles, Not Objects Thursday, September 29, 11
90. Hhttp://www.jmock.org/oopsla2004.pdf Thursday, September 29, 11
91. Wanting To Mock Concrete Objects Is A Design Smell Thursday, September 29, 11
92. Well Designed Objects Don’t Know Who They’re Talking To Thursday, September 29, 11
93. Why? Thursday, September 29, 11
94. Because Who They’re Talking To Can Change Thursday, September 29, 11
95. They Should Only Know The ROLE Their Collaborator Is Playing Thursday, September 29, 11
96. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Thursday, September 29, 11
97. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Ticket Machine Request Handler Thursday, September 29, 11
98. Ticket Machine Interface Number Pressed Delete Pressed Submit Request Ticket Reserve System Reserve Tickets Persist Ticket Requests Reserve Tickets Ticket Machine Request Handler Thursday, September 29, 11
99. When Mocking Roles, TDD Becomes A Design Process Thursday, September 29, 11
100. class TicketMachineInterface def number_pressed(number) end def delete_pressed end def submit_request end end I ask myself what is a scenario that describes the domain logic for this implementation of this role. Thursday, September 29, 11
101. describe TicketMachineInterface do it "reserves the correct number of tickets when a number is pressed two times before submitting" do machine = TicketMachineInterface.new machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end This is the scenario, so I then ask myself what is the responsibility of THIS object, and what is not. I decide that this object is not responsible for handling the request, just for sending it. Thursday, September 29, 11
102. describe TicketMachineInterface do it "reserves the correct number of tickets inputted when the user submits a request" do request_handler = double('request_handler') request_handler.should_receive(:reserve).with(55) machine = TicketMachineInterface.new(request_handler) machine.number_pressed(5) machine.number_pressed(5) machine.submit_request end end So I create a mock object for the role that this object’s peer will play. Thursday, September 29, 11
103. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler end def number_pressed(number) end def delete_pressed end def submit_request @request_handler.reserve(@current_display) end end Thursday, September 29, 11
104. class TicketMachineInterface def initialize(request_handler) @request_handler = request_handler @current_display = ‘’ end def number_pressed(number) @current_display += number.to_s end def delete_pressed end def submit_request @request_handler.reserve(@current_display.to_i) end end Thursday, September 29, 11
105. Only Mock Types You Own Thursday, September 29, 11
106. http://www.mockobjects.com/2008/11/only-mock-types-you- own-revisited.html http://www.jmock.org/oopsla2004.pdf Thursday, September 29, 11
107. If You Don’t Own The API, There Is No Design Feedback Thursday, September 29, 11
108. Instead Of Mocking A Role, Your Mocking An Implementation Thursday, September 29, 11
109. Duplicating Your Production Code In Your Test Is A Test Smell Thursday, September 29, 11
110. This Means Don’t Mock Boundary Objects Thursday, September 29, 11
111. module Codebreaker describe Game do describe "#start" do it "sends a welcome message" do output = double('output') game = Game.new(output) output.should_receive(:puts) .with('Welcome to Codebreaker!') game.start end end end end Thursday, September 29, 11
112. module Codebreaker class Game def initialize(output) @output = output end def start @output.puts(“Welcome to Codebreaker”) end end end Thursday, September 29, 11
113. What Should We Do? Thursday, September 29, 11
114. Mock The Role In The Domain Object Thursday, September 29, 11
115. module Codebreaker describe Game do describe "#start" do it "displays a welcome message" do displayer = double('displayer') game = Game.new(displayer) displayer.should_receive(:display) .with('Welcome to Codebreaker!') game.start end end end end Thursday, September 29, 11
116. module Codebreaker class Game def initialize(displayer) @displayer = displayer end def start @displayer.display(“Welcome to Codebreaker”) end end end Thursday, September 29, 11
117. Implement The Display Object Using Puts Thursday, September 29, 11
118. How Is This Better? The point of the object is not to output to the command line, but to display the welcome. That could be to a GUI or a CLI, or something else. That is not the object’s responsibility to know. Thursday, September 29, 11
119. What About Testing The Display Object? Thursday, September 29, 11
120. Integration Tests / Acceptance Tests Thursday, September 29, 11
121. Only Mock Peers, Not Internals Thursday, September 29, 11
122. http://groups.google.com/group/growing-object-oriented- software/browse_thread/thread/ 5e84aa01d5e8854/8599991b3b1a390b?lnk=gst&q=internal +peer#8599991b3b1a390b “Growing Object Oriented Software Guided By Tests” Book Thursday, September 29, 11
123. Decide What Is Inside, And What Is Outside Your Object This is part of the DESIGN process! You have to decide where the border of your encapsulation is going to be. Thursday, September 29, 11
124. Not Everything Belongs To A Peer Thursday, September 29, 11
125. Key Question: Thursday, September 29, 11
126. Is This My Role? Thursday, September 29, 11
127. Example: Thursday, September 29, 11
128. Auction Server Auction Message Translator Process Message Thursday, September 29, 11
129. Different Types Of Messages Come In Thursday, September 29, 11
130. Translator Translates Them Into Domain Thursday, September 29, 11
131. describe AuctionMessageTranslator do it “notifies bid details when current price message received” do listener = double(‘event_listener’) listener.should_receive(:current_price) .with(192, 7) translator = AuctionMessageTranslator.new(listener) message = Message.new message.set_body(“SOLVersion: 1.1; Event: Price; CurrentPrice: 192; Increment: 7; Bidder: Someone else”) translator.process_message(message) end end Thursday, September 29, 11
132. class AuctionMessageTranslator def initialize(listener) @listener = listener end def process_message(message) event = unpack_event_from(message) if event.type == “CLOSE” @listener.auction_closed else @listener.current_price(event.current_price, event.increment) end end private: def unpack_event(message) AuctionMessageEvent.new(message) end end Thursday, September 29, 11
133. class AuctionMessageTranslator def initialize(listener) @listener = listener end def process_message(message) event = unpack_event_from(message) if event.type == “CLOSE” @listener.auction_closed else @listener.current_price(event.current_price, event.increment) end end private: def unpack_event(message) AuctionMessageEvent.new(message) end end Thursday, September 29, 11
134. class AuctionMessageTranslator def initialize(listener) @listener = listener end def process_message(message) event = unpack_event_from(message) if event.type == “CLOSE” @listener.auction_closed else @listener.current_price(event.current_price, event.increment) end end private: def unpack_event(message) AuctionMessageEvent.new(message) end end Thursday, September 29, 11
135. class AuctionMessageTranslator def initialize(listener) @listener = listener end def process_message(message) event = unpack_event_from(message) if event.type == “CLOSE” @listener.auction_closed else @listener.current_price(event.current_price, event.increment) end end private: def unpack_event(message) AuctionMessageEvent.new(message) end end Thursday, September 29, 11
136. class AuctionMessageTranslator def initialize(listener) @listener = listener end def process_message(message) event = unpack_event_from(message) if event.type == “CLOSE” @listener.auction_closed else @listener.current_price(event.current_price, event.increment) end end private: def unpack_event(message) AuctionMessageEvent.new(message) end end Thursday, September 29, 11
137. Internal Object Thursday, September 29, 11
138. Don’t Mock It Thursday, September 29, 11
139. Don’t Stub It Thursday, September 29, 11
140. Why? Thursday, September 29, 11
141. It’s An Implementation Detail Thursday, September 29, 11
142. Further Resources Thursday, September 29, 11
143. Thursday, September 29, 11
144. @gregmoeck gmoeck@gmail.com Questions? Thursday, September 29, 11