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Mock Objects and Smalltalk

E
ESUG

The document discusses test-driven development (TDD) techniques using mock objects. It introduces the Mocketry framework for Smalltalk, which allows specifying behavior expectations for mock objects within test scenarios. Mock objects enable cutting dependencies to focus tests on the system under development. They help address problems with novel collaborators by revealing responsibilities through tests before implementation. The document provides examples of using mock objects to enable seamless TDD, avoiding upfront analysis or design speculation.

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Overview §  Why Mock Objects? §  “Mockist” vs. “Classic” TDD §  “Mockist” and “Classic” TDD §  Mocks and Smalltalk: §  The Mocketry framework introduction §  Examples Mock Objects and Smalltalk 1
Why Mock Objects? Do we need Mocks at all (in Smalltalk)? Mock Objects and Smalltalk 2
Public Opinion §  Smalltalk vs. Mock Objects? §  Few/rare special cases §  With mock you don’t test real thing §  Use mocks for external objects only §  Use other means to involve complex external objects §  Speed up by other means Do we need Mock Objects at all? 3
Public Opinion …seems to be about testing 4
Smalltalk and Mock Objects §  “Mock Objects” is a TDD technique §  … about developing systems §  … not just testing §  … useful in all languages §  Smalltalk makes mocks §  much easier to use Why Mock Objects? 5
Why Mock Objects? §  Cut off dependencies in tests §  Test-Driven Decomposition §  Discover Responsibility (for collaborators) §  Thinking vs. Speculating/Fantasizing Seamless TDD Why Mock Objects? 6
What Is The Problem? §  Dependencies §  How to cut them off? §  Novel Collaborators §  Where to cut? Seamless TDD 7
What Is The problem? §  Dependencies §  How to cut them off? §  Novel Collaborators §  Where to cut? Seamless TDD 8
Dependencies We have: §  System Under Development (SUD) §  Collaborators §  Collaborators’ collaborators … Complex Test Seamless TDD — What’s the Problem? 9
So What? We have to implement collaborators §  … without tests §  … loosing focus on SUD Digression Seamless TDD — Dependencies 10
Dependencies: Example §  Mocks Aren’t Stubs by Martin Fowler §  Filling orders from warehouse Seamless TDD — Dependencies 11
Filling Order OrderTests >> ! testIsFilledIfEnoughInWarehouse! ! | order |! order:= Order on: 50 of: #product.! order fillFrom: warehouse.! self assert: order isFilled! Seamless TDD — Dependencies 12
Filling Order OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order warehouse |! ! warehouse := Warehouse new.! “Put #product there” ! “…but how?!” ! ! order := Order on: 50 of: #product.! order fillFrom: warehouse.! …! Seamless TDD — Dependencies 13
Digression Detected! §  I develop Order §  I don’t want to think about Warehouse Seamless TDD — Dependencies 14
Filling Order OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order warehouse |! warehouse := Warehouse new.! warehouse add: 50 of: #product.! ! order := Order on: 50 of: #prod.! order fillFrom: warehouse.! …! Seamless TDD — Dependencies 15
…And Even More Digression Reduce amount of #product at the warehouse test…! …! self assert: ! (warehouse ! amountOf: #product)! isZero! Seamless TDD — Dependencies 16
… And Even More Digression Another test case: If there isn’t enough #product in the warehouse, §  do not fill order §  do not remove #product from warehouse Seamless TDD — Dependencies 17
… Much More Digression More complex test cases Collaborators’ logic becomes more and more complex… This can engulf Seamless TDD — Dependencies 18
Not-So-Seamless TDD §  SUD is Order §  Warehouse blures SUD §  #add:of: §  #amountOf: §  No explicit tests for Warehouse Seamless TDD — Dependencies 19
Mocking Warehouse OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order |! order := Order on: 50 of: #product.! [! :warehouse| ! [order fillFrom: warehouse]! should satisfy:! [“expectations”]! ] runScenario.! self assert: order isFilled ! Seamless TDD — Dependencies 20
Mocking Warehouse …! [:warehouse| ! [order fillFrom: warehouse]! should satisfy:! [(warehouse ! has: 50 of: #product)! willReturn: true. ! warehouse ! remove: 50 of: #product]! ] runScenario.! …! Seamless TDD — Dependencies 21
The Mocketry Framework Mock Objects in Smalltalk World Mock Objects and Smalltalk 22
Behavior Expectations When you do this with SUD, expect that to happen with collaborators Collaborators are mocked Mocketry 23
Behavior Expectations Do this Exercise Expect that Verify Scenario Mocketry 24
Mocketry Scenario Pattern SomeTestCases >> testCase [ testScenario ] runScenario Mocketry – Behavior Expectations 25
Mocketry Scenario Pattern SomeTestCases >> testCase [ [exercise] should strictly satisfy: [behaviorExpectations] ] runScenario Mocketry – Behavior Expectations 26
Behavior Expectations Just send mock objects the messages §  ! they should receive warehouse ! has: 50 of: #product! Specify their reaction §  (warehouse ! has: 50 of: #product) willReturn: true Mocketry — Behavior Expectations 27
Mocketry Scenario Pattern SomeTestCases >> testCase [ [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 28
Mocketry Scenario Pattern SomeTestCases >> testCase [ :mock | [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 29
Mocketry Scenario Pattern SomeTestCases >> testCase [ :mock | [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 30
Mocketry Scenario Pattern SomeTestCases >> testCase [ :mock1 :mock2 :mock3 | [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 31
Trivial Example 1 TrueTests >> testDoesNotExecuteIfFalseBlock [ :block | [true ifFalse: block ] should satisfiy: [“nothing expected”] ] runScenario Mocketry – Behavior Expectations 32
Trivial Example 2 TrueTests >> testExecutesIfTrueBlock [ :block | [true ifTrue: block] should satisfiy: [block value] ] runScenario Mocketry – Behavior Expectations 33
State Specification DSL §  resultObject should <expectation> §  result should be: anotherObject §  result should equal: anotherObject §  … Mocketry 34
Mocketry §  There is much more… §  Ask me §  …or Dennis Kudryashov (the Author) 35
Mocking Warehouse OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order |! order := Order on: 50 of: #product.! ! [:warehouse| ! [order fillFrom: warehouse]! should satisfy:! [(warehouse has: 50 of: #product)! willReturn: true. ! warehouse remove: 50 of: #product]! ] runScenario.! ! self assert: order isFilled ! Seamless TDD — Dependencies — Example 36
Mocking Warehouse OrderTests >> ! testIsNotFilledIfNotEnoughInWarehouse! | order |! order := Order on: #amount of: #product.! [:warehouse| ! [order fillFrom: warehouse]! should satisfy:! [(warehouse has: 50 of: #product)! willReturn: false. ! “Nothing else is expected” ]! ] runScenario.! self assert: order isFilled ! Seamless TDD — Dependencies 37
What We Get §  No need to implement Warehouse §  Just specify expectations §  … right in the test §  Focus on the SUD Why Mock Objects 38
What’s the problem? §  Dependencies §  Novel Сollaborators Seamless TDD 39
Where to Start? A novel system to implement Object 2 Object 1 Object N … Object 3 Seamless TDD — Novel Collaborators 40
Where to Cut? A novel system to implement Feature Feature Feature Feature Feature Feature Feature Feature Feature FeatureFeature Seamless TDD — Novel Collaborators 41
Where to Start? §  Try to guess §  … and be ready to abandon test(s) §  … or get a mess Or §  Analyze thoroughly §  … up-front decomposition §  … without tests — just a fantasy Seamless TDD — Novel Collaborators 42
Novel Collaborators: Example Bulls and Cows Game §  Computer generates a secret key §  e.g., a 4-digit number §  Human player tries to disclose it Seamless TDD — Novel Collaborators 43
Bulls and Cows Game Scenario: §  User creates a game object §  User starts the game §  Game should generate a key §  … Seamless TDD — Novel Collaborators 44
testGeneratesKeyOnStart ! ! self assert: key …?! “How to represent key?!”! Seamless TDD — Novel Collaborators 45
What Can I Do? §  Spontaneous representation §  Do you feel lucky? §  Analyze thoroughly §  Give up TDD §  Postpone the test §  Not a solution Seamless TDD — Novel Collaborators 46
What Can I Do? §  … §  Create a new class for key §  Unnecessary complexity? Seamless TDD — Novel Collaborators 47
What Can I Do? That was a Digression! Seamless TDD — Novel Collaborators 48
testGeneratesKeyOnStart |key|! game start.! key := game key.! self assert: ! !key isKindOf: Code! Seamless TDD — Novel Collaborators 49
testGeneratesKeyOnStart [ :keyGen |! game keyGenerator: keyGen.! [ game start ]! should satisfy:! [keyGen createKey! willReturn: #key]! game key should be: #key! ] runScenario.! Seamless TDD — Novel Collaborators 50
What We Get §  Key generation functionality §  is revealed §  moved to another object §  Dependency Injection §  fake key can be created for tests §  KeyGenerator refactored to Turn §  No risk of incorrect decision Seamless TDD — Novel Collaborators 51
What We Get Seamless TDD: §  No digression §  No up-front decomposition §  No up-front design §  No speculating / fantasizing Seamless TDD — Novel Collaborators 52
Complete Example Mock Objects For Top-Down Design by Bulls-and-Cows Example Just ask! 53
Classic vs. Mockist TDD State vs. Behavior? Result vs. Intention? No contradiction! Mockist approach complements “classic” TDD Seamless TDD 54
Classic and Mockist TDD §  Top-Down with Mockist TDD §  Analysis and Decomposition §  Bottom-Up with Classic TDD §  Synthesis and “real-object” testing 55

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Mock Objects and Smalltalk

  • 1. Overview §  Why Mock Objects? §  “Mockist” vs. “Classic” TDD §  “Mockist” and “Classic” TDD §  Mocks and Smalltalk: §  The Mocketry framework introduction §  Examples Mock Objects and Smalltalk 1
  • 2. Why Mock Objects? Do we need Mocks at all (in Smalltalk)? Mock Objects and Smalltalk 2
  • 3. Public Opinion §  Smalltalk vs. Mock Objects? §  Few/rare special cases §  With mock you don’t test real thing §  Use mocks for external objects only §  Use other means to involve complex external objects §  Speed up by other means Do we need Mock Objects at all? 3
  • 4. Public Opinion …seems to be about testing 4
  • 5. Smalltalk and Mock Objects §  “Mock Objects” is a TDD technique §  … about developing systems §  … not just testing §  … useful in all languages §  Smalltalk makes mocks §  much easier to use Why Mock Objects? 5
  • 6. Why Mock Objects? §  Cut off dependencies in tests §  Test-Driven Decomposition §  Discover Responsibility (for collaborators) §  Thinking vs. Speculating/Fantasizing Seamless TDD Why Mock Objects? 6
  • 7. What Is The Problem? §  Dependencies §  How to cut them off? §  Novel Collaborators §  Where to cut? Seamless TDD 7
  • 8. What Is The problem? §  Dependencies §  How to cut them off? §  Novel Collaborators §  Where to cut? Seamless TDD 8
  • 9. Dependencies We have: §  System Under Development (SUD) §  Collaborators §  Collaborators’ collaborators … Complex Test Seamless TDD — What’s the Problem? 9
  • 10. So What? We have to implement collaborators §  … without tests §  … loosing focus on SUD Digression Seamless TDD — Dependencies 10
  • 11. Dependencies: Example §  Mocks Aren’t Stubs by Martin Fowler §  Filling orders from warehouse Seamless TDD — Dependencies 11
  • 12. Filling Order OrderTests >> ! testIsFilledIfEnoughInWarehouse! ! | order |! order:= Order on: 50 of: #product.! order fillFrom: warehouse.! self assert: order isFilled! Seamless TDD — Dependencies 12
  • 13. Filling Order OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order warehouse |! ! warehouse := Warehouse new.! “Put #product there” ! “…but how?!” ! ! order := Order on: 50 of: #product.! order fillFrom: warehouse.! …! Seamless TDD — Dependencies 13
  • 14. Digression Detected! §  I develop Order §  I don’t want to think about Warehouse Seamless TDD — Dependencies 14
  • 15. Filling Order OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order warehouse |! warehouse := Warehouse new.! warehouse add: 50 of: #product.! ! order := Order on: 50 of: #prod.! order fillFrom: warehouse.! …! Seamless TDD — Dependencies 15
  • 16. …And Even More Digression Reduce amount of #product at the warehouse test…! …! self assert: ! (warehouse ! amountOf: #product)! isZero! Seamless TDD — Dependencies 16
  • 17. … And Even More Digression Another test case: If there isn’t enough #product in the warehouse, §  do not fill order §  do not remove #product from warehouse Seamless TDD — Dependencies 17
  • 18. … Much More Digression More complex test cases Collaborators’ logic becomes more and more complex… This can engulf Seamless TDD — Dependencies 18
  • 19. Not-So-Seamless TDD §  SUD is Order §  Warehouse blures SUD §  #add:of: §  #amountOf: §  No explicit tests for Warehouse Seamless TDD — Dependencies 19
  • 20. Mocking Warehouse OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order |! order := Order on: 50 of: #product.! [! :warehouse| ! [order fillFrom: warehouse]! should satisfy:! [“expectations”]! ] runScenario.! self assert: order isFilled ! Seamless TDD — Dependencies 20
  • 21. Mocking Warehouse …! [:warehouse| ! [order fillFrom: warehouse]! should satisfy:! [(warehouse ! has: 50 of: #product)! willReturn: true. ! warehouse ! remove: 50 of: #product]! ] runScenario.! …! Seamless TDD — Dependencies 21
  • 22. The Mocketry Framework Mock Objects in Smalltalk World Mock Objects and Smalltalk 22
  • 23. Behavior Expectations When you do this with SUD, expect that to happen with collaborators Collaborators are mocked Mocketry 23
  • 24. Behavior Expectations Do this Exercise Expect that Verify Scenario Mocketry 24
  • 25. Mocketry Scenario Pattern SomeTestCases >> testCase [ testScenario ] runScenario Mocketry – Behavior Expectations 25
  • 26. Mocketry Scenario Pattern SomeTestCases >> testCase [ [exercise] should strictly satisfy: [behaviorExpectations] ] runScenario Mocketry – Behavior Expectations 26
  • 27. Behavior Expectations Just send mock objects the messages §  ! they should receive warehouse ! has: 50 of: #product! Specify their reaction §  (warehouse ! has: 50 of: #product) willReturn: true Mocketry — Behavior Expectations 27
  • 28. Mocketry Scenario Pattern SomeTestCases >> testCase [ [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 28
  • 29. Mocketry Scenario Pattern SomeTestCases >> testCase [ :mock | [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 29
  • 30. Mocketry Scenario Pattern SomeTestCases >> testCase [ :mock | [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 30
  • 31. Mocketry Scenario Pattern SomeTestCases >> testCase [ :mock1 :mock2 :mock3 | [exercise] should strictly satisfy: [behaviorExpectations] [exercise] should strictly satisfy: [behaviorExpectations] … do anything ] runScenario Mocketry – Behavior Expectations 31
  • 32. Trivial Example 1 TrueTests >> testDoesNotExecuteIfFalseBlock [ :block | [true ifFalse: block ] should satisfiy: [“nothing expected”] ] runScenario Mocketry – Behavior Expectations 32
  • 33. Trivial Example 2 TrueTests >> testExecutesIfTrueBlock [ :block | [true ifTrue: block] should satisfiy: [block value] ] runScenario Mocketry – Behavior Expectations 33
  • 34. State Specification DSL §  resultObject should <expectation> §  result should be: anotherObject §  result should equal: anotherObject §  … Mocketry 34
  • 35. Mocketry §  There is much more… §  Ask me §  …or Dennis Kudryashov (the Author) 35
  • 36. Mocking Warehouse OrderTests >> ! testIsFilledIfEnoughInWarehouse! | order |! order := Order on: 50 of: #product.! ! [:warehouse| ! [order fillFrom: warehouse]! should satisfy:! [(warehouse has: 50 of: #product)! willReturn: true. ! warehouse remove: 50 of: #product]! ] runScenario.! ! self assert: order isFilled ! Seamless TDD — Dependencies — Example 36
  • 37. Mocking Warehouse OrderTests >> ! testIsNotFilledIfNotEnoughInWarehouse! | order |! order := Order on: #amount of: #product.! [:warehouse| ! [order fillFrom: warehouse]! should satisfy:! [(warehouse has: 50 of: #product)! willReturn: false. ! “Nothing else is expected” ]! ] runScenario.! self assert: order isFilled ! Seamless TDD — Dependencies 37
  • 38. What We Get §  No need to implement Warehouse §  Just specify expectations §  … right in the test §  Focus on the SUD Why Mock Objects 38
  • 39. What’s the problem? §  Dependencies §  Novel Сollaborators Seamless TDD 39
  • 40. Where to Start? A novel system to implement Object 2 Object 1 Object N … Object 3 Seamless TDD — Novel Collaborators 40
  • 41. Where to Cut? A novel system to implement Feature Feature Feature Feature Feature Feature Feature Feature Feature FeatureFeature Seamless TDD — Novel Collaborators 41
  • 42. Where to Start? §  Try to guess §  … and be ready to abandon test(s) §  … or get a mess Or §  Analyze thoroughly §  … up-front decomposition §  … without tests — just a fantasy Seamless TDD — Novel Collaborators 42
  • 43. Novel Collaborators: Example Bulls and Cows Game §  Computer generates a secret key §  e.g., a 4-digit number §  Human player tries to disclose it Seamless TDD — Novel Collaborators 43
  • 44. Bulls and Cows Game Scenario: §  User creates a game object §  User starts the game §  Game should generate a key §  … Seamless TDD — Novel Collaborators 44
  • 45. testGeneratesKeyOnStart ! ! self assert: key …?! “How to represent key?!”! Seamless TDD — Novel Collaborators 45
  • 46. What Can I Do? §  Spontaneous representation §  Do you feel lucky? §  Analyze thoroughly §  Give up TDD §  Postpone the test §  Not a solution Seamless TDD — Novel Collaborators 46
  • 47. What Can I Do? §  … §  Create a new class for key §  Unnecessary complexity? Seamless TDD — Novel Collaborators 47
  • 48. What Can I Do? That was a Digression! Seamless TDD — Novel Collaborators 48
  • 49. testGeneratesKeyOnStart |key|! game start.! key := game key.! self assert: ! !key isKindOf: Code! Seamless TDD — Novel Collaborators 49
  • 50. testGeneratesKeyOnStart [ :keyGen |! game keyGenerator: keyGen.! [ game start ]! should satisfy:! [keyGen createKey! willReturn: #key]! game key should be: #key! ] runScenario.! Seamless TDD — Novel Collaborators 50
  • 51. What We Get §  Key generation functionality §  is revealed §  moved to another object §  Dependency Injection §  fake key can be created for tests §  KeyGenerator refactored to Turn §  No risk of incorrect decision Seamless TDD — Novel Collaborators 51
  • 52. What We Get Seamless TDD: §  No digression §  No up-front decomposition §  No up-front design §  No speculating / fantasizing Seamless TDD — Novel Collaborators 52
  • 53. Complete Example Mock Objects For Top-Down Design by Bulls-and-Cows Example Just ask! 53
  • 54. Classic vs. Mockist TDD State vs. Behavior? Result vs. Intention? No contradiction! Mockist approach complements “classic” TDD Seamless TDD 54
  • 55. Classic and Mockist TDD §  Top-Down with Mockist TDD §  Analysis and Decomposition §  Bottom-Up with Classic TDD §  Synthesis and “real-object” testing 55