Treating Objects Like People (Baruco, Sept 2013)

Cucumber core developer Matt Wynne uses the example of re-developing Cucumber's internals to explain important lessons about object-oriented design.

Transcript

1. TREATING
   OBJECTS
   LIKE PEOPLE
   @mattwynne | @baruco | Setiembre 2013
   

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2. (c) David Shrigley
   

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13. Feature: Update card details
    Need to be able to update my card, e.g. if I decide
    to use a different one for billing.
    Scenario: Update card details successfully
    Given I have started a paid subscription on the pro plan
    When I choose to update which card is billed
    Then I should see that my card details have been saved
    Scenario: Use a bad card
    Given I have started a paid subscription on the pro plan
    When I try to change to an invalid card
    Then I should see that my card has been declined
    

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14. Feature Scenario Step
    * *
    Formatter
    

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15. Result
    

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16. Feature Scenario
    Formatter Step
    accept(self)
    before_feature
    accept(formatter)
    before_scenario
    accept(formatter)
    before_step
    after_step
    after_scenario
    after_feature
    execute(step)
    

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17. Feature: Update card details
    Need to be able to update my card, e.g. if I decide
    to use a different one for billing.
    Scenario: Update card details successfully
    Given I have started a paid subscription on the pro plan
    When I choose to update which card is billed
    Then I should see that my card details have been saved
    Scenario: Use a bad card
    Given I have started a paid subscription on the pro plan
    When I try to change to an invalid card
    Then I should see that my card has been declined
    

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18. Feature: Update card details
    Need to be able to update my card, e.g. if I decide
    to use a different one for billing.
    Scenario: Update card details successfully
    Given I have started a paid subscription on the pro plan
    When I choose to update which card is billed
    Then I should see that my card details have been saved
    Scenario: Use a bad card
    Given I have started a paid subscription on the pro plan
    When I try to change to an invalid card
    Then I should see that my card has been declined
    

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19. Feature: Update card details
    Need to be able to update my card, e.g. if I decide
    to use a different one for billing.
    Background:
    Given I have started a paid subscription on the pro plan
    Scenario: Update card details successfully
    When I choose to update which card is billed
    Then I should see that my card details have been saved
    Scenario: Use a bad card
    When I try to change to an invalid card
    Then I should see that my card has been declined
    

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20. Feature Scenario Step
    * *
    Background Step
    *
    0..1
    

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21. Feature: Update card details
    Need to be able to update my card, e.g. if I decide
    to use a different one for billing.
    Scenario Outline: Update card details
    Given I have started a paid subscription on the pro plan
    When I update my card details with a card
    Then I should see ""
    Examples:
    | card status | message |
    | valid | Your card details have been saved. |
    | expired | This card has expired. |
    | stolen | The police are on their way. |
    

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22. Feature Scenario Step
    * *
    Background Step
    *
    0..1
    Scenario
    Outline
    Step
    *
    *
    Example
    *
    

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23. module Cucumber
    module Ast
    class Background
    def accept(visitor)
    return if @already_visited
    visitor.visit_background(self) do
    comment.accept(visitor)
    visitor.visit_background_name(@keyword, name, file_colon_li
    with_visitor(feature_elements.first || self, visitor) do
    visitor.runtime.before(hook_context)
    skip_invoke! if failed?
    step_invocations.accept(visitor)
    @failed = step_invocations.any? do |step_invocation|
    step_invocation.exception ||
    step_invocation.status != :passed
    end
    if @failed || feature_elements.empty?
    visitor.runtime.after(hook_context)
    end
    end
    end
    @already_visited = true
    end
    

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24. Our First Mistake:
    Data-Centric Domain Model
    

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26. Problem
    Domain
    Solution
    Domain
    

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27. "Model the problem
    domain and the solution
    will take care of itself."
    DAVID WEST, OBJECT THINKING
    

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29. SIMULA 67
    1967
    http://www.jot.fm/issues/issue_2002_09/eulogy/
    

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30. "It should be problem oriented and
    not computer oriented, even if this
    implies an appreciable increase in the
    amount of work that has to be done
    by the computer"
    SIMULA 67
    1967
    http://www.jot.fm/issues/issue_2002_09/eulogy/
    

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33. C++
    1979
    "SIMULA's class-based system was a
    huge plus, but it's run-time
    performance was hopeless."
    

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34. SMALLTALK-80
    Called "Smalltalk"--as in
    "programming should be a matter
    of ..." [Smalltalk] and "children should
    program in ..." [Smalltalk].
    -- Alan Kay
    1980
    

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35. SIMULA 67
    1967
    C++
    1979
    Smalltalk-80
    1980
    

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36. Two philosophies
    

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37. Two cultures
    

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38. C++
    Smalltalk-80
    

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39. "Good" OOP "Bad" OOP
    FOCUS ON DATA
    FOCUS ON MESSAGES
    FOCUS ON BEHAVIOUR,
    RESPONSIBILITIES
    FOCUS ON STRUCTURE, LAYERS
    ABSORBED IN THE PROBLEM DISTRACTED BY THE SOLUTION
    CULTURE OF FLEXIBILITY,
    COLLABORATION, FUN, CHAOS
    CULTURE OF FORMALITY,
    HIERARCHY, CENTRALISED CONTROL
    

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41. 2.0
    

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43. Our first fix:
    A responsibility-centric
    model
    

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44. Compiler
    Runner
    Parser
    Report
    

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45. A design principle:
    Tell objects, ask values*
    * (thank GOOS)
    

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46. Compiler Runner
    Parser
    feature
    scenario
    step
    test_case
    step
    scenario
    step
    step
    test_case
    

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47. Compiler Runner
    test_case
    Report
    before_test_case(case)
    after_test_case(case, result)
    before_test_step(step)
    after_test_step(step, result)
    execute(step)
    

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48. LESS STATE,
    LESS BUGS
    

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49. YOUR PROGRAMS
    ARE PROBABLY
    TOO STUPID TO BE
    ALLOWED TO
    REMEMBER STUFF
    

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50. So remember...
    MODEL ACTIVITY, THEN PUSH
    DATA THROUGH THAT MODEL
    MAKE YOUR DATA IMMUTABLE
    TELL OBJECTS, ASK VALUES
    TREAT OBJECTS LIKE PEOPLE
    

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51. http://cucumber.pro
    

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