mutatingキーワードから学ぶCQS / 20180309 #tryswift_aftertalks

try!Swift Tokyo 2018 After talks Day 2 - connpass
https://tryswifttokyo-aftertalks.connpass.com/event/79690/
での発表資料です。

# 参考リンク

オブジェクト指向入門 第2版 方法論・実践 (IT Architects' Archiveクラシックモダン・コンピューティング) | バートランド・メイヤー, 酒匂 寛 |本 | 通販 | Amazon
https://www.amazon.co.jp/dp/4798111120

CommandQuerySeparation
https://martinfowler.com/bliki/CommandQuerySeparation.html

Swift.org - Documentation
https://swift.org/documentation/

Swift.org - API Design Guidelines
https://swift.org/documentation/api-design-guidelines/

Transcript

1. Learning CQS
   from mutating keyword
   by.
   2018/3/9 try! Swift After Talks
   1
   

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2. takasek
   iOS Developer
   @takasek
   OSS
   ActionClosurable౳
   Xcode Extension
   PasteTheType
   2
   

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3. CQS
   Command Query Separation
   ίϚϯυΫΤϦ෼཭ݪଇ
   3
   

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4. Commands 1
   • Change the state of a system
   • but do not return a value
   Queries
   • Return a result
   • do not change the observable state of the system
   • (are free of side effects)
   1
   https://martinfowler.com/bliki/CommandQuerySeparation.html
   4
   

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5. Side effects
   ෭࡞༻
   5
   

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6. ෭࡞༻͸ɺιϑτ΢ΣΞͷਖ਼͠͞ͷ
   ࠜڌʢʹ਺ֶతଐੑʣΛࣦΘͤΔ
   Side effects ruin a ground for
   mathematical validity of the software.
   6
   

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7. Referential transparency
   ࢀরಁաੑ
   Queries shouldn't change answers.
   ʮ໰͍Λൃ͢Δ͜ͱʹΑͬͯ
   ౴͕͑ม͑ΒΕΔ΂͖Ͱ͸ͳ͍ʯ
   7
   

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8. Martin Fowler says... 1
   It would be nice if the language itself would support
   this notion. I could imagine a language that would
   detect state changing methods, or at least allow the
   programmer to mark them.
   ʮCQSΛαϙʔτͯ͘͠ΕΔݴޠ͕͋Ε͹͍͍ͷʹʯ
   1
   https://martinfowler.com/bliki/CommandQuerySeparation.html
   8
   

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10. mutating keyword
    “if you need to modify the properties of your
    structure or enumeration within a particular method,
    you can opt in to mutating behavior for that method” 3
    func ͕஋ܕΦϒδΣΫτʹมߋΛՃ͑Δ͜ͱΛ໌ࣔ
    3 "The Swift Programming Language (Swift 4.1)"
    10
    

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11. 4
    4
    https://swift.org/documentation/api-design-guidelines/
    11
    

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12. Commands 1
    • Change the state of a system
    • but do not return a value
    Queries
    • Return a result
    • do not change the observable state of the system
    • (are free of side effects)
    1
    https://martinfowler.com/bliki/CommandQuerySeparation.html
    12
    

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13. Commands 1
    • Change the state of a system
    • but do not return a value
    !
    Queries
    • Return a result
    • do not change the observable state of the system
    • (are free of side effects)
    1
    https://martinfowler.com/bliki/CommandQuerySeparation.html
    13
    

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14. find ./stdlib/public/core | grep .swift | xargs cat | grep
    "public mutating func"
    public mutating func append(_ newElement: Element)
    public mutating func insert(_ newElement: Element, at i: Int)
    public mutating func removeFirst()
    public mutating func sort()
    public mutating func subtract(_ other: Self)
    public mutating func multiply(by other: Self)
    public mutating func divide(by other: Self)
    public mutating func negate()
    public mutating func round(_ rule: FloatingPointRoundingRule)
    public mutating func write(_ other: String)
    public mutating func encode(_ value: T, forKey key: Key) throws
    14
    

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15. Using Swift value types
    Swiftͷ஋ܕΛ࢖͏
    =
    Geting along with CQS
    CQSʹ׳Ε਌͠Ή
    15
    

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16. How about reference types
    (classes)?
    16
    

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17. Do queries have side effects?
    ΫΤϦ͕෭࡞༻Λ͍࣋ͬͯͳ͍͔ʁ
    Do commands have return value?
    ίϚϯυ͕໭Γ஋Λ͍࣋ͬͯͳ͍͔ʁ
    17
    

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18. Learn from Swift.
    Swiftʹֶ΅͏
    Stand on the shoulders of giants.
    աڈͷڊਓʹֶ΅͏
    18
    

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19. ׬
    19
    

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20. !
    20
    

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21. find ./stdlib/public/core | grep .swift | xargs cat | grep
    "public mutating func"
    public mutating func popFirst() -> Element?
    public mutating func popLast() -> Element?
    public mutating func update(with newMember: Element) -> Element?
    public mutating func remove(at index: Int) -> Element
    public mutating func remove(_ member: Element) -> Element?
    public mutating func removeFirst() -> Element
    public mutating func removeValue(forKey key: Key) -> Value?
    public mutating func next() -> Element?
    public mutating func decode(_ input: inout I)
    -> UnicodeDecodingResult
    public mutating func superEncoder() -> Encoder
    public mutating func nestedContainer(keyedBy keyType: NestedKey.Type)
    -> KeyedEncodingContainer
    21
    

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22. Exceptions.
    ྫ֎΋͋Δɻ
    22
    

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23. Popping a stack is a good example of a query that modifies
    state. Meyer correctly says that you can avoid having this
    method, but it is a useful idiom.
    So I prefer to follow this principle when I can, but I'm
    prepared to break it to get my pop.1
    ʮ෭࡞༻ආ͚ΒΕΔͬͯMeyer͸ݴͬͯΔ͚Ͳɺ
    popʹ໭Γ஋͕͋ͬͨΒਖ਼௚ศརͳͷͰ
    ʰCQSݪଇ͸कΕΔͳΒकΔʱʯ
    1
    https://martinfowler.com/bliki/CommandQuerySeparation.html
    23
    

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24. कഁ཭
    24
    

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25. ͓͖ͯ͞ɺجຊతʹ͸༗ޮͳͷ͕
    ʮݪଇʯ
    Principles work fine principally.
    25
    

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26. ϝϦοτҙࣝͯ͠
    ࢖͍͜ͳ͠·͠ΐ͏
    Get along with them!
    26
    

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