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iOSAdvent Calendar 2020Day 9
@417_72ki

SwiftのKeyPathと戯れる

Last updated at Posted at 2020-12-08
1 / 24

KeyPathとは?

KeyPath とは?

Swift4で追加された、プロパティに動的にアクセスするための記法

Use key-path expressions to access properties dynamically.1

Swift3.xまでの #keyPath 

class Hoge: NSObject { // 🤔
    @objc var foo: String // 🤔
    let bar: String

    init(foo: String, bar: String) {
        self.foo = foo
        self.bar = bar
    }
}

let foo = hoge.value(forKeyPath: #keyPath(Hoge.foo))
print(type(of: foo)) // Optional<Any>

print(#keyPath(hoge.bar)) // error: argument of '#keyPath' refers to non-'@objc' property 'bar'

Obj-C時代からあるKVOでのtypoを防ぐ程度の役割しかない

  • NSObject にしか使えない
  • @objc 宣言されたプロパティのみ
  • 返却値はAny! (型情報が消える😫)
  • 解析が遅い、Darwinでしか使えない2といった問題点も

Swift4からの KeyPath

  • Swiftの強力な型システムに基づいた機構
    • structにもenumにも使える
    • 型情報が維持される
    • Swiftのstdlibに含まれているのでプラットフォームに依存しない

NSObjectも@objc宣言もいらない

class Hoge {
    var foo: String
    let bar: String
}
let hogeFoo: ReferenceWritableKeyPath<Hoge, String> = \.foo
let hogeBar = \Hoge.bar // KeyPath<Hoge, String>

let hoge = Hoge(foo: "foo", bar: "bar")
print(hoge.foo) // foo
hoge[keyPath: hogeFoo] = "hoge-foo"
print(hoge.foo) // hoge-foo
hoge[keyPath: hogeBar] = "bar" // error: cannot assign through subscript: 'hogeBar' is a read-only key path

structにも使える

struct Fuga {
    var foo: String
    let bar: String
}
let fugaFoo: WritableKeyPath<Fuga, String> = \.foo
let fugaBar = \Fuga.bar // KeyPath<Fuga, String>

var fuga = Fuga(foo: "foo", bar: "bar")
print(fuga.foo) // foo
fuga[keyPath: fugaFoo] = "fuga-foo"
print(fuga.foo) // fuga-foo
fuga[keyPath: fugaBar] = "bar" // error: cannot assign through subscript: 'fugaBar' is a read-only key path

KeyPath関連クラス

_AppendKeyPath
└ AnyKeyPath
  └ PartialKeyPath<Root>
    └ KeyPath<Root, Value>
      └ WritableKeyPath<Root, Value> // Value is variable
        └ ReferenceWritableKeyPath<Root, Value> // Root is class

小ネタ

TupleもRootになれる

let hogeFuga = (hoge, fuga)

let hogeKeyPath = \(Hoge, Fuga).0 // no label
let fugaKeyPath = \(hoge: Hoge, fuga: Fuga).fuga // with labels

hogeFuga[keyPath: hogeKeyPath] // hoge
hogeFuga[keyPath: fugaKeyPath] // fuga

プロパティのネストも可能

struct FooBar {
    var hoge: Hoge
    var fuga: Fuga
}

let fooBarHogeFoo = (\FooBar.hoge).appending(path: \Hoge.foo)
\FooBar.hoge.foo == fooBarHogeFoo // true

print(fooBar.hoge.foo) // hoge-foo
fooBar[keyPath: fooBarHogeFoo] = "foo-bar-hoge-foo"
print(fooBar.hoge.foo) // foo-bar-hoge-foo
fooBar[keyPath: \.hoge.foo] = "foo"
print(fooBar.hoge.foo) // foo

let fooBarFugaFoo = (\FooBar.fuga).appending(path: \Fuga.foo)
fooBar[keyPath: fooBarFugaFoo] = "foo-bar-fuga-foo" // error: cannot assign through subscript: 'fooBar' is a 'let' constant

KeyPathの実用例

プロパティでソートする3 

public extension Sequence {
    func sorted<T: Comparable>(by keyPath: KeyPath<Element, T>) -> [Element] {
        sorted { $0[keyPath: keyPath] < $1[keyPath: keyPath] }
    }

    func sorted<T: Comparable>(by keyPath: KeyPath<Element, T?>) -> [Element] {
        sorted {
            guard let l = $0[keyPath: keyPath],
                let r = $1[keyPath: keyPath] else { return false }
            return l < r
        }
    }

    func min<T: Comparable>(by keyPath: KeyPath<Element, T>) -> Element? {
        self.min { $0[keyPath: keyPath] < $1[keyPath: keyPath] }
    }
}

struct Person {
    var id: Int
    var name: String
    var age: Int
}

extension Person: CustomStringConvertible {
    var description: String { name }
}

let people: [Person] = [
    .init(id: 3, name: "Bob", age: 28),
    .init(id: 1, name: "Emma", age: 40),
    .init(id: 4, name: "Amelia", age: 18),
    .init(id: 2, name: "George", age: 22),
]

// Before
people.sorted(by: { $0.id < $1.id })      // ["Emma", "George", "Bob", "Amelia"]
people.sorted(by: { $0.name < $1.name })  // ["Amelia", "Bob", "Emma", "George"]

// After
people.sorted(by: \.id)   // ["Emma", "George", "Bob", "Amelia"]
people.sorted(by: \.name) // ["Amelia", "Bob", "Emma", "George"]

同じAnchor同士へのConstraintを貼る

public extension UIView {
    func equal<Axis, Anchor: NSLayoutAnchor<Axis>>(_ anchor: KeyPath<UIView, Anchor>,
                                                   to target: UIView) -> NSLayoutConstraint {
        self[keyPath: anchor].constraint(equalTo: target[keyPath: anchor])
    }

    func equal<Axis, Anchor: NSLayoutAnchor<Axis>>(_ anchor: KeyPath<UIView, Anchor>,
                                                   to target: UIView,
                                                   constant: CGFloat) -> NSLayoutConstraint {
        self[keyPath: anchor].constraint(equalTo: target[keyPath: anchor], constant: constant)
    }
}

// Before
NSLayoutConstraint.activate([
    view.topAnchor.constraint(equalTo: target.topAnchor),
    view.leadingAnchor.constraint(equalTo: target.leadingAnchor),
    view.trailingAnchor.constraint(equalTo: target.trailingAnchor),
    view.bottomAnchor.constraint(equalTo: target.bottomAnchor)
])

// After
NSLayoutConstraint.activate([
    view.equal(\.topAnchor, to: target),
    view.equal(\.bottomAnchor, to: target),
    view.equal(\.leadingAnchor, to: target),
    view.equal(\.trailingAnchor, to: target)
])

更に関数チックに書いた例
https://www.objc.io/blog/2018/10/30/auto-layout-with-key-paths/

Key Path Expressions as Functions

Key Path Expressions as Functions

  • Swift5.2で追加(SE-0249)
  • KeyPath 表現をクロージャを引数とする関数に渡すことができる
    • コレクション操作などが簡潔に書けるように
  • Xcode12.xならコード補完が効く

例

struct Department {
    var name: String
    var member: [Person]
}

extension Department {
    var boss: Person {
        member.min(by: \.id)!
    }
}

let departments: [Department] = [
    .init(name: "General Affairs Department",
          member: [
            .init(id: 3, name: "Bob", age: 28),
            .init(id: 1, name: "Emma", age: 40)
    ]),
    .init(name: "Development Department",
          member: [
            .init(id: 4, name: "Amelia", age: 18),
            .init(id: 2, name: "George", age: 22)
    ])
]

Before

departments.flatMap { $0.member }
    .map { $0.name } // ["Bob", "Emma", "Amelia", "George"]
departments.map { $0.boss }
    .map { $0.name } // ["Emma", "George"]
departments.map { $0.boss.name }

After

departments.flatMap(\.member)
    .map(\.name) // ["Bob", "Emma", "Amelia", "George"]
departments.flatMap(\.member.name) // Value of type '[Person]' has no member 'name'
departments.map(\.boss)
    .map(\.name) // ["Emma", "George"]
departments.map(\.boss.name) // ["Emma", "George"]

Performance4

image.png

extension Person {
    var canDrink: Bool { age >= 20 }
}

注意

変数/定数化したKeyPathを渡すことはできない(2020/12/9現在)

let bossKeyPath = \Department.boss
let nameKeyPath = \Person.name
let bossNameKeyPath = bossKeyPath.appending(path: nameKeyPath)

departments.map(bossKeyPath) // Cannot convert value of type 'KeyPath<Department, Person>' to expected argument type '(Department) throws -> T'
    .map(nameKeyPath) // Cannot convert value of type 'WritableKeyPath<Person, String>' to expected argument type '(T) throws -> T'

departments.map(bossNameKeyPath) // Cannot convert value of type 'KeyPath<Department, String>' to expected argument type '(Department) throws -> T'

最後に

  • KeyPathによってSwiftコードがより簡潔に書きやすくなった
  • RxSwift / Combine のストリーム変換にも使えるのでより宣言的に書ける
  • Genericsと併用することで処理の汎用化も大幅に進みそう
  • Sample in Xcode Playground

  1. https://developer.apple.com/documentation/swift/swift_standard_library/key-path_expressions ↩

  2. https://github.com/apple/swift-evolution/blob/master/proposals/0161-key-paths.md#we-can-do-better-than-string ↩

  3. https://qiita.com/417_72ki/items/de4a7e49c32d39d3bfb6 ↩

  4. measure関数 ↩

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