Swift: copier-modifier

Il arrive souvent que nous devions copier un objet, en changeant certaines de ses propriétés, mais en gardant le reste inchangé. Il existe une fonction pour cette tâche copy().

Ceci est un extrait de la description de la méthode copy()de la documentation Kotlin . Dans notre langue Swift native, cela signifie quelque chose comme ceci:

struct User {
    let id: Int
    let name: String
    let age: Int
}

let steve = User(id: 1, name: "Steve", age: 21)

//  ,   `name`  `age`
let steveJobs = steve.changing { newUser in
    newUser.name = "Steve Jobs"
    newUser.age = 41
}

Ça a l'air délicieux, n'est-ce pas?

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struct User {
    let id: Int
    var name: String
    var age: Int
}

let steve = User(id: 1, name: "Steve", age: 21)

...

var steveJobs = steve

steveJobs.name = "Steve Jobs"
steveJobs.age = 41

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//   ,   `name`
let steveJobs = User(
    id: steve.id, 
    name: "Steve Jobs",
    age: steve.age
)

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, generic-:

@dynamicMemberLookup
struct ChangeableWrapper<Wrapped> {
    private let wrapped: Wrapped
    private var changes: [PartialKeyPath<Wrapped>: Any] = [:]

    init(_ wrapped: Wrapped) {
        self.wrapped = wrapped
    }

    subscript<T>(dynamicMember keyPath: KeyPath<Wrapped, T>) -> T {
        get { 
            changes[keyPath].flatMap { $0 as? T } ?? wrapped[keyPath: keyPath] 
        }

        set {
            changes[keyPath] = newValue
        }
    }
}

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Changeable

, , Changeable :

protocol Changeable {
    init(copy: ChangeableWrapper<Self>)
}

extension Changeable {
    func changing(_ change: (inout ChangeableWrapper<Self>) -> Void) -> Self {
        var copy = ChangeableWrapper<Self>(self)
        change(&copy)
        return Self(copy: copy)
    }
}

changing(:) , , .

, , Changeable:

extension User: Changeable {
    init(copy: ChangeableWrapper<Self>) {
        self.init(
            id: copy.id,
            name: copy.name,
            age: copy.age
        )
    }
}

, , — :

let steve = User(id: 1, name: "Steve", age: 21)

let steveJobs = steve.changing { newUser in
    newUser.name = "Steve Jobs"
    newUser.age = 30
}

, , …

changing(:) , , , :

struct Company {
    let name: String
    let country: String
}

struct User {
    let id: Int
    let company: Company
}

let user = User(
    id: 1, 
    company: Company(
        name: "NeXT", 
        country: "USA"
    )
)

user, company.name, :

let appleUser = user.changing { newUser in
    newUser.company = newUser.company.changing { newCompany in
        newCompany.name = "Apple"
    }
}

, .

. — ChangeableWrapper:

subscript<T: Changeable>(
    dynamicMember keyPath: KeyPath<Wrapped, T>
) -> ChangeableWrapper<T> {
    get {
        ChangeableWrapper<T>(self[dynamicMember: keyPath])
    }

    set { 
        self[dynamicMember: keyPath] = T(copy: newValue)
    }
}

, Changeable. Swift - . , .

, :

let appleUser = user.changing { newUser in
    newUser.company.name = "Apple"
}

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 . . !