Rápido: copiar-modificar

A menudo sucede que necesitamos copiar un objeto, cambiando algunas de sus propiedades, pero manteniendo el resto sin cambios. Hay una función para esta tarea copy().

Este es un extracto copy()de la documentación de Kotlin para el método . En nuestro idioma Swift nativo, esto significa algo como esto:

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
}

Parece delicioso, ¿no?

, Swift " ". .

 

, var let?

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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• "". , willSet didSet .
• , .

, — , :

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

, , . - , .

, , "" , .

: 

• , . 
• Changeable -, , .
• , .

, , . Key-Path , Key-Path Dynamic Member Lookup Swift 5.1 .

, 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
        }
    }
}

, KeyPath. , , . .

changes[keyPath] as? T, T . nil, , . , flatMap(:), , changes .

@dynamicMemberLookup , , var.

Xcode . : .

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"
}

, .

, , , , . Swift , .

. , , . Stencil- Sourcery, .

, , , Swift 5.1 .

 . . !