Swift: Copy-modify

It often happens that we need to copy an object, changing some of its properties, but keeping the rest unchanged. There is a function for this task copy().

This is an excerpt copy()from the Kotlin documentation for the method . In our native Swift language, this means something like this:

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
}

Looks delicious, doesn't it?

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, 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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• , .

, — , :

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

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• , . 
• 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
        }
    }
}

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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"
}

, .

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, , , Swift 5.1 .

 . . !