Can a Swift Property Wrapper reference the owner of the property its wrapping?

Question

From within a property wrapper in Swift, can you someone refer back to the instance of the class or struck that owns the property being wrapped? Using self doesn\'t

Answer 1

You cannot do this out of the box currently.

However, the proposal you refer to discusses this as a future direction in the latest version: https://github.com/apple/swift-evolution/blob/master/proposals/0258-property-wrappers.md#referencing-the-enclosing-self-in-a-wrapper-type

For now, you would be able to use a projectedValue to assign self to. You could then use that to trigger some action after setting the wrappedValue.

As an example:

import Foundation

@propertyWrapper
class Wrapper {
    let name : String
    var value = 0
    weak var owner : Owner?

    init(_ name: String) {
        self.name = name
    }

    var wrappedValue : Int {
        get { value }
        set {
            value = 0
            owner?.wrapperDidSet(name: name)
        }
    }

    var projectedValue : Wrapper {
        self
    }
}


class Owner {
    @Wrapper("a") var a : Int
    @Wrapper("b") var b : Int

    init() {
        $a.owner = self
        $b.owner = self
    }

    func wrapperDidSet(name: String) {
        print("WrapperDidSet(\(name))")
    }
}

var owner = Owner()
owner.a = 4 // Prints: WrapperDidSet(a)

Answer 2

The answer is no, it's not possible with the current specification.

I wanted to do something similar. The best I could come up with was to use reflection in a function at the end of init(...). At least this way you can annotate your types and only add a single function call in init().

fileprivate protocol BindableObjectPropertySettable {
    var didSet: () -> Void { get set }
}

@propertyDelegate
class BindableObjectProperty<T>: BindableObjectPropertySettable {
    var value: T {
        didSet {
            self.didSet()
        }
    }
    var didSet: () -> Void = { }
    init(initialValue: T) {
        self.value = initialValue
    }
}

extension BindableObject {
    // Call this at the end of init() after calling super
    func bindProperties(_ didSet: @escaping () -> Void) {
        let mirror = Mirror(reflecting: self)
        for child in mirror.children {
            if var child = child.value as? BindableObjectPropertySettable {
                child.didSet = didSet
            }
        }
    }
}

Answer 3

The answer is yes! See this answer

Example code for calling ObservableObject publisher with a UserDefaults wrapper:

import Combine
import Foundation

class LocalSettings: ObservableObject {
  static var shared = LocalSettings()

  @Setting(key: "TabSelection")
  var tabSelection: Int = 0
}

@propertyWrapper
struct Setting<T> {
  private let key: String
  private let defaultValue: T

  init(wrappedValue value: T, key: String) {
    self.key = key
    self.defaultValue = value
  }

  var wrappedValue: T {
    get {
      UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
    }
    set {
      UserDefaults.standard.set(newValue, forKey: key)
    }
  }

  public static subscript<EnclosingSelf: ObservableObject>(
    _enclosingInstance object: EnclosingSelf,
    wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, T>,
    storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Setting<T>>
  ) -> T {
    get {
      return object[keyPath: storageKeyPath].wrappedValue
    }
    set {
      (object.objectWillChange as? ObservableObjectPublisher)?.send()
      UserDefaults.standard.set(newValue, forKey: object[keyPath: storageKeyPath].key)
    }
  }
}

Answer 4

My experiments based on : https://github.com/apple/swift-evolution/blob/master/proposals/0258-property-wrappers.md#referencing-the-enclosing-self-in-a-wrapper-type

protocol Observer: AnyObject {
    func observableValueDidChange<T>(newValue: T)
}

@propertyWrapper
public struct Observable<T: Equatable> {
    public var stored: T
    weak var observer: Observer?

    init(wrappedValue: T, observer: Observer?) {
        self.stored = wrappedValue
    }

    public var wrappedValue: T {
        get { return stored }
        set {
            if newValue != stored {
                observer?.observableValueDidChange(newValue: newValue)
            }
            stored = newValue
        }
    }
}

class testClass: Observer {
    @Observable(observer: nil) var some: Int = 2

    func observableValueDidChange<T>(newValue: T) {
        print("lol")
    }

    init(){
        _some.observer = self
    }
}

let a = testClass()

a.some = 4
a.some = 6