Persistable.swift

import Combine
import Foundation

let preferenceKey = "com.bermudalocket.myapp"

@propertyWrapper
struct Persistable<T> {

    let key: String

    let publisher: CurrentValueSubject<T, Never>

    var projectedValue: Persistable<T> { return self }

    var wrappedValue: T {
        get {
            self.publisher.value
        }
        set {
            publisher.send(newValue)
            UserDefaults.standard.set(newValue, forKey: self.key)
        }
    }

    init(_ key: String, defaultValue: T) {
        self.key = "\(preferenceKey).\(key)"
        var value: T = defaultValue
        if let currentValue = UserDefaults.standard.object(forKey: self.key) as? T {
            value = currentValue
        }
        self.publisher = CurrentValueSubject<T, Never>(value)
    }
}

Preferences.swift

class Preferences {

    static let shared = Preferences()

    @Persistable("blur", defaultValue: 10) var blur: Double
    @Persistable("vibrancy", defaultValue: 17.5) var vibrancy: Double
    @Persistable("tint", defaultValue: .someEnumValue) var tint: SomeEnumType
  
}

SomePreferenceImplClass.swift

class SomePreferenceImplClass {
    
    var cancellables = [Cancellable]()
    
    var blur: Double
  
    init() {
        let cancellable = Preferences.shared.$blur.publisher.sink(receiveValue: { blur in
            self.blur = blur
        })
        self.cancellables.append(cancellable)
    }
    
    deinit {
        self.cancellables.forEach { $0.cancel() }
    }

}
      

Using an Enum type will result in a fatal error. ([User Defaults] Attempt to set a non-property-list object). Regards.

Crashes as well when optionals are used since you can not set userdefaults to nil, though that's easy to circumvent with the widely adopted AnyOptional protocol.