user defaults backed property wrapper (credit: john sundell)

UserDefaultsBacked.swift

import Foundation

public protocol AnyOptional {
  var isNil: Bool { get }
}

extension Optional: AnyOptional {
  public var isNil: Bool { self == nil }
}

@propertyWrapper public struct UserDefaultsBacked<Value, Key: StorageKey> {
  private let key: Key
  private let defaultValue: Value

  public init(wrappedValue defaultValue: Value, key: Key) {
    self.defaultValue = defaultValue
    self.key = key
  }

  public var wrappedValue: Value {
    get {
      UserDefaults.standard.value(forKey: key.string) as? Value ?? defaultValue
    }
    set {
      if let optional = newValue as? AnyOptional, optional.isNil {
        UserDefaults.standard.removeObject(forKey: key.string)
      } else {
        UserDefaults.standard.setValue(newValue, forKey: key.string)
      }
    }
  }
}

public extension UserDefaultsBacked where Value: ExpressibleByNilLiteral {
  init(key: Key) {
    self.init(wrappedValue: nil, key: key)
  }
}