UserDefault.swift

@propertyWrapper public struct UserDefault<T> {

    public let key: String
    public let defaultValue: T
    private let container: UserDefaults

    public init(key: String, defaultValue: T, container: UserDefaults) {
        self.key = key
        self.defaultValue = defaultValue
        self.container = container
    }

    public var wrappedValue: T {
        get {
            return container.object(forKey: key) as? T ?? defaultValue
        }
        set {
            container.set(newValue, forKey: key)
        }
    }
}


class SomeClass<T> {
    @UserDefault var someStoredValue: T

    init(key: String, defaultValue: T, container: UserDefaults) {
        self._someStoredValue = UserDefault(key: key, defaultValue: defaultValue, container: container)
    }
}

let someClass = SomeClass(key: "secret-key", defaultValue: "1", container: .standard)
print(someClass.someStoredValue)
// prints "1"

let anotherClass = SomeClass(key: "secret-key", defaultValue: "2", container: .standard)
print(anotherClass.someStoredValue)
// prints "2"

someClass.someStoredValue = "hehe"
print(anotherClass.someStoredValue)
// prints "hehe"