Singleton and their synchronization in swift - ios

Question

I am working on project where I have requirements of using singleton.
while using them I stuck on following code
struct User{
var someUserProperty: String?{
willSet{
print(UserManager.shared.sharedUser ?? "")
}
}
}
class UserManager{
static var shared: UserManager = UserManager()
public var sharedUser: User?
/* Private to make this class singleton */
private init() {
self.sharedUser = User(someUserProperty: "someInitialValue")
}
}
class Managers{
static var userManager = UserManager.shared
}
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
Managers.userManager.sharedUser?.someUserProperty = "someDifferentValue"
}
}
This code throws error in viewDidLoad as it is setting User's property and in that property's observer we are getting same User object.
To solve this, I searched and get solution, which led me to modification in UserManager as follows
class UserManager{
private let concurrentQueue = DispatchQueue(label: "ConcurrentQueue", attributes: .concurrent, target: nil)
static var shared: UserManager = UserManager()
public var sharedUser: User?{
get{
return concurrentQueue.sync {
return self.privateSharedUser
}
}set{
concurrentQueue.async(flags: .barrier){[unowned self] in
self.privateSharedUser = newValue
}
}
}
private var privateSharedUser: User?
/* Private to make this class singleton */
private init() {
self.sharedUser = User(someUserProperty: "someInitialValue")
}
}
I have used dispatch queue to allow read from different places but write in barrier mode so that it will not interrupt read operations. and it worked.
But also following solution works, and that is making me wonder because now I have removed all queuing task and just put get and set.
class UserManager{
private let concurrentQueue = DispatchQueue(label: "ConcurrentQueue", attributes: .concurrent, target: nil)
static var shared: UserManager = UserManager()
public var sharedUser: User?{
get{
return self.privateSharedUser
}set{
self.privateSharedUser = newValue
}
}
private var privateSharedUser: User?
/* Private to make this class singleton */
private init() {
self.sharedUser = User(someUserProperty: "someInitialValue")
}
}
My thinking is that what above code does is that just creating wrapper named sharedUser around actual variable privateUser. So when we set something on sharedUser behind the scenes, it will give you private privateSharedUser whose address is different than sharedUser. Is this the reason ? Or Swift is doing something great behind the scenes ? Please tell me, if any one knows