presenting whose view is not in the window hierarchy warning - ios

Question

I am new in Programming and swift, and I have tried to read some solutions in stack overflow, but to be honest I don't really grasp with the answer :(
I have 2 view controllers. a homeVC and a LoginVC. homeVC is my initial view controller. in viewDidLoad I have firebase function that can check if the user has logged in before or not. if not, then the user will be send to loginVC. here is my simplified code in the HomeVC
import UIKit
import Firebase
class HomeVC: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
// to check whether the user has already logged in or not
Auth.auth().addStateDidChangeListener { (auth, user) in
if user == nil {
let login = self.storyboard?.instantiateViewController(withIdentifier: "login") as! LoginVC
self.present(login, animated: true, completion: nil)
}
}
print("user enter homeVC")
}
}
and here is my loginVC
import UIKit
import Firebase
import GoogleSignIn
class LoginVC : UIViewController, GIDSignInUIDelegate {
#IBOutlet weak var googleButton: GIDSignInButton!
#IBOutlet weak var emailButton: UIButton!
override func viewDidLoad() {
super.viewDidLoad()
// delegate declaration
GIDSignIn.sharedInstance().uiDelegate = self
}
#IBAction func googleButtonDidPressed(_ sender: Any) {
GIDSignIn.sharedInstance().signIn()
}
}
the app can perform as I expected. but there is a warning in my debugging area :
Warning: Attempt to present LoginVC: 0x7fc315714f40 on
HomeVC: 0x7fc3155095c0 whose view is not in the window
hierarchy!
of course the problem is in this lines of code
let login = self.storyboard?.instantiateViewController(withIdentifier: "login") as! LoginVC
self.present(login, animated: true, completion: nil)
as far as I know, if the view is stacked in the layer of navigation controller, then if I want to move to another view controller I have to use perform segue method.
But for this case, between homeVC and LoginVC are not stacked in the same navigation controller. so no hierarchy. thats why I use that line of code to move to another view controller (loginVC). but I don't understand why it is said "view is not in the window hierarchy!"
So what should I do to omit that warning?

Answer 1

Move code to viewDidAppear
override func viewDidAppear(_ animated:Bool) {
super.viewDidAppear(true)
// to check whether the user has already logged in or not
Auth.auth().addStateDidChangeListener { (auth, user) in
if user == nil {
let login = self.storyboard?.instantiateViewController(withIdentifier: "login") as! LoginVC
self.present(login, animated: true, completion: nil)
}
}
print("user enter homeVC")
}

Answer 2

Your LoginVC is perfectly fine.
However, you need to change your HomeVC as #Sh_Khan suggested and move the testing code from viewDidLoad to viewDidAppear:
import UIKit
import Firebase
class HomeVC: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
}
// HomeVC.view was added to a view hierarchy
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
// to check whether the user has already logged in or not
Auth.auth().addStateDidChangeListener { (auth, user) in
if user == nil {
let login = self.storyboard?.instantiateViewController(withIdentifier: "login") as! LoginVC
self.present(login, animated: true, completion: nil)
}
}
}
}
Explanation
Your viewDidLoad method gets called before the viewController gets presented, so it at that moment it cannot really present another view controller (since it itself is not presented), viewDidLoad documentation:
Called after the controller's view is loaded into memory.
This method is called after the view controller has loaded its view hierarchy into memory. This method is called regardless of whether the view hierarchy was loaded from a nib file or created programmatically in the loadView() method. You usually override this method to perform additional initialization on views that were loaded from nib files.
In that moment the viewController is not in the window hierarchy yet.
viewDidAppear however gets called when the view is presented and becomes a part of the window hierarchy, viewDidAppear documentation:
Notifies the view controller that its view was added to a view hierarchy.
You can override this method to perform additional tasks associated with presenting the view. If you override this method, you must call super at some point in your implementation.
Don't forget to call super.viewDidAppear during overriding it.

Answer 3

TLDR; You should move your code to viewDidAppear
viewDidLoad()
This method is called after the view controller has loaded its view hierarchy into memory. This method is called regardless of whether the view hierarchy was loaded from a nib file or created programmatically in the loadView() method. You usually override this method to perform additional initialization on views that were loaded from nib files.
Apple docs
So the view is only in memory yet and not in the hierarchy. You should move it to viewDidAppear
viewDidAppear()
Notifies the view controller that its view was added to a view hierarchy. Apple docs

Answer 4

As Sh_Khan said, move the lines:
let login = self.storyboard?.instantiateViewController(withIdentifier: "login") as! LoginVC
self.present(login, animated: true, completion: nil)
in viewDidAppear() method; when viewDidLoad() gets called, the view controller it's not added on the view hierarchy yet, it's not visible and it cannot present another view controller.

Answer 5

The reason for such kind of error is: You are trying present (open) two view controllers simultaneously (view of first presenting view controller is just started and you may be trying to present second view controller).
You should move your code (for view controller presentation/navigation) to viewDidAppear. Your main view of existing view controller (from where you are presenting new view controller) is not ready/loaded.
You should move it to viewDidAppear.
Here is sample code:
Swift 4
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
// instantiate your view controller either using storyboard or class or any other way....
if let newVC = self.storyboard?.instantiateViewController(withIdentifier: "NewViewController") as? NewViewController {
self.present(newVC, animated: true, completion: nil)
}
}
In your case/code, solution is:
override func viewDidLoad() {
super.viewDidLoad()
// Move your code from here (viewDidLoad) to viewDidAppear
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
// to check whether the user has already logged in or not
Auth.auth().addStateDidChangeListener { (auth, user) in
if user == nil {
let login = self.storyboard?.instantiateViewController(withIdentifier: "login") as! LoginVC
self.present(login, animated: true, completion: nil)
}
}
print("user enter homeVC")
}
Look at the difference between both view controller life cycle.
viewDidLoad
This method is called after the view controller has loaded its view hierarchy into memory. This method is called regardless of whether the view hierarchy was loaded from a nib file or created programmatically in the loadView() method. You usually override this method to perform additional initialization on views that were loaded from nib files.
See more about: viewDidLoad
viewDidAppear
Notifies the view controller that its view was added to a view hierarchy.
See more about: viewDidAppear

Answer 6

At this point in your code, the view controller's view has only been created but not added to any view hierarchy. If you want to present from that view controller as soon as possible you should do it in viewDidAppear to be safest.