Swift Firebase Check if user exists - swift

Question

What am i doing wrong? I have a database structure like the one shown in this image.
In appleDelegate.swift i just want to check if a certain user token actually exists under the "users" node. that is, if "users" has the child currentUserID (a string token). I understand observeSingleEvent is executed asynchronously.I get this error in swift: 'Application windows are expected to have a root view controller at the end of application launch'. in "func application(_ application: UIApplication" i have this code. I also have my completion handler function below.
if let user = Auth.auth().currentUser{
let currentUserID = user.uid
ifUserIsMember(userId:currentUserID){(exist)->() in
if exist == true{
print("user is member")
self.window?.rootViewController = CustomTabBarController()
} else {
self.window?.rootViewController = UINavigationController(rootViewController: LoginController())
}
}
return true
} else {
self.window?.rootViewController = UINavigationController(rootViewController: LoginController())
return true
}
}
func ifUserIsMember(userId:String,completionHandler:#escaping((_ exists : Bool)->Void)){
print("ifUserIsMember")
let ref = Database.database().reference()
ref.child("users").observeSingleEvent(of: .value, with: { (snapshot) in
if snapshot.hasChild(userId) {
print("user exists")
completionHandler(true)
} else {
print("user doesn't exist")
completionHandler(false)
}
})
}

Answer 1

I would suggest moving the code out of the app delegate and into an initial viewController. From there establish if this is an existing user and send the user to the appropriate UI.
.observeSingleEvent loads all of the nodes at a given location - one use would be to iterate over them to populate a datasource. If there were 10,000 users they would all be loaded in if you observe the /users node.
In this case it's really not necessary. It would be better to just observe the single node you are interested in and if it exists, send the user to a UI for existing users.
here's the code to do that
if let user = Auth.auth().currentUser {
let ref = self.ref.child("users").child(user.uid)
ref.observeSingleEvent(of: .value, with: { snapshot in
self.presentUserViewController(existing: snapshot.exists() )
})
}
snapshot.exists will be either true if the user node exists or false if not so the function presentUserViewController would accept a bool to then set up the UI depending on the user type.