Assign username value from firestore - swift

Question

i'm creating my first app (and newbie in swift). When i login from Facebook, the name and email are saved in Firestore. I'm trying to set the name from facebook to a variable to use it in other places, but i can't assign the value, always shows "nil" in the console. Anyone can help me please?
I set the variable
var userN: String?
I get the data from Firestore
func readDatabase(){
let db = Firestore.firestore()
let docRef = db.collection("users").document("email")
docRef.getDocument { (document, error) in
if let document = document, document.exists {
let dataDescription = document.data().map(String.init(describing:)) ?? "nil"
print("Document data: \(dataDescription)")
let data = document.data()
let userName = data!["name"]! as! String
print(userName)
let userEmail = data!["email"]! as! String
print(userEmail)
let containerController = ContainerController()
let containerController.userN = userName;
return
}
}
}
i want to assign userN = userName, to use it in other view
How can i do that? thanks

Answer 1

If you are using StoryBoards you can pass this through the segue function;
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "YourStoryBoardSegue" {
if let viewController = segue.destination as? ContainerController {
viewController.userN = userName
}
}
}
otherwise, best practice would be to use a delegate method.
Search stack overflow for best practices using delegates to pass data.

Answer 2

The question is extremely broad and without knowing details the only way to address it is with a general answer that specifically addresses how to read data from Firestore and save it in a variable to be used later.
Suppose your Firestore looks like this
root
users
uid_0
name: "users name"
uid_1
name: "another users name"
and when the app loads, we want to read the users name and store it in in a variable per your question:
a variable to use it in other places
Here's what that could look like
class ViewController: NSViewController {
var usersName = ""
override func viewDidLoad() {
super.viewDidLoad()
FirebaseApp.configure()
self.db = Firestore.firestore()
let settings = self.db.settings
self.db.settings = settings
self.readUserName()
}
func readUsersName() {
let users = self.db.collection("users")
let thisUser = users.document("uid_1")
thisUser.getDocument(completion: { documentSnapshot, error in
if let error = error {
print(error.localizedDescription)
return
}
let name = documentSnapshot?.get("name") as! String
self.usersName = name
})
}
The code sets up Firestore, reads the user name from the uid_1 document and stores it in a variable where it could be used later.
Suppose we want to let the user change their name. There's 100 ways to do it; passing data via a segue, use a delegate method or open a detail view controller and before it closes, have this master controller read the updated name from a textField and save the data. You could even pass the users uid and then in the detail viewcontroller read the document via that uid and then update it upon closing. However, all of those go beyond the scope of the question.