Query Data from Firebase and write in UITextfield - ios

Question

I'm trying to pull data from my firebase project and write the field value in a UITextfield. I can't seem to figure out how to query the data needed for the UITextfields from the firebase field values correctly.
Is there anyway to do this? I have only seen videos of people adding firebase documents to a tableview, not any for direct pull of a firebase value to a UITextfield.
I've tried:
Switching the constants to instance the UITextfield Outlets and optionally unwrap as text fields let username = data[USERNAME] as? UITextField ?? "anonymous"
Creating variables of the UITextfields and equal the collections array data. So something kind like... let self.username = Userdata[0] but I keep getting "expected pattern error"
I know these may not be appropriate syntax but this is the first time I've ever coded and attempted to make an app, but I can't find any tutorial to follow for this scenario. So any information greatly appreciated, Thanks.
Heres what I've got so far...
import UIKit
import FirebaseDatabase
import Firebase
class ProfileVC: UIViewController {
#IBOutlet weak var userImage: UIImageView!
#IBOutlet weak var username: UITextField!
#IBOutlet weak var firstname: UITextField!
#IBOutlet weak var userEmail: UITextField!
#IBOutlet weak var lastname: UITextField!
#IBOutlet private weak var bgView: UIView!
//variables
private var Userdatas = [Userdata]()
private var usersCollectionRef: CollectionReference!
override func viewDidLoad() {
super.viewDidLoad()
usersCollectionRef = Firestore.firestore().collection(USERS_REF)
}
override func viewWillAppear(_ animated: Bool){
usersCollectionRef.getDocuments { (snapshot, error) in
if let err = error {
debugPrint("Error fetching docs: \(err) ")
} else {
guard let snap = snapshot else{return}
for document in snap.documents {
let data = document.data()
let username = data[USERNAME] as? String ?? "anonymous"
let firstname = data[FIRST_NAME] as? String ?? "Anonymous"
let lastname = data[LAST_NAME] as? String ?? "Anonymous"
let email = data[EMAIL] as? String ?? "Anonymous"
let documentID = document.documentID
let newuserData = Userdata(username: username, email: email, firstname: firstname, lastname: lastname, documentID: documentID)
}
}
}
}