Where in Firebase login process, do I check/migrate existing content? - swiftui

Question

I have a SwiftUI/Firebase project, where I allow users to create and upload content while logged in with anonymous. I also have a Firebase rule that prevent editing data that isn't tagged with the same UID as you're logged in with.
My problem is that, when users log in with Google or Apple login, I don't know where to insert any logic for migrating their content from their old anonymous UID to their Apple/Google UID. (Update: Yes, I can link accounts, but that only works if they haven't previously used their account on a different device).
As far as I can tell, I don't get their new Apple/Google UID until after they're authenticated, and by then, they can no longer modify data tagged with the Anonymous UID.
I've tried linking the accounts, but I get an "Account is already linked" error, so I'm assuming that approach is a dead end?
As an example, here is my code for the Google login with a note where I'm trying to insert my migration logic:
import SwiftUI
import Firebase
import GoogleSignIn
struct GoogleSignInButton: View {
#EnvironmentObject var viewModel: GoogleSignInViewModel
var body: some View {
Button("Sign in with Google") {
viewModel.signIn()
}
.foregroundColor(Color.greyZ)
.padding()
.frame(maxWidth: .infinity)
.background(Color.greyB)
.cornerRadius(5)
.padding()
}
}
struct GoogleSignInButton_Previews: PreviewProvider {
static var previews: some View {
GoogleSignInButton()
}
}
class GoogleSignInViewModel: NSObject, ObservableObject {
enum SignInState {
case signedIn
case signedOut
}
#Published var state: SignInState = .signedOut
override init() {
super.init()
setupGoogleSignIn()
}
func signIn() {
if GIDSignIn.sharedInstance().currentUser == nil {
GIDSignIn.sharedInstance().presentingViewController = UIApplication.shared.windows.first?.rootViewController
GIDSignIn.sharedInstance().signIn()
}
}
func signOut() {
GIDSignIn.sharedInstance().signOut()
do {
try Auth.auth().signOut()
state = .signedOut
} catch let signOutError as NSError {
print(signOutError.localizedDescription)
}
}
private func setupGoogleSignIn() {
GIDSignIn.sharedInstance().delegate = self
}
}
extension GoogleSignInViewModel: GIDSignInDelegate {
func sign(_ signIn: GIDSignIn!, didSignInFor user: GIDGoogleUser!, withError error: Error!) {
if error == nil {
// Get UID of existing user
if let previousUID:String = Auth.auth().currentUser?.uid {
// migrate Firestore data for old uid to new uid
// Firebase rule prevent modifying data if you're logged in with different uid so it has to be before logging in with Google
// But I don't seem to have the new Google UID yet, so what do I migrate it to?
}
// Log in with new user
firebaseAuthentication(withUser: user)
} else {
print(error.debugDescription)
}
}
private func firebaseAuthentication(withUser user: GIDGoogleUser) {
if let authentication = user.authentication {
let credential = GoogleAuthProvider.credential(withIDToken: authentication.idToken, accessToken: authentication.accessToken)
Auth.auth().signIn(with: credential) { (_, error) in
if let error = error {
print(error.localizedDescription)
self.state = .signedOut
} else {
self.state = .signedIn
}
}
}
}
}
UPDATE: As requested, here is the Link-function that invariably results in a "This credential is already associated with a different user account" error. I have checked the account in Firebase, and the account already exists, so that is why I assumed the "link" approach is a dead end, and tried migrating the data instead.
private func firebaseAuthentication(withUser user: GIDGoogleUser) {
if let authentication = user.authentication {
let credential = GoogleAuthProvider.credential(withIDToken: authentication.idToken, accessToken: authentication.accessToken)
if let currentUser = Auth.auth().currentUser {
// User already logged in
currentUser.link(with: credential) { result, error in
if let error = error {
print(error.localizedDescription)
} else {
print(result ?? "Success")
}
}
} else {
// User not logged in (shouldn't happen as they're always anonymous
Auth.auth().signIn(with: credential) { (_, error) in
if let error = error {
print(error.localizedDescription)
self.state = .signedOut
} else {
self.state = .signedIn
}
}
}
}
}

Answer 1

Instead of migrating the data, consider linking the user's new Google or Apple credentials to their existing Firebase account by filling the process outlines in linking multiple Auth providers to an account on iOS.