Funzione per il login con Apple

sign_in_apple.dart

static Future<UserModel> signInWithApple() async {
  final OAuthProvider appleOAuth = OAuthProvider("apple.com");
  
  final AuthorizationCredentialAppleID appleCredential = await SignInWithApple.getAppleIDCredential(
    scopes: [
      AppleIDAuthorizationScopes.email,
      AppleIDAuthorizationScopes.fullName,
    ],
  );
  
  OAuthCredential appleRemappedCredential = appleOAuth.credential(idToken: appleCredential.identityToken, accessToken: appleCredential.authorizationCode);
  UserCredential result = await FirebaseAuth.instance.signInWithCredential(appleRemappedCredential);

  if (result.user != null) {
    // debugPrint(result.user.uid);
    if ((await References.usersCollection.doc(result.user.uid).get()).exists) return UserModel.fromJson((await References.usersCollection.doc(result.user.uid).get()).data());

    while (!(await References.usersCollection.doc(result.user.uid).get()).exists) await Future.delayed(Duration(seconds: 2));

    UserModel currentUser = UserModel.fromJson((await References.usersCollection.doc(result.user.uid).get()).data());
    currentUser.reference = References.usersCollection.doc(result.user.uid);

    currentUser.displayName = appleCredential.givenName + " " + appleCredential.familyName;

    await result.user.updateProfile(displayName: appleCredential.givenName + " " + appleCredential.familyName);
    await currentUser.updateOnServer();

    return currentUser;
  }

  throw ("Errore nel login con Apple!");
}