Flutter Firebase Auth - Catching expired token

Question

I'm using Firebase Auth plugin for authentication of my Flutter app.

Until the upgrade (not sure if relevant) to the latest Firebase auth version:

firebase_core: ^0.5.0
firebase_auth: ^0.18.0+1

everything worked fine.

Now I got, for the first time, Sentry error:

FirebaseAuthException: [firebase_auth/user-token-expired] The user's credential is no longer valid. The user must sign in again.
  File "exception.dart", line 20, in catchPlatformException
  File "zone.dart", line 1198, in _rootRunUnary
  File "zone.dart", line 1100, in _CustomZone.runUnary
  File "future_impl.dart", line 160, in _FutureListener.handleError
  File "future_impl.dart", line 708, in Future._propagateToListeners.handleError
  File "future_impl.dart", line 729, in Future._propagateToListeners
  File "future_impl.dart", line 537, in Future._completeError
  File "async_patch.dart", line 47, in _AsyncAwaitCompleter.completeError
  File "platform_channel.dart", in MethodChannel.invokeMapMethod
  File "<asynchronous suspension>"
  File "unparsed"

How can this happen? The user said, that he didn't use this app for a few days. As I understand Firebase Authentication documentation, the auth token automatically gets refreshed with the refresh token.

How can I mitigate this issue?

Where/how can I catch this exception to redirect a user to the login screen?

Anurag Tripathi Anurag Tripathi · Accepted Answer · 2020-09-18T14:47:00

You can do couple of things ,

First:

when a user starts your app , you should check first whether the user is already signed in or not , if he is ,then sign in silently ,if not , then send the user to sign in page , below is an example from an app I build four months ago (you will have to add google_sign_in dependency to your app)

class _StartingPageState extends State<StartingPage> {

  Future<dynamic> decideStartingPage() async {
    bool isUserSignedIn  = await googleSignIn.isSignedIn();

    if (isUserSignedIn == true) {
      FirebaseUser futurefbuser =await getCurrentFirebaseUser();
      assignFireBaseUser(futurefbuser);
      await googleSignIn.signInSilently();
      return HomeTabView();
    }
    else
      return LoginPage();
  }

  Future<dynamic> startingpage;

  @override
  void initState(){
    super.initState();
    startingpage=decideStartingPage();
    FirebaseAdMob.instance.initialize(appId: "ca-app-pub-...........");
  }

   @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future:startingpage,
      builder: (BuildContext ctx, AsyncSnapshot<dynamic> snapshot) {
        if (snapshot.connectionState != ConnectionState.done) {
          return Center( child : const CircularProgressIndicator());
        }
        else
          return snapshot.data;
      }
    );
  }
}

Second:

The second thing you can do is to save the access token and(or) refresh token you get when a user signs in, when that expires you can get an another access token using the refresh token . That means a little extra work for you as you will have to save the token in shared preferences or in a json file .

Now where to add the code for that handles the above procedure ? You have to identify the first thing that only an authenticated user can do like writing to the database , it is quite possible that your app is throwing exception at that because the user is unauthenticated for that operation . Using try , catch and finally you can sign in the user again without needing the user to do anything . for more info visit this

In first case user would need to explicitly sign in again , the second method requires extra work for you but more convenient for the user . Choice is yours .

Flutter Firebase Auth - Catching expired token

1 Answers