Handling oauth in flutter app without browser

Question

I have a rails backend server using doorkeeper to handle authentication on the api, and a flutter app (android/ios) to show some data to the user. Just for getting started I've used flutter_appauth to get a token for making api-requests, but I have the following problem/requirement:

1) Email/password fields should be handled entirely within the app, I don't want to handle the actual authentication with a drop-in browser or similar.

2) I don't want to use a 3rd party oauth provider.

I used flutter_appauth because it was really easy to set up, but I can't figure out if it supports my requirement of not leaving the app for authentication.

Can someone advice on the best possible library for handling this scenario, and possibly point towards a working example? Best, Jan

With flutter_appauth I've used the following configuration

_serviceConfiguration = AuthorizationServiceConfiguration(
    authorizationEndpoint: '$baseUrl/oauth/authorize',
    tokenEndpoint: '$baseUrl/oauth/token',
);

and this

Future<void> _signInWithAutoCodeExchange() async {
    bool preferEphemeralSession = false;
    if (Platform.isIOS || Platform.isMacOS) {
        preferEphemeralSession = true;
    }

    try {
        final AuthorizationTokenResponse? result =
            await _appAuth.authorizeAndExchangeCode(
            AuthorizationTokenRequest(
                clientId,
                Constants.redirectUrl,
                serviceConfiguration: _serviceConfiguration,
                preferEphemeralSession: preferEphemeralSession,
           ),
       );
    if (result != null) {
        _processAuthTokenResponse(result);
        Navigator.pushReplacement(context,
            MaterialPageRoute(builder: (context) => TeamsSelectorScreen()));
        } else {
            showDialog(
            context: context,
            builder: (context) => AlertDialog(
            title: const Text('Login Failed'),
            content: const Text('Something went wrong. Please try again.'),
            actions: [
              TextButton(
                child: const Text('OK'),
                onPressed: () => Navigator.pop(context),
              ),
            ],
          ),
        );
      }
    } catch (error, stackTrace) {
      //print('error in handling the response: $error');
    }
  }