What is the logic behind this authentication in Firebase for Flutter? - firebase

Question

I'm following the Firebase for Flutter Codelab and on the 8th step there's this _ensureLoggedIn() function:
final _googleSignIn = new GoogleSignIn();
final _auth = FirebaseAuth.instance;
Future<Null> _ensureLoggedIn() async {
GoogleSignInAccount user = _googleSignIn.currentUser;
if (user == null)
user = await _googleSignIn.signInSilently();
if (user == null) {
await _googleSignIn.signIn();
analytics.logLogin();
}
if (await auth.currentUser() == null) {
GoogleSignInAuthentication credentials =
await _googleSignIn.currentUser.authentication;
await auth.signInWithGoogle(
idToken: credentials.idToken,
accessToken: credentials.accessToken,
);
}
}
As a newbie to both Flutter and Firebase frameworks, i'm really struggling to understand the logic behind it: First we attempt to log the user with the GoogleSignIn package, and then regardless of what we will have in user we try to auth the user again with FirebaseAuth, which in turn will also use the GoogleSignIn.
Could you explain me why we do both?
My target is to have two separate screens for users who open my app - one for unauthorized/anonymous (which will have Log In and Register options), and one for authorized users that will see the normal app interface.

Answer 1

The login example in that Codelab seems poorly written, as the user can potentially cancel the non-silent signIn() and then googleSignIn.currentUser will be null when they try to access googleSignIn.currentUser.authentication. I think a better way to handle it is trigger the Google sign in and handle the Firebase auth in in the googleSignIn.onAuthStateChanged listener.
As for why both are used in that example: If you want to authenticate a user on Firebase with a Google account you have to provide an idToken and accessToken, which must be obtained from a valid Google login. So first, you have them sign in to their Google account (via googleSignIn) and use the tokens from that to authenticate with Firebase (via auth).
Using googleSignIn is only required if you want to authenticate with Firebase using a Google account; you can also use Firebase Auth with a username/password combination (which requires that the account be created on Firebase first) or with a token from a Facebook login, or some other OAuth token.