flutter - How to keep user signed-in with Google

Users already stay authenticated. After you restart the app, Firebase reads the credentials from disk, and refreshes the user's token. Since this requires a roundtrip to the server, it happens asynchronously. So be sure to await _auth.currentUser() to get notified of the user's status.

Whenever I'm wondering how to do such things, I look at the FlutterFire sample app. This specific line can be found here.