Flutter: Move to a new screen without providing navigate back to previous screen

I'm implementing an authentication flow in my Flutter app.

After a sign in attempt, the CheckAuth (which checks whether a user is signed in or not and then opens home screen or sign up screen accordingly) is opened with this code:

  void _signIn() async {
    await _auth
        .signInWithEmailAndPassword(
            email: _userEmail.trim(), password: _userPassword.trim())
        .then((task) {
      // go to home screen
      if (task.getIdToken() != null) {
        setState(() {
          Navigator.pushReplacement(
              context,
              new MaterialPageRoute(
                  builder: (BuildContext context) => new CheckAuth()));
        });
      } else {
        print("Authentication failed");
      }
    });
  }

Problem: I can successfully sign in to the app, but if I tap back button after I sign in, it goes back to the sign in screen (while I expect it to exit from the app).

Question: How to move from one screen to another in Flutter without the way back?

Do I need to somehow delete the navigator history? Or don't use navigator at all? I tried Navigator.replace method, but it didn't seem to work.

You need to use Navigator.pushReplacement when leaving the auth screen too. Not just when redirecting to login page.

You can use the pushAndRemoveUntil method:

Push the given route onto the navigator that most tightly encloses the given context, and then remove all the previous routes until the predicate returns true. To remove all the routes below the pushed route, use a [RoutePredicate] that always returns false (e.g. (Route<dynamic> route) => false).

Navigator.pushAndRemoveUntil(
  context,
  MaterialPageRoute(builder: (context) => MainPage()),
  (Route<dynamic> route) => false,
);