[Flutter Web - Authentication] What is the right way to redirect not logged in users?

[xcsob]

I'm developing a web app using Flutter Web and Firebase.
I have to handle the Firebase Login.
Let's assume we have two screens (login and homepage), the situation that I want to achieve is the following:

if the user is not logged in:

• Whatever url he types in the web, redirect the user on the login page

If the user is not logged in:

• if he lands on the login page, redirect him on the homepage
• if he lands on any other pages, stay on that page

I've implemented a functions that checks the current user in firebase and acts as following:

void checkAuthentication() {
var url = window.location.href;

var navigationService = locator<NavigationService>();

var loggedIn = this.isUserLoggedIn();
if (!loggedIn) {
  navigationService.replaceWith(Routes.login);
} else {
  if (url.contains("Login")) {
    navigationService.replaceWith(Routes.homepage);
  }
}
}

This solution works, but has two problems:

1. This is not the right approach to do this. It's not possible that I have to call this in each screen page

2. When you are redirected you can see a transition with the new page presented.

My question: How would you manage this in Flutter Web in a unique point in the code? Is there a better way to achieve this different from the one I shown here?

Thanks