how to ubblock the users in firebase code example

Example 1: restrict the user from going to signup or login page if the user is already logged in firebase auth

var user = firebase.auth().currentUser;

if (user) {
  // User is signed in.
} else {
  // No user is signed in.
}

Example 2: firebase auth update

In the newest version of firebase_auth:

FirebaseUser has been changed to User

AuthResult has been changed to UserCredential

GoogleAuthProvider.getCredential() has been changed to GoogleAuthProvider.credential()

onAuthStateChanged which notifies about changes to the user's sign-in state was replaced with authStateChanges()

currentUser() which is a method to retrieve the currently logged in user, was replaced with the property currentUser and it no longer returns a Future<FirebaseUser>