Firebase 如何在 Auth 中定义当前用户？

Question

I'm using Firebase Auth with Google and doing everyrhing through Firebase docs, and in my SigninActivity , I have onStart method, which check is there are current user or no.

@Override
    protected void onStart() {
        super.onStart();
        Log.d(TAG, "onStart");
        // Check if user is signed in (non-null) and update UI accordingly.
        FirebaseUser currentUser = mAuth.getCurrentUser();
        updateUI(currentUser);
    }

and also have

FirebaseAuth.getInstance().signOut();

which signs out user.

And i cannot understand, how Firebase defines current user, when he not logged in yet. Is there are unique ID for app or how?

Answer 1

According to your comments:

Yes, And also i dont understand, how Firebase knows current user, when he not signed in yet? I mean, which criteria Firebase checks in this method FirebaseUser currentUser = mAuth.getCurrentUser()

Firebase "knows" when the current user is logged in or not by checking the FirebaseUser object for nullity. If the user is logged in, when using the following line of code:

FirebaseUser currentUser = mAuth.getCurrentUser();

It returns a non nullable FirebaseUser object. This is the reason why we always check it for nullity. If you are using the following line of code:

FirebaseAuth.getInstance().signOut();

It means that the FirebaseUser becomes null , which also means that the user has signed out.