Listen for authentication state in Android

Listen for authentication state.java

// In Firebase it is often better to *react* to authentication state 
// changes, instead getting the current authentication state everywhere.
// To respond to auth state changes, use an auth state change listener 
// like this:
FirebaseAuth auth = FirebaseAuth.getInstance();
auth.addAuthStateListener(new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        if (firebaseAuth.getCurrentUser() == null) {
            Log.i("firebase", "AuthState changed to null");
        }
        else {
            Log.i("firebase", "AuthState changed to "+firebaseAuth.getCurrentUser().getUid());
        }
    }
});

Listen for authentication state.kt

// In Firebase it is often better to *react* to authentication state 
// changes, instead getting the current authentication state everywhere.
// To respond to auth state changes, use an auth state change listener 
// like this:
val auth = FirebaseAuth.getInstance();
auth.addAuthStateListener {
    Log.i("firebase", "AuthState changed to ${it.currentUser?.uid}")
    if (it.currentUser != null) {
      ... do something with the signed in user
    }
}