How to Implement One-Time Login with Firebase AuthStateListener?

One-Time Login with Firebase Authentication

In mobile applications that utilize Firebase for authentication, ensuring a seamless login experience is crucial. A common requirement is to implement a one-time login mechanism that maintains the user's logged-in status even after closing and relaunching the app.

Solution: Firebase AuthStateListener

To achieve one-time login, we can leverage the Firebase AuthStateListener. This listener monitors changes in the user's authentication status and provides real-time updates.

Implementation:

1. Create an AuthStateListener instance:

FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
    }
};

2. Instantiate the FirebaseAuth object:

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();

3. Register the AuthStateListener in both LoginActivity and MainActivity:

// LoginActivity
firebaseAuth.addAuthStateListener(authStateListener);

// MainActivity
firebaseAuth.addAuthStateListener(authStateListener);

4. In the LoginActivity, if the user is logged in, redirect them to the MainActivity:

if (firebaseUser != null) {
    Intent intent = new Intent(LoginActivity.this, MainActivity.class);
    startActivity(intent);
    finish();
}

5. In the MainActivity, if the user is not logged in, redirect them to the LoginActivity:

if (firebaseUser == null) {
    Intent intent = new Intent(MainActivity.this, LoginActivity.class);
    startActivity(intent);
}

Finally, remember to remove the listener in both activities' onStop() method to prevent memory leaks:

@Override
protected void onStop() {
    super.onStop();
    firebaseAuth.removeAuthStateListener(authStateListener);
}

By implementing this mechanism, users will only need to sign in once, ensuring a convenient and seamless login experience across app restarts.

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