How to Implement a One-Time Phone Number Login with Firebase Authentication?

Firebase Authentication: Implementing One-Time Login with Phone Number Authentication

In Firebase authentication, achieving one-time login for users who have signed in via phone numbers involves maintaining a persistent login state even after closing and reopening the app. This eliminates the need for a logout feature.

Solution:

Utilizing a Firebase AuthStateListener can effectively establish this functionality. Here's how to implement it:

1. Create an Auth State Listener:

FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
        if (firebaseUser != null) {
            // User is logged in, proceed to MainActivity
            Intent intent = new Intent(LoginActivity.this, MainActivity.class);
            startActivity(intent);
            finish();
        }
    }
};

This listener monitors changes in the authentication state. If a user is logged in, it initiates navigation to the MainActivity.

2. Instantiate FirebaseAuth and Register Listener:

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();
firebaseAuth.addAuthStateListener(authStateListener);

Instantiate the FirebaseAuth object and start listening for changes in the onStart() method.

3. Implement Auth State Listener in MainActivity:

In the MainActivity, create a similar AuthStateListener that handles the case when the user is not logged in and redirects them to the LoginActivity.

4. Remove Listener on Activity Pause:

When the activity pauses, remove the listener to avoid unnecessary callbacks:

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

By following these steps, you can ensure one-time login for users who have signed in with their phone numbers using Firebase Authentication.

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