How to Implement One-Time Login with Firebase Authentication in an Android App?

Implement One-Time Login in an App Using FirebaseAuth

In mobile applications, seamless and convenient user authentication is crucial. This article explores a technique to achieve one-time login in an app utilizing Firebase Authentication.

Background

Firebase Authentication simplifies user registration and authentication processes. By integrating phone number sign-in, you can ensure secure access to your app. However, traditional approaches involve logging in each time the app is launched or restarted.

Challenge

The goal is to implement a one-time login functionality, where users remain logged in even after closing and relaunching the app. Moreover, a logout option should not be available.

Solution

To achieve this, we leverage Firebase AuthStateListeners, which monitor changes in user authentication status. Here's how to implement it:

1. FirebaseAuth Initialization:

   • Instantiate a FirebaseAuth object in the MainActivity and LoginActivity.

2. AuthStateListener:

   • Define a FirebaseAuth.AuthStateListener in both activities.
   • If the user is logged in in the LoginActivity, redirect to MainActivity.
   • If the user is not logged in in the MainActivity, redirect to LoginActivity.

3. Start Listening:

   • Add the AuthStateListener in the onStart() method of both activities.

4. Stop Listening:

   • Remove the AuthStateListener in the onStop() method of both activities.

Implementation Details

LoginActivity:

FirebaseAuth auth = FirebaseAuth.getInstance();
FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser user = firebaseAuth.getCurrentUser();
        if (user != null) {
            startActivity(new Intent(LoginActivity.this, MainActivity.class));
            finish();
        }
    }
};

@Override
protected void onStart() {
    super.onStart();
    auth.addAuthStateListener(authStateListener);
}

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

MainActivity:

FirebaseAuth auth = FirebaseAuth.getInstance();
FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser user = firebaseAuth.getCurrentUser();
        if (user == null) {
            startActivity(new Intent(MainActivity.this, LoginActivity.class));
        }
    }
};

@Override
protected void onStart() {
    super.onStart();
    auth.addAuthStateListener(authStateListener);
}

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

This approach ensures that users are only required to log in once, providing a seamless experience while maintaining security.

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