How to Implement Single-Time Login with Firebase Authentication in Android?

Single-Time Login Implementation in an App with Firebase Authentication

Introduction:

Achieving a one-time login mechanism in an app using Firebase Authentication ensures that users remain logged in even after the app is closed and reopened. This simplifies the user experience and eliminates the need for repetitive login screens.

Implementation:

Using FirebaseAuth AuthStateListener

To implement single-time login, a FirebaseAuth AuthStateListener can be employed. This listener monitors changes in the authentication state, allowing you to handle user login and logout events.

LoginActivity:

1. Create an AuthStateListener:

FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
        if (firebaseUser != null) {
            // If user logged in, redirect to MainActivity
            // Remove current view to prevent re-displaying LoginActivity
            startActivity(new Intent(LoginActivity.this, MainActivity.class));
            finish();
        }
    }
};

2. Start listening in onStart():

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

MainActivity:

1. Create an AuthStateListener:

FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
        if (firebaseUser == null) {
            // If user not logged in, redirect to LoginActivity
            // Remove background activity to ensure single-time login
            startActivity(new Intent(MainActivity.this, LoginActivity.class));
        }
    }
};

2. Start listening in onStart():

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

3. Stop listening in onStop():

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

Note:

• Both activities require a FirebaseAuth instance:

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();

• In both activities, listener removal in onStop() ensures proper resource management.

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