Firebase Authentication in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when a user tries to sign in with an invalid email format?

Consider this Flutter code snippet using Firebase Authentication to sign in a user. What will be the app's behavior if the user enters an invalid email format?

Flutter

try {
  await FirebaseAuth.instance.signInWithEmailAndPassword(
    email: userEmail,
    password: userPassword
  );
  print('Sign in successful');
} catch (e) {
  print('Error: $e');
}

A The app crashes with a null pointer exception.

B The app prints 'Sign in successful' even with invalid email format.

C The app throws a FirebaseAuthException with code 'invalid-email'.

D The app ignores the error and continues without signing in.

💡 Hint

Firebase validates email format and throws specific exceptions for invalid inputs.

Firebase Authentication checks the email format and throws a FirebaseAuthException with code 'invalid-email' if the format is wrong. This exception can be caught and handled.

❓ lifecycle

intermediate

When does FirebaseAuth.instance.authStateChanges() emit a new event?

In Flutter, FirebaseAuth provides authStateChanges() stream. When does this stream emit a new event?

A Every second regardless of user state.

B Only when the app starts.

C Only when the user changes their password.

D When the user signs in or signs out.

💡 Hint

Think about what changes the authentication state.

The authStateChanges() stream emits a new event whenever the user's sign-in state changes, such as signing in or signing out.

📝 Syntax

advanced

What is the output of this Flutter Firebase sign-up code snippet?

Analyze the following code and select the correct output printed to the console.

Flutter

try {
  final userCredential = await FirebaseAuth.instance.createUserWithEmailAndPassword(
    email: '[e-mail]',
    password: '123'
  );
  print('User ID: ${userCredential.user?.uid}');
} catch (e) {
  print('Error: $e');
}

A Error: FirebaseAuthException with code 'weak-password'

B User ID: a valid UID string

C User ID: null

D Error: FormatException

💡 Hint

Firebase requires passwords to be at least 6 characters.

Firebase throws a FirebaseAuthException with code 'weak-password' if the password is less than 6 characters.

❓ navigation

advanced

What happens to the navigation stack after a successful Firebase sign-out?

In a Flutter app, after calling FirebaseAuth.instance.signOut(), what is the best practice for navigation to reflect the signed-out state?

A Pop all routes and push the login screen as the new root.

B Push a new home screen on top of the current stack.

C Do nothing and stay on the current screen.

D Replace the current screen with a loading spinner.

💡 Hint

Think about preventing the user from returning to authenticated screens after sign-out.

After sign-out, the app should clear the navigation stack and push the login screen as the new root to prevent back navigation to protected screens.

🔧 Debug

expert

Why does this Flutter Firebase sign-in code cause an unhandled exception?

Review the code below. Why does it cause an unhandled exception when the user enters wrong credentials?

Flutter

void signInUser(String email, String password) async {
  final userCredential = await FirebaseAuth.instance.signInWithEmailAndPassword(
    email: email,
    password: password
  );
  print('Signed in: ${userCredential.user?.email}');
}

// Called somewhere without try-catch

A Because signInWithEmailAndPassword returns null on failure, causing a null pointer.

B Because exceptions from signInWithEmailAndPassword are not caught, causing unhandled exceptions.

C Because the print statement uses string interpolation incorrectly.

D Because the async function is not awaited, causing timing issues.

💡 Hint

Think about error handling with async calls.

If signInWithEmailAndPassword fails (e.g., wrong password), it throws an exception. Without try-catch, this exception is unhandled and crashes the app.