Firebase with Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Firebase Authentication UI Behavior

What will happen when the following Flutter code tries to sign in a user with Firebase Authentication using an email and password that do not exist in Firebase?

Firebase

try {
  UserCredential userCredential = await FirebaseAuth.instance.signInWithEmailAndPassword(
    email: '[e-mail]',
    password: 'wrongpassword'
  );
  print('Signed in: ${userCredential.user?.uid}');
} catch (e) {
  print('Error: $e');
}

A The app prints the user ID of the signed-in user.

B The app crashes with a null pointer exception.

C The app prints an error message indicating wrong credentials.

D The app signs in anonymously instead.

💡 Hint

Think about what Firebase does when credentials are invalid.

Firebase Authentication throws a FirebaseAuthException when the email or password is incorrect. The catch block prints the error message.

🧠 Conceptual

intermediate

Firebase Firestore Data Structure

In Firebase Firestore, which of the following best describes how data is organized?

A Data is stored in collections containing documents, which can have nested collections.

B Data is stored only as key-value pairs without any hierarchy.

C Data is stored as plain text files on the server.

D Data is stored in tables with rows and columns like SQL databases.

💡 Hint

Think about how Firestore groups related data.

Firestore organizes data in collections and documents. Documents can contain fields and nested collections, allowing flexible hierarchical data.

❓ lifecycle

advanced

Firebase StreamBuilder Lifecycle

Consider this Flutter widget using StreamBuilder to listen to Firestore changes. What happens if the widget is removed from the widget tree?

Firebase

StreamBuilder<QuerySnapshot>(
  stream: FirebaseFirestore.instance.collection('messages').snapshots(),
  builder: (context, snapshot) {
    if (!snapshot.hasData) return CircularProgressIndicator();
    return ListView(
      children: snapshot.data!.docs.map((doc) => Text(doc['text'])).toList(),
    );
  },
)

A The app crashes because the stream is not closed manually.

B The stream subscription is automatically cancelled to avoid memory leaks.

C The stream keeps running and causes a memory leak.

D The stream pauses but does not cancel, so updates are delayed.

💡 Hint

Think about how StreamBuilder manages streams internally.

StreamBuilder automatically cancels its stream subscription when the widget is disposed, preventing memory leaks.

❓ navigation

advanced

Firebase Auth State and Navigation

In a Flutter app using Firebase Authentication, which approach correctly navigates the user to the home screen only after successful login?

Firebase

FirebaseAuth.instance.authStateChanges().listen((User? user) {
  if (user != null) {
    Navigator.pushReplacementNamed(context, '/home');
  } else {
    Navigator.pushReplacementNamed(context, '/login');
  }
});

A This code works correctly and navigates based on auth state changes.

B This code does not navigate because authStateChanges() never emits events.

C This code causes infinite navigation loops because it listens continuously.

D This code causes navigation errors because context is not valid inside the listener.

💡 Hint

Consider when and where you can safely use Navigator with context.

Using Navigator inside a stream listener without checking widget lifecycle or context validity can cause errors. The context may be invalid if the widget is disposed.

🔧 Debug

expert

Debugging Firestore Query Results

Given this Firestore query in Flutter, why does the app always show an empty list even though the collection has documents?

Firebase

StreamBuilder<QuerySnapshot>(
  stream: FirebaseFirestore.instance.collection('tasks').where('done' == true).snapshots(),
  builder: (context, snapshot) {
    if (!snapshot.hasData) return CircularProgressIndicator();
    return ListView(
      children: snapshot.data!.docs.map((doc) => Text(doc['title'])).toList(),
    );
  },
)

A The where clause is incorrect; 'done' == true is a boolean expression, not a field filter.

B The collection 'tasks' does not exist in Firestore.

C The StreamBuilder is missing a required key parameter.

D The snapshot data is null because the app lacks Firestore read permissions.

💡 Hint

Look carefully at the syntax inside the where() method.

The expression 'done' == true evaluates immediately to false, so the where clause becomes .where(false), which is invalid. The correct syntax is .where('done', isEqualTo: true).