Firebase Authentication in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens after a successful Firebase email/password sign-in?

Consider this Swift code snippet for Firebase Authentication sign-in:

Auth.auth().signIn(withEmail: email, password: password) { authResult, error in
  if let user = authResult?.user {
    print("User signed in: \(user.email ?? "unknown")")
  } else {
    print("Sign-in failed")
  }
}

What will be printed if the sign-in is successful?

iOS Swift

Auth.auth().signIn(withEmail: email, password: password) { authResult, error in
  if let user = authResult?.user {
    print("User signed in: \(user.email ?? \"unknown\")")
  } else {
    print("Sign-in failed")
  }
}

A User signed in: [e-mail]

B Sign-in failed

C User signed in: unknown

D No output printed

💡 Hint

Check what the 'user.email' property contains after successful sign-in.

When sign-in succeeds, 'authResult?.user' is non-nil and 'user.email' contains the signed-in user's email. The print statement outputs that email.

🧠 Conceptual

intermediate

What error is returned if Firebase sign-in is attempted with wrong password?

In Firebase Authentication for iOS, if a user tries to sign in with an email that exists but enters the wrong password, what error code will the completion handler receive?

A AuthErrorCode.networkError

B AuthErrorCode.userNotFound

C AuthErrorCode.invalidEmail

D AuthErrorCode.wrongPassword

💡 Hint

Think about the error that specifically means the password is incorrect.

Firebase returns 'AuthErrorCode.wrongPassword' when the password does not match the stored password for the email.

❓ lifecycle

advanced

When is the Firebase Auth state listener triggered?

Given this Swift code:

Auth.auth().addStateDidChangeListener { auth, user in
  if let user = user {
    print("User is signed in: \(user.uid)")
  } else {
    print("No user signed in")
  }
}

At which moments will this listener be called?

iOS Swift

Auth.auth().addStateDidChangeListener { auth, user in
  if let user = user {
    print("User is signed in: \(user.uid)")
  } else {
    print("No user signed in")
  }
}

A Only when the user signs in

B When the app starts and whenever the user signs in or out

C Only when the user signs out

D Only when the app is terminated

💡 Hint

Think about when the authentication state changes.

The listener triggers immediately on adding and whenever the user signs in or out, reflecting current auth state.

📝 Syntax

advanced

What is the correct way to create a new Firebase user with email and password?

Which of the following Swift code snippets correctly creates a new user with Firebase Authentication?

A

Auth.auth().createUser(withEmail: email, password: password) { authResult, error in
  if let user = authResult?.user {
    print("User created: \(user.email ?? "")")
  }
}

B

Auth.auth().signUp(withEmail: email, password: password) { authResult, error in
  print("User created")
}

C

Auth.auth().createUser(email: email, password: password) { authResult, error in
  print("User created")
}

D

Auth.createUser(withEmail: email, password: password) { user, error in
  print("User created")
}

💡 Hint

Check the exact method name and parameters in Firebase iOS SDK.

The correct method is 'createUser(withEmail:password:completion:)' on 'Auth.auth()'. Other options use wrong method names or signatures.

🔧 Debug

expert

Why does this Firebase sign-out code cause a crash?

Consider this Swift code snippet:

try? Auth.auth().signOut()
print("User signed out")

Sometimes this code crashes the app. What is the most likely cause?

iOS Swift

try? Auth.auth().signOut()
print("User signed out")

A Firebase requires a completion handler for signOut()

B The print statement is called before signOut completes

C The signOut() method can throw an error that is not handled properly

D The Auth object is nil causing a crash

💡 Hint

Check how Swift handles errors from throwing functions.

signOut() is a throwing method. Using 'try?' ignores errors but if an error occurs, it can cause unexpected behavior or crash if not handled properly.

Practice

1. What is the main purpose of Firebase Authentication in an iOS app?

easy

A. To send push notifications to users

B. To store large files in the cloud

C. To design the app's user interface

D. To let users sign up and log in easily

Solution

1. Step 1: Understand Firebase Authentication's role

   Firebase Authentication helps manage user sign-up and sign-in processes.

2. Step 2: Compare options with this role

   Only To let users sign up and log in easily matches this purpose; others relate to different Firebase features or app functions.

3. Final Answer:

   To let users sign up and log in easily -> Option D

4. Quick Check:

   Firebase Authentication = User sign-in [OK]

Hint: Firebase Auth is for user login/signup only [OK]

Common Mistakes:

• Confusing Authentication with storage or UI design
• Thinking it handles notifications
• Assuming it manages app layout

2. Which Swift code snippet correctly creates a new user with Firebase Authentication?

easy

A. Auth.signIn(withEmail: email, password: password)

B. Auth.auth().createUser(withEmail: email, password: password) { result, error in }

C. FirebaseAuth.createUser(email, password)

D. Auth.createUser(email: email, password: password)

Solution

1. Step 1: Recall correct Firebase method for user creation

   The method is Auth.auth().createUser(withEmail:password:completion:).

2. Step 2: Check each option's syntax

   Only Auth.auth().createUser(withEmail: email, password: password) { result, error in } uses the correct method and parameters; others are invalid or for sign-in.

3. Final Answer:

   Auth.auth().createUser(withEmail: email, password: password) { result, error in } -> Option B

4. Quick Check:

   Create user method = Auth.auth().createUser(withEmail: email, password: password) { result, error in } [OK]

Hint: Use Auth.auth().createUser for new users [OK]

Common Mistakes:

• Using signIn method instead of createUser
• Wrong method call without Auth.auth()
• Missing completion handler closure

3. What will be printed if the following Swift code runs and the user enters a wrong password?

Auth.auth().signIn(withEmail: email, password: wrongPassword) { authResult, error in
  if let error = error {
    print("Login failed: \(error.localizedDescription)")
  } else {
    print("Login successful")
  }
}

medium

A. Login failed: The password is invalid or the user does not have a password.

B. No output

C. Login successful

D. Crash due to missing error handling

Solution

1. Step 1: Understand signIn completion handler behavior

   If sign-in fails, error is non-nil and prints the error message.

2. Step 2: Analyze wrong password case

   Wrong password triggers error with message about invalid password.

3. Final Answer:

   Login failed: The password is invalid or the user does not have a password. -> Option A

4. Quick Check:

   Error message prints on wrong password [OK]

Hint: Errors print message; success prints success [OK]

Common Mistakes:

• Assuming success prints even on error
• Ignoring error optional unwrapping
• Expecting no output on failure

4. Identify the error in this Swift Firebase sign-in code:

Auth.auth().signIn(withEmail: email, password: password) { authResult, error in
  if error != nil {
    print("Error signing in")
  }
  print("User signed in")
}

medium

A. Missing else block causes "User signed in" to print even on error

B. Incorrect method name for signIn

C. No completion handler provided

D. Password parameter is missing

Solution

1. Step 1: Review control flow in completion handler

   Code prints "User signed in" regardless of error presence.

2. Step 2: Identify missing else block

   Without else, success message prints even if sign-in failed.

3. Final Answer:

   Missing else block causes "User signed in" to print even on error -> Option A

4. Quick Check:

   Print success only if no error [OK]

Hint: Use else to separate success from error prints [OK]

Common Mistakes:

• Printing success message unconditionally
• Not checking error properly
• Confusing method names

5. You want to create a Swift function that registers a user and shows an alert with success or error message. Which approach correctly handles this using Firebase Authentication?

hard

A. Call createUser, then immediately show success alert without waiting for completion

B. Call signIn instead of createUser for registration

C. Call createUser with completion handler, show alert inside handler based on error or success

D. Use createUser without email or password parameters

Solution

1. Step 1: Understand asynchronous nature of createUser

   createUser runs asynchronously; alerts must be shown after completion.

2. Step 2: Choose correct alert timing and method

   Showing alert inside completion handler ensures correct success or error message.

3. Step 3: Eliminate incorrect options

   Call createUser, then immediately show success alert without waiting for completion shows alert too early; C uses wrong method; D misses required parameters.

4. Final Answer:

   Call createUser with completion handler, show alert inside handler based on error or success -> Option C

5. Quick Check:

   Show alert after createUser completes [OK]

Hint: Show alerts only inside completion handlers [OK]

Common Mistakes:

• Showing alerts before async call finishes
• Using signIn for registration
• Omitting required parameters