Biometric authentication (Face ID, Touch ID) in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when biometric authentication succeeds?

In an iOS app using Face ID or Touch ID, what is the typical behavior after successful biometric authentication?

A The app unlocks the protected content and navigates to the main screen.

B The app closes immediately without any message.

C The app shows an error message and asks to try again.

D The app logs out the user automatically.

💡 Hint

Think about what the user expects after confirming their identity.

When biometric authentication succeeds, the app typically grants access to protected content or features, often by navigating to the main or secured screen.

🧠 Conceptual

intermediate

Which framework is used for biometric authentication in iOS?

Which Apple framework provides the APIs to implement Face ID and Touch ID authentication in iOS apps?

A UIKit

B CoreData

C AVFoundation

D LocalAuthentication

💡 Hint

It starts with 'Local' and deals with device security.

The LocalAuthentication framework provides classes and methods to evaluate biometric authentication policies like Face ID and Touch ID.

📝 Syntax

advanced

What is the correct way to create a LAContext and check if biometric authentication is available?

Given the following Swift code snippet, which option correctly creates a LocalAuthentication context and checks if biometric authentication can be evaluated?

iOS Swift

import LocalAuthentication

let context = LAContext()
var error: NSError?

// Check if biometric authentication is available
let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error)

A let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error)

B let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthentication, error: error)

C let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: error)

D let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics)

💡 Hint

Check the method signature and parameter types carefully.

The method canEvaluatePolicy requires an UnsafeMutablePointer to an NSError? for the error parameter, so it must be passed as &error. Option A correctly uses this syntax.

❓ lifecycle

advanced

What happens if biometric authentication is interrupted by app going to background?

If the user starts biometric authentication but the app goes to the background (e.g., user presses home button), what is the expected behavior?

A The authentication succeeds immediately without user input.

B The app crashes due to lost context.

C The authentication is cancelled and the app must restart it when active again.

D The authentication continues in the background automatically.

💡 Hint

Think about security and user interaction requirements.

Biometric authentication requires user interaction and cannot continue in the background. If interrupted, it is cancelled and must be restarted when the app is active again.

🔧 Debug

expert

Why does this biometric authentication code always fail with error code -7?

Consider this Swift code snippet for biometric authentication: import LocalAuthentication let context = LAContext() context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Access needed") { success, error in if success { print("Authenticated") } else { print(error?.localizedDescription ?? "Unknown error") } } The error code returned is always -7 (LAError.biometryNotAvailable). What is the most likely cause?

A The LAContext instance must be created inside the closure.

B The device does not have biometric hardware or it is disabled in settings.

C The localizedReason string is missing and causes failure.

D The evaluatePolicy method requires a completion handler on the main thread.

💡 Hint

Error code -7 means biometry is not available on the device.

Error code -7 (LAError.biometryNotAvailable) means the device either lacks biometric hardware or the user has disabled it in system settings. The code is correct but the device environment prevents biometric use.