Biometric authentication (Face ID, Touch ID) in iOS Swift - UI Render Trace

Component - Biometric authentication (Face ID, Touch ID)

This component allows users to log in using their face or fingerprint. It uses the device's built-in biometric sensors to check identity quickly and securely.

Widget Tree

UIViewController
├── UIView (main view)
│   ├── UILabel (instruction text)
│   └── UIButton (Authenticate)
└── UIAlertController (for error or success messages)

The main screen has a label showing instructions and a button to start biometric login. Alerts pop up to show success or errors.

Render Trace - 4 Steps

Step 1: UIViewController

Step 2: UILabel

Step 3: UIButton

Step 4: UIAlertController

State Change - Re-render

Trigger:User taps the 'Authenticate' button

Before

No authentication attempted, screen shows instruction and button

→

After

Biometric prompt appears, then alert shows success or failure message

Re-renders:The UIAlertController is presented modally, main view remains unchanged

UI Quiz - 3 Questions

What happens when the user taps the 'Authenticate' button?

A The app immediately logs in without checking biometrics

B The biometric authentication prompt appears

C The screen background color changes

D The instruction label text changes

Key Insight

Biometric authentication UI uses simple, clear instructions and a single button to start. Feedback is given with alerts so users know if login succeeded or failed. This keeps the flow easy and secure.

Practice

1. What is the main purpose of using biometric authentication like Face ID or Touch ID in an iOS app?

easy

A. To change the app's theme automatically

B. To improve app loading speed

C. To securely verify the user's identity without typing a password

D. To send notifications to the user

Solution

1. Step 1: Understand biometric authentication purpose

   Biometric authentication is designed to verify the user's identity securely using Face ID or Touch ID.

2. Step 2: Compare options with purpose

   Only To securely verify the user's identity without typing a password describes secure identity verification without passwords, which is the main goal.

3. Final Answer:

   To securely verify the user's identity without typing a password -> Option C

4. Quick Check:

   Biometric authentication = Secure identity check [OK]

Hint: Biometrics replace passwords for secure login [OK]

Common Mistakes:

• Thinking biometrics speed up app loading
• Confusing biometric use with UI changes
• Assuming biometrics send notifications

2. Which Swift code snippet correctly checks if biometric authentication is available before using it?

easy

A. let context = LAContext()
if context.evaluatePolicy(.deviceOwnerAuthentication, error: nil) {
// proceed
}

B. let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
// proceed
}

C. let context = LAContext()
if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: nil) {
// proceed
}

D. let context = LAContext()
if context.canAuthenticate() {
// proceed
}

Solution

1. Step 1: Identify correct method to check biometric availability

   The method canEvaluatePolicy with .deviceOwnerAuthenticationWithBiometrics checks if biometrics are available.

2. Step 2: Verify code syntax and usage

   let context = LAContext() var error: NSError? if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) { // proceed } uses canEvaluatePolicy with correct policy and proper &error parameter, matching Apple's guidelines.

3. Final Answer:

   let context = LAContext()\nvar error: NSError?\nif context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {\n // proceed\n} -> Option B

4. Quick Check:

   Check biometrics availability = canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics) [OK]

Hint: Use canEvaluatePolicy with biometric policy to check availability [OK]

Common Mistakes:

• Using evaluatePolicy instead of canEvaluatePolicy for availability
• Using wrong policy like .deviceOwnerAuthentication
• Calling non-existent methods like canAuthenticate

3. Given this Swift code snippet, what will be printed if biometric authentication succeeds?

let context = LAContext()
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Access needed") { success, error in
  if success {
    print("Authentication succeeded")
  } else {
    print("Authentication failed")
  }
}

medium

A. Authentication succeeded

B. No output

C. Authentication failed

D. Syntax error

Solution

1. Step 1: Understand evaluatePolicy callback behavior

   The closure receives a success boolean indicating if authentication passed.

2. Step 2: Check the if condition for success

   If success is true, it prints "Authentication succeeded"; otherwise, it prints failure.

3. Final Answer:

   Authentication succeeded -> Option A

4. Quick Check:

   Success true prints "Authentication succeeded" [OK]

Hint: Success true means authentication passed and prints success message [OK]

Common Mistakes:

• Confusing success and failure branches
• Assuming no output from async closure
• Thinking code has syntax errors

4. Identify the error in this Swift code for biometric authentication:

let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: error) {
  context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Login") { success, error in
    if success {
      print("Access granted")
    }
  }
}

medium

A. The error parameter should be passed as &error to be mutable

B. evaluatePolicy should be called before canEvaluatePolicy

C. The localizedReason string is missing

D. LAContext must be declared as a class, not a let constant

Solution

1. Step 1: Check error parameter usage in canEvaluatePolicy

   The error parameter must be passed as an inout pointer using &error to capture errors.

2. Step 2: Verify other code parts

   evaluatePolicy is correctly called after canEvaluatePolicy, localizedReason is provided, and LAContext can be a let constant.

3. Final Answer:

   The error parameter should be passed as &error to be mutable -> Option A

4. Quick Check:

   Error param must be &error for canEvaluatePolicy [OK]

Hint: Pass error as &error to canEvaluatePolicy for error capture [OK]

Common Mistakes:

• Passing error without & causing no error info
• Calling evaluatePolicy before checking availability
• Missing localizedReason string
• Misunderstanding let vs var for LAContext

5. You want to allow users to authenticate using Face ID or Touch ID, but fallback to device passcode if biometrics fail or are unavailable. Which Swift code snippet correctly implements this behavior?

hard

A. let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Authenticate") { success, error in
if success {
print("Biometric success")
} else {
print("Authentication failed")
}
}
} else {
print("Biometrics not available")
}

B. let context = LAContext()
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Authenticate") { success, error in
if success {
print("Authentication success")
} else {
print("Authentication failed")
}
}

C. let context = LAContext()
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil) {
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Authenticate") { success, error in
if success {
print("Biometric success")
} else {
context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Fallback") { success, error in
if success {
print("Passcode success")
} else {
print("Authentication failed")
}
}
}
}
}

D. let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) {
context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Authenticate") { success, error in
if success {
print("Authentication success")
} else {
print("Authentication failed")
}
}
}

Solution

1. Step 1: Understand fallback requirement

   The app should try biometrics first, then fallback to device passcode if biometrics fail or are unavailable.

2. Step 2: Analyze code options for fallback logic

   let context = LAContext() var error: NSError? if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) { context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Authenticate") { success, error in if success { print("Authentication success") } else { print("Authentication failed") } } } uses .deviceOwnerAuthentication policy which automatically prefers biometrics (Face ID/Touch ID) and falls back to passcode if unavailable or fails. It also properly checks availability first per Apple's guidelines.

3. Final Answer:

   let context = LAContext()\nvar error: NSError?\nif context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) {\n context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: \"Authenticate\") { success, error in\n if success {\n print(\"Authentication success\")\n } else {\n print(\"Authentication failed\")\n }\n }\n} -> Option D

4. Quick Check:

   .deviceOwnerAuthentication policy = biometric fallback to passcode [OK]

Hint: Use .deviceOwnerAuthentication policy with canEvaluatePolicy check for fallback [OK]

Common Mistakes:

• Using biometrics-only policy without fallback (Options A, B)
• Direct evaluation without availability check
• Syntax error with error: nil and incomplete fallback (Option D)