Biometric authentication (Face ID, Touch ID) in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a LocalAuthentication context.

iOS Swift

let context = LAContext()
let reason = "Authenticate to access your account"
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: reason) { success, error in
    if success {
        print("Authentication [1]")
    }
}

Drag options to blanks, or click blank then click option'

Apending

Bcancelled

Cfailed

Dsucceeded

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'failed' instead of 'succeeded'

Confusing the success boolean meaning

The success boolean is true when authentication succeeds, so the message should say "succeeded".

2fill in blank

medium

Complete the code to check if biometric authentication is available.

iOS Swift

let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &[1]) {
    print("Biometrics available")
} else {
    print("Biometrics not available")
}

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Aerror

Berr

Cfailure

DauthError

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not declared

Forgetting the & before the variable

The canEvaluatePolicy method requires an NSError pointer variable named error to store any error details.

3fill in blank

hard

Fix the error in the code to handle authentication failure.

iOS Swift

context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: reason) { success, [1] in
    if !success {
        print("Authentication failed: \([1]?.localizedDescription ?? \"Unknown error\")")
    }
}

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Aerror

Berr

Cfailure

DauthError

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong parameter name causing compile errors

Not handling optional error safely

The closure parameter for the error is conventionally named error, matching the API signature.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension filtering biometrics types.

iOS Swift

let biometrics = ["FaceID": context.biometryType == [1], "TouchID": context.biometryType == [2]]
print(biometrics)

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ALABiometryType.faceID

BLABiometryType.touchID

CLABiometryType.none

DLABiometryType.fingerprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong enum names

Confusing FaceID with TouchID constants

The biometryType property is compared to LABiometryType.faceID and LABiometryType.touchID to check which biometric is available.

5fill in blank

hard

Fill all three blanks to create a biometric authentication function with completion handler.

iOS Swift

func authenticateUser(completion: @escaping (Bool) -> Void) {
    let context = LAContext()
    let reason = "Please authenticate"
    context.evaluatePolicy([1], localizedReason: reason) { success, error in
        DispatchQueue.main.async {
            if success {
                completion([2])
            } else {
                completion([3])
            }
        }
    }
}

Drag options to blanks, or click blank then click option'

A.deviceOwnerAuthenticationWithBiometrics

Btrue

Cfalse

D.deviceOwnerAuthentication

Attempts:

3 left

💡 Hint

Common Mistakes

Using only biometrics policy without fallback

Not calling completion on main thread

Swapping true and false in completion

Using .deviceOwnerAuthentication allows biometrics or passcode fallback. The completion handler receives true on success and false on failure.