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Using Touch ID and Face ID In Your App (Swift 5)

Jules Lee Jules Lee Follow Oct 28, 2019 · 1 min read
Using Touch ID and Face ID In Your App (Swift 5)
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To use Face ID, or even Touch ID, we need to import LocalAuthentication. The code below works for both Face and Touch ID. Whichever is enabled in the user’s settings.

let context = LAContext()
var error: NSError?

if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
	// This reason is only for Touch ID, not Face ID
    // The reason for accessing Face ID, is found in Info.plist
    let reason: String = "Please use Touch ID to unlock"
    
    context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: reason) { [weak self] success, authenticationError in
    	Dispatch.main.async {
        	if success {
            	// do some unlocking of data or
                // present new view
            } else {
            	// Face wasn't detected
                // Maybe user was wearing sunglasses
                if error = error {
                  let ac = UIAlertController(title: "Authetication Failed!"),
                              message: self.reportTouchIDError(error: error),
                              preferredStyle: .alert)
                  ac.addAction(UIAlertAction(title: "OK", style: .default))
                  self?.present(ac, animated: true)
                }
            }
        }
    }
} else {
	// Biometric is not supported by your iOS device
    let ac = UIAlertController(title: "Biometric Authentication Unavailable"),
                  message: "Your device is not configured for biometric authentication.",
                  preferredStyle: .alert)
    ac.addAction(UIAlertAction(title: "OK", style: .default))
    self?.present(ac, animated: true)
}

Before this can work, we must add a key value in our Info.plist to alert the user that we are going to be using this biometric feature.

Key: Privacy - Face ID Usage Description

Type: String

Value: This app uses Face ID to confirm your identity

There’s a more specific way of producing an error. This example was explained by David Tran:

func reportTouchIDError(error: NSError) -> String {
	switch error.code {
    case LAError.AuthenticationFailed.rawValue:
    	return "Authentication Failed!"
    case LAError.PasscodeNotSet.rawValue:
    	return "Passcode not set!"
    case LAError.SystemCancel.rawValue:
    	return "Authentication was cancelled by the system!"
    case LAError.UserCancel.rawValue:
    	return "User cancel auth"
    case LAError.TouchIDNotEnrolled.rawValue:
    	return "User has not enrolled any finger with touch ID"
    case LAError.TouchIDNotAvailable.rawValue:
    	return "Touch ID is not available"
    case LAError.UserFallBack.rawValue:
    	return "User tapped enter password"
    default:
    	return error.localizedDescription
    }
}
Jules Lee
Written by Jules Lee
Hi, I am Jules, the author of iOS Junkie. Demystifying complex instructions from the internet.