Touch ID and Face Id Authentication in Swift 4

let localAuthenticationContext = LAContext()

if localAuthenticationContext.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &authError) {
     //Write the your code here
}

import UIKit
import LocalAuthentication

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        authenticationWithTouchID()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

extension ViewController {
    
    func authenticationWithTouchID() {
        let localAuthenticationContext = LAContext()
        localAuthenticationContext.localizedFallbackTitle = "Use Passcode"

        var authError: NSError?
        let reasonString = "To access the secure data"

        if localAuthenticationContext.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &authError) {
            
            localAuthenticationContext.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: reasonString) { success, evaluateError in
                
                if success {
                    
                    //TODO: User authenticated successfully, take appropriate action
                    
                } else {
                    //TODO: User did not authenticate successfully, look at error and take appropriate action
                    guard let error = evaluateError else {
                        return
                    }
                    
                    print(self.evaluateAuthenticationPolicyMessageForLA(errorCode: error._code))
                    
                    //TODO: If you have choosen the 'Fallback authentication mechanism selected' (LAError.userFallback). Handle gracefully
                    
                }
            }
        } else {
            
            guard let error = authError else {
                return
            }
            //TODO: Show appropriate alert if biometry/TouchID/FaceID is lockout or not enrolled
            print(self.evaluateAuthenticationPolicyMessageForLA(errorCode: error.code))
        }
    }
    
    func evaluatePolicyFailErrorMessageForLA(errorCode: Int) -> String {
        var message = ""
        if #available(iOS 11.0, macOS 10.13, *) {
            switch errorCode {
                case LAError.biometryNotAvailable.rawValue:
                    message = "Authentication could not start because the device does not support biometric authentication."
                
                case LAError.biometryLockout.rawValue:
                    message = "Authentication could not continue because the user has been locked out of biometric authentication, due to failing authentication too many times."
                
                case LAError.biometryNotEnrolled.rawValue:
                    message = "Authentication could not start because the user has not enrolled in biometric authentication."
                
                default:
                    message = "Did not find error code on LAError object"
            }
        } else {
            switch errorCode {
                case LAError.touchIDLockout.rawValue:
                    message = "Too many failed attempts."
                
                case LAError.touchIDNotAvailable.rawValue:
                    message = "TouchID is not available on the device"
                
                case LAError.touchIDNotEnrolled.rawValue:
                    message = "TouchID is not enrolled on the device"
                
                default:
                    message = "Did not find error code on LAError object"
            }
        }
        
        return message;
    }
    
    func evaluateAuthenticationPolicyMessageForLA(errorCode: Int) -> String {
        
        var message = ""
        
        switch errorCode {
            
        case LAError.authenticationFailed.rawValue:
            message = "The user failed to provide valid credentials"
            
        case LAError.appCancel.rawValue:
            message = "Authentication was cancelled by application"
            
        case LAError.invalidContext.rawValue:
            message = "The context is invalid"
            
        case LAError.notInteractive.rawValue:
            message = "Not interactive"
            
        case LAError.passcodeNotSet.rawValue:
            message = "Passcode is not set on the device"
            
        case LAError.systemCancel.rawValue:
            message = "Authentication was cancelled by the system"
            
        case LAError.userCancel.rawValue:
            message = "The user did cancel"
            
        case LAError.userFallback.rawValue:
            message = "The user chose to use the fallback"

        default:
            message = evaluatePolicyFailErrorMessageForLA(errorCode: errorCode)
        }
        
        return message
    }
}