let context = LAContext()
context.localizedFallbackTitle = "Fall back button"
if #available(iOS 10.0, *) {
 context.localizedCancelTitle = "Cancel button"
}
var authError: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &authError) {
 context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Localized reason for authentication with biometrics", reply: { (success, evaluateError) in
 // NOT in main thread
 DispatchQueue.main.async {
 if success {
 self.label.text = "Success"
 // Do something success
 } else if let error = evaluateError {
 self.label.text = error.localizedDescription
 // Deal with error
 if let code = LAError.Code(rawValue: (error as NSError).code) {
  switch code {
  case .userFallback:
  print("fall back button clicked")
  default:
  break
  }
 }
 }
 } 
 })
} else if let error = authError {
 label.text = error.localizedDescription
 // Deal with error
}

if #available(iOS 9.0, *) {
 let context = LAContext()
 context.localizedFallbackTitle = "Fall back button"
 if #available(iOS 10.0, *) {
 context.localizedCancelTitle = "Cancel button"
 }
 var authError: NSError?
 if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &authError) {
 context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Localized reason for authentication", reply: { (success, evaluateError) in
 // NOT in main thread
 DispatchQueue.main.async {
 if success {
  self.label.text = "Success"
  // Do something success
 } else if let error = evaluateError {
  self.label.text = error.localizedDescription
  // When fall back button clicked, user is required to enter PIN. Error code will not be "userFallback"
  // Deal with error
 }
 }
 })
 } else if let error = authError {
 label.text = error.localizedDescription 
 // Deal with error
 }
} else {
 let alert = UIAlertController(title: nil, message: "Authentication is available on iOS 9.0 or later", preferredStyle: .alert)
 alert.addAction(UIAlertAction(title: "OK", style: .default, handler: nil))
 present(alert, animated: true, completion: nil)
}