faceid_evaluate.swift

self.authenticationContext = LAContext()
        
var errorPolicy: NSError?

if let authenticationContext = self.authenticationContext, 
   authenticationContext.canEvaluatePolicy(.deviceOwnerAuthentication, error: &errorPolicy)
{
	authenticationContext.localizedCancelTitle = NSLocalizedString("CONTEXT_CANCEL_TITLE", comment: "")
	authenticationContext.localizedFallbackTitle = NSLocalizedString("CONTEXT_FALLBACK_TITLE", comment: "")

	let authenticationReason = NSLocalizedString("CONTEXT_REASON", comment: "")

	authenticationContext.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: authenticationReason ) { (success: Bool, error: Error?) -> Void in
		if success
		{
			print(NSLocalizedString("AUTHENTICATION_SUCCESS_MESSAGE", comment: ""))

			DispatchQueue.main.async
			{
				self.presentImage()
			}
		}
		else
		{
			if let error = error
			{
				print(error.localizedDescription)

				if let contextError = LAError.Code(rawValue: error._code)
				{
					print(contextError.localizedDescription)
				}
			}
			else
			{
				print(NSLocalizedString("AUTHENTICATION_ERROR_MESSAGE", comment: ""))
			}
		}
	}
}
else
{
	let message = errorPolicy?.localizedDescription ?? NSLocalizedString("AUTHENTICATION_POLICY_ERROR_MESSAGE", comment: "")
	print(message)
}