Error handling Beacon scanning

Question

I've recently began trying to put more error handling in to my code so I thought I'd post a use-case to check I'm on the right path. I feel like this is going to make my functions much larger and more indented/nested than before. Is this the right way to go about things or should I be utilising try? instead? Where do people generally put these Error enum definitions?

fileprivate enum LocationError: Error {
 case noAuthorization
 case noBeaconSupport
 case rangingUnavailable
}
class ViewController: UIViewController, CLLocationManagerDelegate {
 @IBOutlet weak var distanceReading: UILabel!
 var locationManager: CLLocationManager!
override func viewDidLoad() {
 super.viewDidLoad()
 locationManager = CLLocationManager()
 locationManager.delegate = self
 locationManager.requestWhenInUseAuthorization()
 view.backgroundColor = UIColor.gray
}
func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
 do {
 guard status == .authorizedWhenInUse else {
 throw LocationError.noAuthorization
 }
 guard CLLocationManager.isMonitoringAvailable(for: CLBeaconRegion.self) else {
 throw LocationError.noBeaconSupport
 }
 guard CLLocationManager.isRangingAvailable() else {
 throw LocationError.rangingUnavailable
 }
 startScanning()
 }
 catch LocationError.noAuthorization {
 print("User has not authorized us to use location")
 }
 catch LocationError.noBeaconSupport {
 print("User's device does not support Beacons")
 }
 catch LocationError.rangingUnavailable {
 print("User's device does not support ranging")
 }
 catch {
 fatalError()
 }
}

Answer 1

Throwing Swift errors is a mechanism how a function/method can report a failure to its caller. Your code throws and catches the error within the same method, and I can see no advantage of using try/catch in that situation. Your code is equivalent to

func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
 guard status == .authorizedWhenInUse else {
 print("User has not authorized us to use location")
 return
 }
 guard CLLocationManager.isMonitoringAvailable(for: CLBeaconRegion.self) else {
 print("User has not authorized us to use location")
 return
 }
 guard CLLocationManager.isRangingAvailable() else {
 print("User's device does not support ranging")
 return
 }
 startScanning()
}

which is

• shorter and easy to understand,
• makes the enum LocationError obsolete (which is fileprivate, and therefore apparently not used anywhere else),
• makes the catch-all with fatalError() obsolete.

I would even go a step further: guard is useful in connection with optional binding (to avoid the "optional binding pyramid of doom"). In your case, the same can be achieved with a simple if/else if/.../else statement:

func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
 if status != .authorizedWhenInUse {
 print("User has not authorized us to use location")
 } else if !CLLocationManager.isMonitoringAvailable(for: CLBeaconRegion.self) {
 print("User has not authorized us to use location")
 } else if !CLLocationManager.isRangingAvailable() {
 print("User's device does not support ranging")
 } else {
 startScanning()
 }
}

• Declan McKenna: Thanks. In what case would I want to use Error and try catch? When I do more than just print?
• Martin R: @Deco: A function throws an Error to report a failure to its caller. You use try/catch when calling functions that throw an Error.