GPS Accuracy Resetting on its own on iOS - ios

Question

In an iPhone app that I'm building, GPS accuracy is a user-controlled value that can be set to either KCLLocationAccuracyNearestTenMeters or KCLLocationAccuracyHundred Meters. The code works fine to capture a track, but when monitoring the app in the Location Energy Impact Instrument I’m seeing some unexpected behavior. When accuracy is set to nearest 10 meters the app works as expected, however, if it’s set to 100 meters the setting remains OK for between a half and two seconds, as shown by the Instrument, but then it switches to KCLLocationAccuracy Best. This setting isn't an option in the app, or even a String anywhere in the code.
One twist here, I am running this as Swift 3.0 using Xcode 8, but it's working as it did before and the new platform doesn't seem to be an issue (and shouldn't have this kind of impact). It does mean that some API's have changed and may look different (and to my taste, better) below.
Obviously, this resetting has a very counterproductive impact on energy consumption, which shows in the Instrument which goes from Low to High energy usage at the point where it switches. I can't identify any point where the app would be doing this unintentionally - in fact, during this one-second period it should only be appending points returned by didUpdateLocations to a pending buffer for later processing, as shown below.
In a shared constants declaration:
let defaultTrackingAccuracy = kCLLocationAccuracyNearestTenMeters
let alternateTrackingAccuracy = kCLLocationAccuracyHundredMeters
var trackingAccuracy = kCLLocationAccuracyNearestTenMeters
var waypointInterval = 100
In a settings ViewController (can also reset waypointInterval but did not):
#IBAction func accuracySwitched(_ sender: UISwitch) {
if accuracySwitch.isOn {
Set.shared.trackingAccuracy = Set.shared.defaultTrackingAccuracy
} else {
Set.shared.trackingAccuracy = Set.shared.alternateTrackingAccuracy
}
}
LocationManager instantiation in a Model portion of code:
lazy var locationManager: CLLocationManager = {
var _locationManager = CLLocationManager()
_locationManager.delegate = self
_locationManager.desiredAccuracy = Set.shared.trackingAccuracy
_locationManager.allowsBackgroundLocationUpdates = true
_locationManager.distanceFilter = Double(Set.shared.waypointInterval)
return _locationManager
}()
Starting the location manager updates in the Model:
func setupLocationManager()
{
if CLLocationManager.authorizationStatus() != .authorizedAlways {
locationManager.requestAlwaysAuthorization()
if CLLocationManager.authorizationStatus() != .authorizedAlways {
delegate?.displayNotice("Unable to Capture Track", alertMessage: "This device requires authorization to use location services in order to capture a track. \n\nPress Continue to return to the Track List.", buttonText: "Continue")
}
}
if !CLLocationManager.locationServicesEnabled() {
delegate?.abortTrackCapture(.gpsUnavailable)
}
locationManager.startUpdatingLocation()
}
Processing points returned by the location manager instance, in the same Model portion:
#objc(locationManager:didUpdateLocations:) func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation])
{
assert(locations.first != nil, "didUpdateLocations called with empty array.")
if recordStatus == .record {
pendingPointsBuffer = pendingPointsBuffer + locations
}
lastPosition.latitude = locations.last!.coordinate.latitude
lastPosition.longitude = locations.last!.coordinate.longitude
lastPosition.elevation = locations.last!.altitude
}
Every few seconds, there is a routine that takes points from the buffer and posts them into a CoreData database, but that only accesses the buffer - it has no interaction with the location manager (so it can sleep through periods when the app is in background while the code above keeps loading points into the buffer).
The user interface is in a separate ViewController module, but shouldn’t be doing anything during the two seconds in question.
I'm thinking that there may be some OS based setting or process that could be doing this, but I haven't found anything that seems to do that.
Thanks for your input - any thoughts are appreciated.