MapBox didUpdateUserLocation is never called - swift

Question

I'm trying to respond to changes in the user's location in an iOS app using MapBox, but didUpdateUserLocation is not being called. Why isn't didUpdateUserLocation being called?
ViewController.swift
import UIKit
import Mapbox
class ViewController: UIViewController, MGLMapViewDelegate {
#IBOutlet weak var upButton: UIButton!
override func viewDidLoad() {
super.viewDidLoad()
let styleURL = NSURL(string: "mapbox://styles/jmeyers919/cj8w00yxvfrqr2rpehxd47up1") // MGLStyle.darkStyleURL()
let mapView = MGLMapView(frame: view.bounds, styleURL: styleURL as URL?)
mapView.delegate = self
mapView.logoView.isHidden = true
mapView.attributionButton.isHidden = true
mapView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
// Note that we have changed the center coordinate to New York City for this guide
// mapView.setCenter(CLLocationCoordinate2D(latitude: 44.0475276, longitude: -123.08927319), zoomLevel: 16, animated: false)
mapView.userTrackingMode = .follow
mapView.showsUserLocation = true
view.addSubview(mapView)
self.view.bringSubview(toFront: self.upButton)
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
func mapView(mapView: AnyObject!, didUpdateUserLocation userLocation: AnyObject!) {
print("didUpdateUserLocation")
}
}

Answer 1

While the showsUserLocation property is set to YES, this method is called
whenever a new location update is received by the map view. This method is also
called if the map view’s user tracking mode is set to
MGLUserTrackingModeFollowWithHeading and the heading changes, or if it is set
to MGLUserTrackingModeFollowWithCourse and the course changes.
This method is not called if the application is currently running in the background. If you want to receive location updates while running in the
background, you must use the Core Location framework.