When/ How does swift call user created functions - swift

Question

Im currently learning how to code using swift. I created an app that simple displays the users location using map view.
In this code block, when does swift call the function locationManager()? I seem to have never called this function, yet it is running all the logic within it
import UIKit
import MapKit
class ViewController: UIViewController,
CLLocationManagerDelegate{
#IBOutlet weak var map: MKMapView!
#IBOutlet weak var AreaLbl: UILabel!
var locationManager = CLLocationManager()
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
self.locationManager.requestWhenInUseAuthorization()
if CLLocationManager.locationServicesEnabled(){
locationManager.delegate = self
locationManager.desiredAccuracy = kCLLocationAccuracyNearestTenMeters
locationManager.startUpdatingLocation()
}
}
func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
let locValue:CLLocationCoordinate2D = manager.location!.coordinate
print("locations = \(locValue.latitude) \(locValue.longitude)")
let userLocation = locations.last
let viewRegion = MKCoordinateRegion(center: (userLocation?.coordinate)!, latitudinalMeters: 600, longitudinalMeters: 600)
self.map.showsUserLocation = true
self.map.setRegion(viewRegion, animated: true)
}
If there are any resources you recommend for learning swift please let me know!

Answer 1

First, a point of order. In Swift, the name of a function includes parameters. Your function isn't named locationManager(). It's named locationManager(_:didUpdateLocations:) (One unnamed parameter and one parameter named "didUpdateLocations".)
Now to your question.
iOS and Mac OS make pretty common use of the delegate design pattern. In that pattern, one object sets itself up to be another object's "delegate". That basically means that the delegate object agrees to wait by the phone for incoming phonecalls.
When you are an object's delegate, you agree to respond to a set of messages that might be sent to you (or functions that might be called.) You "conform to a protocol" that defines the messages you need to respond to. Some of those messages might be required and some might be optional.
It's not Swift calling your function, it's the OS. Specifically the location manager.
Your viewDidLoad() function includes the line locationManager.delegate = self.
That makes you the location manager's delegate. That means that you conform to the CLLocationManagerDelegate protocol. It tells the location manager that you are ready to accept messages (function calls) from the location manager that are defined by that protocol.
When you do that, and then you tell the location manager to start updating your location with the line locationManager.startUpdatingLocation(), the location manager will start calling your locationManager(_:didUpdateLocations:) function at some future time when it detects a change to the device's location.
That is what is causing your locationManager(_:didUpdateLocations:) function to be called.