MapViewController.swift

func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool) {
    let center = mapView.region.center
    let latitudeDelta = mapView.region.span.latitudeDelta

    let topPin = CLLocation(latitude: center.latitude - (latitudeDelta / 2),
                                longitude: center.longitude)
    let bottomPin = CLLocation(latitude: center.latitude + (latitudeDelta / 2),
                                   longitude: center.longitude)
    let centerLocation = CLLocation(latitude: center.latitude,
                                        longitude: center.longitude)
    let distance = max(topPin.distance(from: centerLocation),
                           bottomPin.distance(from: centerLocation))
    
    reloadData()

}