let clusterManager = ClusterManager()

let annotation = Annotation()
annotation.coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
clusterManager.add(annotation)

func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    var view = mapView.dequeueReusableAnnotationView(withIdentifier: identifier)
    if view == nil {
        view = ClusterAnnotationView(annotation: annotation, reuseIdentifier: identifier, type: .color(color: color, radius: radius))
    } else {
        view?.annotation = annotation
    }
    return view
}

func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool) {
    clusterManager.reload(mapView)
}