mapkit_clustering2.swift

func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    // 1
    if let item = annotation as? MapItem {
        // 2
        let annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: "mapItem")
            ?? MKAnnotationView(annotation: annotation, reuseIdentifier: "mapItem")
        
        annotationView.annotation = item
        annotationView.image = UIImage(named: "annotation")
        // 3
        annotationView.clusteringIdentifier = "mapItemClustered"
        
        return annotationView
    } else if let cluster = annotation as? MKClusterAnnotation {
        // 4
        let clusterView = mapView.dequeueReusableAnnotationView(withIdentifier: "clusterView")
            ?? MKAnnotationView(annotation: annotation, reuseIdentifier: "clusterView")
        
        clusterView.annotation = cluster
        clusterView.image = UIImage(named: "cluster")
        
        return clusterView
    } else {
        return nil
    }
}