Cluster
Easy Map Annotation Clustering
...Next, add the annotation object to an instance of ClusterManager with add(annotation:). Implement the map view’s mapView(_:viewFor:) delegate method to configure the annotation view. Return an instance of MKAnnotationView to display as a visual representation of the annotations. The ClusterAnnotationView class exposes a countLabel property. You can subclass ClusterAnnotationView to provide custom behavior as needed. Here's an example of subclassing the ClusterAnnotationView and customizing the layer borderColor.