title: MapView clustering with custom annotations
body: <p>I'm trying to cluster my mapView with custom annotations but cluster is never being called. This is my code:</p>
  <pre><code>func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -&gt; MKAnnotationView? {
      if annotation is MKUserLocation{
          return nil
      }
      
      let an = MKAnnotationView.init(annotation: annotation, reuseIdentifier: &quot;AD&quot;)
      an.clusteringIdentifier = (annotation as! StopAnnotation).id!.joined(separator: &quot;_&quot;)
      an.image = #imageLiteral(resourceName: &quot;face&quot;)
      return an
  }
  </code></pre>
  <p>This never triggers clustering, I also implemented this delegate:</p>
  <pre><code>func mapView(_ mapView: MKMapView, clusterAnnotationForMemberAnnotations memberAnnotations: [MKAnnotation]) -&gt; MKClusterAnnotation {
      return MKClusterAnnotation(memberAnnotations: memberAnnotations)
  }
  </code></pre>
  <p>Which is also never called. What am I doing wrong?</p>