可重用MKAnnotationView显示不同图像的问题

func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    let annotationIdentifier = "AnnotationIdentifier"
    
    var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: annotationIdentifier)
    
    if annotationView == nil {
        annotationView = MKAnnotationView(annotation: annotation, reuseIdentifier: annotationIdentifier)
        annotationView?.canShowCallout = true
    }
    else {
        annotationView?.annotation = annotation
    }
    
    // 设置标注视图的图像
    if let customAnnotation = annotation as? CustomAnnotation {
        annotationView?.image = customAnnotation.image
    }
    
    return annotationView
}

class CustomAnnotation: NSObject, MKAnnotation {
    let coordinate: CLLocationCoordinate2D
    let title: String?
    let subtitle: String?
    let image: UIImage
    
    init(coordinate: CLLocationCoordinate2D, title: String?, subtitle: String?, image: UIImage) {
        self.coordinate = coordinate
        self.title = title
        self.subtitle = subtitle
        self.image = image
    }
}

let annotation1 = CustomAnnotation(coordinate: CLLocationCoordinate2D(latitude: 37.786971, longitude: -122.399677), title: "San Francisco", subtitle: "California", image: UIImage(named: "sf_icon")!)
let annotation2 = CustomAnnotation(coordinate: CLLocationCoordinate2D(latitude: 34.052235, longitude: -118.243683), title: "Los Angeles", subtitle: "California", image: UIImage(named: "la_icon")!)

mapView.addAnnotations([annotation1, annotation2])