如何将字幕和自定义图片添加到MKMapView中的特定位置引脚？

import MapKit

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

class ViewController: UIViewController, MKMapViewDelegate {
    @IBOutlet weak var mapView: MKMapView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 设置地图的中心位置和缩放级别
        let initialLocation = CLLocation(latitude: 37.7749, longitude: -122.4194)
        let regionRadius: CLLocationDistance = 1000
        let coordinateRegion = MKCoordinateRegion(center: initialLocation.coordinate, latitudinalMeters: regionRadius, longitudinalMeters: regionRadius)
        mapView.setRegion(coordinateRegion, animated: true)
        
        // 创建自定义标注对象
        let annotation = CustomAnnotation(coordinate: initialLocation.coordinate, title: "San Francisco", subtitle: "California")
        
        // 添加标注到地图上
        mapView.addAnnotation(annotation)
    }
    
    func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
        if annotation is MKUserLocation {
            return nil
        }
        
        let reuseIdentifier = "CustomAnnotationView"
        var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: reuseIdentifier)
        
        if annotationView == nil {
            annotationView = MKAnnotationView(annotation: annotation, reuseIdentifier: reuseIdentifier)
        } else {
            annotationView?.annotation = annotation
        }
        
        // 设置自定义图片
        annotationView?.image = UIImage(named: "custom_pin")
        
        // 设置字幕的位置
        annotationView?.centerOffset = CGPoint(x: 0, y: -annotationView!.image!.size.height / 2)
        
        return annotationView
    }
}