下面的代码是从数据库中获取特定数量的引用，从一些DBSCAN函数给出的一些坐标中绘制圆覆盖图，并在Map上绘制圆覆盖图。每隔10秒，圆从Map中删除，并使用数据库中的相同圆进行更新。问题是圆没有显示出来。我已经尝试过调试，但没有用。任何建议/帮助都很感激。

import MapKit
import SwiftUI
import Firebase
import FirebaseAnalytics
import FirebaseAnalyticsSwift

struct ContentView: View {
    @StateObject private var viewModel = ContentViewModel()
/* Variable region accounts for showing specific coordinate region within span, showcasing input coordinates */
    @State var annotationItems: [User] = []
    @State var locations: [CLLocation] = []
    @State var circleOverlays: [MKCircle] = []
    
    func plotUsers() {
        for i in 0...20
        {
            var latData = 0.0
            var longData = 0.0
            var userName = ""
            var userAge = 0
            var ref: DatabaseReference!

            ref = Database.database().reference().child("users").child(String(i))
            ref.getData(completion:  { error, snapshot in
                guard error == nil else {
                    print("issue")
                    return;
                }
                var a: [String: Any] = [:]
                //turning datasnapshot returned from database into a dictionary
                a = snapshot?.value as! Dictionary<String, Any>
                //assigning values from the dictionary to variables so we don't have to type all the necessary error stuff every time
                latData = (a["locData"] as? [String:Any])?["lat"] as? Double ?? -1
                longData = (a["locData"] as? [String:Any])?["long"] as? Double ?? -1
                userName = (a["name"]) as! String
                userAge = Int((a["age"] as? [String:Any])?["age"] as? String ?? "-1") ?? -5
                annotationItems.append(User(name:userName,age: userAge,latitude: latData,longitude: longData))
                self.locations.append(CLLocation(latitude: latData, longitude: longData))
            });
        }
        print("@@@@@@@@@@@@@@@@@@@@@@@Locations:@@@@@@@@@@@@@@@@@@@@@@@@",locations.count)
        
        let dbscan = DBSCAN(self.locations)
        let (sequence, places) = dbscan.findCluster(eps: 1000.0, minPts: 2)
        print("@@@@@@@@@@@@@@@@@@@@@@@Places:@@@@@@@@@@@@@@@@@@@@@@@@",places)
    
        for place in places {
            print("Cluster:", place.members.count)
            let circle = MKCircle(center: place.location.coordinate, radius: 10000)
            self.circleOverlays.append(circle)
        }
        self.viewModel.mapView.addOverlays(circleOverlays)
        print("@@@@@@@@@@@@@@@@@@@@@@@Circles:@@@@@@@@@@@@@@@@@@@@@@@@",circleOverlays)
        
    }
    
    private var timer: Timer?
    
    var body: some View {
        Map(coordinateRegion: $viewModel.region,
            showsUserLocation: true)
        .onAppear() {
//            viewModel.mapView.delegate = viewModel
            let _ = self.plotUsers()
//            let _ = self.plotUsers()
            timer?.invalidate()
            Timer.scheduledTimer(withTimeInterval: 10, repeats: true) { _ in
                let _ = self.plotUsers()
                self.locations.removeAll()
                self.viewModel.mapView.removeOverlays(circleOverlays)
                self.circleOverlays.removeAll()
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider { /* Shows map */
    static var previews: some View {
        ContentView()
    }
}

final class ContentViewModel: NSObject, ObservableObject, CLLocationManagerDelegate, MKMapViewDelegate {
    @Published var region = MKCoordinateRegion(center: CLLocationCoordinate2D(
                                                   latitude: 38.898022, longitude: -77.050604),
                                                   span: MKCoordinateSpan(
                                                   latitudeDelta: 0.05, longitudeDelta: 0.05)) /* Can substitute region coordinates with longitude,latitude from firebase */

    @Published var map = MKMapView()
    var locationManager: CLLocationManager? /* Enables location services*/
    let mapView: MKMapView!
    
    override init() {
        mapView = MKMapView()
        super.init()
        mapView.delegate = self
    }
    
    func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
        if let circleOverlay = overlay as? MKCircle {
            let circleRenderer = MKCircleRenderer(circle: circleOverlay)
            circleRenderer.fillColor = UIColor(red: 1.0, green: 0.0, blue: 0.0, alpha: 0.3)
            circleRenderer.strokeColor = UIColor(red: 1.0, green: 0.0, blue: 0.0, alpha: 0.7)
            circleRenderer.lineWidth = 2.0
            return circleRenderer
        }
        return MKOverlayRenderer()
    }
}

我是Swift的新手，所以我不完全确定是否应该以某种方式将$viewModel.region设置为在ContentViewModel类中创建的MKMapView。