如何在调用viewDidLoad（）后将MKPolyline添加到mapView？

Question

我目前有一个应用程序，当按下按钮时MKPolyline应显示在我的mapView上。

当我在MKPolyline中调用viewDidLoad()函数时，折线会显示在mapView上。如果我尝试使用按钮操作功能而不是viewDidLoad()来调用该功能，则MKPolyline不会显示。

根据按下的按钮决定应显示哪条折线。

如何显示MKPolyline

我的MapViewController代码：

import UIKit
import MapKit

class MapViewController: UIViewController, MKMapViewDelegate, 
CLLocationManagerDelegate {

    @IBOutlet var mapView: MKMapView?

    let locationManager = CLLocationManager()

    override func viewDidLoad() {
        super.viewDidLoad()

        mapView?.delegate = self
        mapView?.showsUserLocation = true

        locationManager.requestAlwaysAuthorization()
        locationManager.requestWhenInUseAuthorization()

        if CLLocationManager.locationServicesEnabled() {
            locationManager.delegate = self
            locationManager.desiredAccuracy = kCLLocationAccuracyBest
            locationManager.startUpdatingLocation()
        }

        onLoadMapView()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    let location1:CLLocationCoordinate2D = CLLocationCoordinate2DMake(40.7484445, -73.9878531)
    let location2:CLLocationCoordinate2D = CLLocationCoordinate2DMake(40.758899, -73.9873197)
    let location3:CLLocationCoordinate2D = CLLocationCoordinate2DMake(40.7058134, -73.9981622)
    let location4:CLLocationCoordinate2D = CLLocationCoordinate2DMake(40.6892534, -74.0466891)

    func onLoadMapView() {
        let distanceSpan:CLLocationDegrees = 500

    mapView?.setRegion(MKCoordinateRegionMakeWithDistance(
            location1,
            distanceSpan,
            distanceSpan
        ), animated: true)
    }

    func mapPolylineView(buttonNo: NSNumber) {
        let sourceCoordinates = locationManager.location?.coordinate
        var destCoordinates: CLLocationCoordinate2D?

        if buttonNo == 1 {
            destCoordinates = location1
        } else if buttonNo == 2 {
            destCoordinates = location2
        } else if buttonNo == 3 {
            destCoordinates = location3
        } else if buttonNo == 4 {
            destCoordinates = location4
        }

        let sourceItem = MKMapItem(placemark: MKPlacemark(coordinate: sourceCoordinates!)
        let destItem = MKMapItem(placemark: MKPlacemark(coordinate: destCoordinates!)

        let directionRequest = MKDirectionsRequest()
        directionRequest.source = sourceItem
        directionRequest.destination = destItem
        directionRequest.transportType = .walking

        let directions = MKDirections(request: directionRequest)
        directions.calculate(completionHandler: {
            response, error in
            guard let response = response else {
                if error != nil {
                    print("Something went wrong")
                }
                return
            }

            let route = response.routes[0]
            self.mapView?.add(route.polyline, level: .aboveRoads)

            let rekt = route.polyline.boundingMapRect
            self.mapView?.setRegion(MKCoordinateRegionForMapRect(rekt), animated: true)
        })
    }

    func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
        let renderer = MKPolylineRenderer(overlay: overlay)
        renderer.strokeColor = UIColor(displayP3Red: 69/255, green: 0/255, blue: 132/255, alpha: 1)
        renderer.lineWidth = 5.0

        return renderer
    }
}