如何在MKMapView上绘制一条线来显示用户所采用的路线？

import MapKit

var coordinates: [CLLocationCoordinate2D] = [
    CLLocationCoordinate2D(latitude: 37.7749, longitude: -122.4194),
    CLLocationCoordinate2D(latitude: 37.7749, longitude: -122.4194),
    CLLocationCoordinate2D(latitude: 37.7919, longitude: -122.4053),
    CLLocationCoordinate2D(latitude: 37.7919, longitude: -122.4053)
]

let polyline = MKPolyline(coordinates: &coordinates, count: coordinates.count)

mapView.addOverlay(polyline)

func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
    if overlay is MKPolyline {
        let renderer = MKPolylineRenderer(overlay: overlay)
        renderer.strokeColor = UIColor.blue
        renderer.lineWidth = 3
        return renderer
    }
    return MKOverlayRenderer()
}