问Xcode Swift删除MKPolyline

EN

import UIKit
import CoreLocation
import MapKit

class ViewController: UIViewController, CLLocationManagerDelegate, MKMapViewDelegate {

    @IBOutlet weak var theMap: MKMapView!
    @IBOutlet weak var startStopTracking: UISegmentedControl!
    @IBOutlet weak var clearPath: UIButton!
    @IBOutlet weak var label: UILabel!

    var manager:CLLocationManager!
    var myLocations: [CLLocation] = []

    override func viewDidLoad() {
        super.viewDidLoad()

        //Setup our Location Manager
        manager = CLLocationManager()
        manager.delegate = self
        manager.desiredAccuracy = kCLLocationAccuracyBest
        manager.requestAlwaysAuthorization()
        //manager.startUpdatingLocation()

        //Setup our Map View
        theMap.delegate = self
        theMap.mapType = MKMapType.Hybrid
        theMap.showsUserLocation = true

        //Setup startStopTracking button?

        startStopTracking.selectedSegmentIndex = -1
    }

    @IBAction func indexChanged(sender: UISegmentedControl) {
            switch startStopTracking.selectedSegmentIndex
            {
            case 0:
                manager.startUpdatingLocation()
            case 1:
                manager.stopUpdatingLocation()
            default:
                break;
            }
    }

    @IBAction func clearPath(sender: UIButton) {
        //label.text = String(myLocations.count)
        //stopTracking()
        //theMap.removeOverlay(overlay: MKOverlay) -> Void
    }

    func stopTracking() {
        manager.stopUpdatingLocation()
        myLocations = []
    }

    func locationManager(manager:CLLocationManager, didUpdateLocations locations:[AnyObject]) {
        //theLabel.text = "\(locations[0])"
        myLocations.append(locations[0] as CLLocation)

        let spanX = 0.007
        let spanY = 0.007
        var newRegion = MKCoordinateRegion(center: theMap.userLocation.coordinate, span: MKCoordinateSpanMake(spanX, spanY))
        theMap.setRegion(newRegion, animated: true)

        if (myLocations.count > 1){
            var sourceIndex = myLocations.count - 1
            var destinationIndex = myLocations.count - 2

            let c1 = myLocations[sourceIndex].coordinate
            let c2 = myLocations[destinationIndex].coordinate
            var a = [c1, c2]
            var polyline = MKPolyline(coordinates: &a, count: a.count)
            theMap.addOverlay(polyline)
        }
    }


    func mapView(mapView: MKMapView!, rendererForOverlay overlay: MKOverlay!) -> MKOverlayRenderer! {

        if overlay is MKPolyline {
            var polylineRenderer = MKPolylineRenderer(overlay: overlay)
            polylineRenderer.strokeColor = UIColor.blueColor()
            polylineRenderer.lineWidth = 6
            return polylineRenderer
        }
        return nil
    }
}