问Swift:地图视图消失/重新出现后，MapView geocodeAddressString不起作用

EN

class MapViewController: UIViewController, MKMapViewDelegate, CLLocationManagerDelegate {

    @IBOutlet var mapView: MKMapView!

    var searchZipCode:String! = "95110" //used by other segues to set zipcode to show on this page
    let locationManager = CLLocationManager()
    let geocoder = CLGeocoder()
    var dispatchGroup = dispatch_group_create()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        self.locationManager.delegate = self
        self.locationManager.desiredAccuracy = kCLLocationAccuracyBest
        self.locationManager.requestWhenInUseAuthorization()
        self.locationManager.startUpdatingLocation()
        self.mapView.showsUserLocation = true

        dispatchGroup = dispatch_group_create()

        dispatch_group_enter(dispatchGroup)
        self.plotZipCodeInMap(self.searchZipCode)

        dispatch_group_notify(dispatchGroup, dispatch_get_main_queue()){
            print("Zip code plotted.")
    }

    func plotZipCodeInMap(zipCode:String) {
        geocoder.geocodeAddressString(zipCode, completionHandler: {(placemarks, error) -> Void in
            print("Zip code = \(zipCode)")
            if let placemark = placemarks?.first {
                print("Successfully decoded the zip code..")
                let coordinate = placemark.location?.coordinate
                let center = CLLocationCoordinate2D(latitude: coordinate!.latitude, longitude: coordinate!.longitude)
                let region = MKCoordinateRegion(center: center, span: MKCoordinateSpan(latitudeDelta: 0.04, longitudeDelta: 0.04))
                self.mapView.setRegion(region, animated: true)
                dispatch_group_leave(self.dispatchGroup)
            }
        })
    }

    func locationManager(manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        print("Location updated..")
        let locations = locations.last
        let center = CLLocationCoordinate2D(latitude: locations!.coordinate.latitude, longitude: locations!.coordinate.longitude)
        let region = MKCoordinateRegion(center: center, span: MKCoordinateSpan(latitudeDelta: 0.075, longitudeDelta: 0.075))
        self.mapView.setRegion(region, animated: true)

        self.locationManager.stopUpdatingLocation()
}