在CocoaMacOS应用程序中,如何获取mkmapview左下角的度数坐标?

我已经成功地从左下角到右上角找到了可见的地图矩形坐标,调用了地图可见矩形上的mkcoordinateregionformaprect。这是我的代码:

- (IBAction)userDidPressFillCoordinates:(id)sender {

    MKMapRect mapRect = self.mapView.visibleMapRect;

    CLLocationCoordinate2D coordinates = MKCoordinateForMapPoint(mapRect.origin);

    NSLog(@"Latitude %f Longitude: %f", coordinates.latitude, coordinates.longitude);
    NSLog(@"size width = %f, size height = %f",mapRect.size.width,mapRect.size.height);

    NSLog(@"%@",MKStringFromMapRect(mapRect));

    MKCoordinateRegion region =   MKCoordinateRegionForMapRect(mapRect);

    CLLocationCoordinate2D center = region.center;
    MKCoordinateSpan span = region.span;

    CLLocationCoordinate2D topRightCoordinates = coordinates;

    coordinates.latitude -= span.latitudeDelta;

    topRightCoordinates.longitude += span.longitudeDelta;


    self.coordinatesString = [NSString stringWithFormat:@"%f,%f,%f,%f",coordinates.longitude,coordinates.latitude,topRightCoordinates.longitude,topRightCoordinates.latitude];


}