obtain userLocation from mapView ios - ios

Question

I want to retrieve the current position and send it with a POST request to a server.
I use mapView.userLocation.location.coordinate but it doesn't work and shows always 0.0 for lat and 0.0 for lng.
This is .h of my controller:
#import <UIKit/UIKit.h>
#import <MapKit/MapKit.h>
#interface CustomViewController : UIViewController
<MKMapViewDelegate>
#property NSMutableArray * shops;
#property (strong, nonatomic) IBOutlet UIActivityIndicatorView *loadingIcon;
#property (weak, nonatomic) IBOutlet MKMapView *mapView;
#end
and this .m :
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view from its nib.
_mapView.showsUserLocation = YES;
_mapView.delegate = self;
_shops = [[NSMutableArray alloc] init];
[_loadingIcon startAnimating];
CLLocationCoordinate2D coordinate;
coordinate = _mapView.userLocation.location.coordinate;
NSNumber *lat = [NSNumber numberWithDouble:coordinate.latitude];
NSNumber *lng = [NSNumber numberWithDouble:coordinate.longitude];
//here the post method
NSArray *keys = [NSArray arrayWithObjects:#"lat", #"lng", nil];
NSArray *objects = [NSArray arrayWithObjects:lat,lng, nil];
NSDictionary *jsonDictionary = [NSDictionary dictionaryWithObjects:objects forKeys:keys];
NSData *jsonData ;
NSString *jsonString;
if([NSJSONSerialization isValidJSONObject:jsonDictionary])
{
jsonData = [NSJSONSerialization dataWithJSONObject:jsonDictionary options:0 error:nil];
jsonString = [[NSString alloc]initWithData:jsonData encoding:NSUTF8StringEncoding];
}
.....
NSLog(#"posizione %#", jsonString);
}
in Log I always have 0.0 0.0.
I have this method too:
- (void)mapView:(MKMapView *)mapView didUpdateUserLocation: (MKUserLocation *)userLocation
{
[_mapView setCenterCoordinate:userLocation.location.coordinate animated:YES];
userLocation.title = #"Posizione attuale";
MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance(userLocation.location.coordinate, 2000, 2000);
[_mapView setRegion:region animated:YES];
NSLog(#"lat %f", (double)userLocation.location.coordinate.latitude);
}
and in this case the log shows the current position.
Someone have an idea?