Question

I am new to ObjC and I am struggling with the CLGeocoder. I want to be able to use reverseGeocodeLocation to obtain a string that contains the user location that I pass to my delegate when the user presses a Done button.

So the user triggers the display of a MapViewController, I call the reverseGeocodeLocation in the viewDidLoad but the [placemarks count = 0] this first time in, and I have no placemark to get the info that I need. The second time the user triggers the display of the MapViewController the placemarks array has been populated and everything works.

I suspect it is something to do with the reverseGeocodeLocation being an asynchronous call – but I cannot figure out how to solve this problem. I have tried searching online but nothing seems to help me understand what I am doing wrong and how i can solve this issue. Thanks in advance.

@interface MapViewController ()
@property (strong, nonatomic) CLGeocoder *geocoder;
@property (readwrite, nonatomic) NSString *theLocationName;
@end

@implementation MapViewController
@synthesize mapView, geocoder, delegate = _delegate, theLocationName = _theLocationName;

- (void)viewDidLoad
{
[super viewDidLoad];

self.mapView.delegate=self;
self.mapView.showsUserLocation = YES;

[self theUserLocation];
}

-(void)theUserLocation
{
if (!geocoder)
{
    geocoder = [[CLGeocoder alloc] init];
}

MKUserLocation *theLocation;
theLocation = [self.mapView userLocation];

[geocoder reverseGeocodeLocation:theLocation.location 
               completionHandler:^(NSArray* placemarks, NSError* error)
 {
     if ([placemarks count] > 0)
     {
         CLPlacemark *placemark = [placemarks objectAtIndex:0];

         [self setTheLocationName: placemark.locality];

     }
 }];

- (IBAction)done:(id)sender 
{

[[self delegate] mapViewControllerDidFinish:self locationName:[self theLocationName]];

}

@end

Answer 1

So the user triggers the display of a MapViewController, I call the reverseGeocodeLocation in the viewDidLoad but the [placemarks count = 0] this first time in, and I have no placemark to get the info that I need. The second time the user triggers the display of the MapViewController the placemarks array has been populated and everything works.

It’s not because the call is asynchronous – it’s because the first time you call theUserLocation the actual location isn’t available. Getting the user’s location is not instantaneous – it takes time. However, you’re asking for the user’s location as soon as the map loads, which in most circumstances won’t work.

What you need to do is hook into the MKMapViewDelegate methods, which provide you with callbacks when the location is updated. You can use this to check the location’s accuracy, and decide whether it is accurate enough for you to reverse geolocate.