How to calculate distance from user location to annotation when user moves

Question

I have an user location (blue dot) and annotations on mapView. When annotation is selected, I setting text to distLabel - \"Distance to point %4.0f

Answer 1

The map view has a selectedAnnotations property that you can use in the didUpdateToLocation method to tell which annotation to get the distance from.

(By the way, if you are using the map view's userLocation, you might want to use the map view's didUpdateUserLocation delegate method instead of didUpdateToLocation which is a CLLocationManager delegate method.)

In the delegate method, you can check if there is any currently selected annotation and, if so, show the distance to that annotation (otherwise say "no annotation selected").

You might want to write a common method that can be called from both didSelectAnnotationView and didUpdateUserLocation to reduce code duplication.

For example:

-(void)updateDistanceToAnnotation:(id<MKAnnotation>)annotation
{
    if (annotation == nil) 
    {
        distLabel.text = @"No annotation selected";
        return;
    }

    if (mapView.userLocation.location == nil) 
    {
        distLabel.text = @"User location is unknown";
        return;
    }

    CLLocation *pinLocation = [[CLLocation alloc] 
        initWithLatitude:annotation.coordinate.latitude
               longitude:annotation.coordinate.longitude];

    CLLocation *userLocation = [[CLLocation alloc] 
        initWithLatitude:mapView.userLocation.coordinate.latitude 
               longitude:mapView.userLocation.coordinate.longitude];

    CLLocationDistance distance = [pinLocation distanceFromLocation:userLocation];

    [distLabel setText: [NSString stringWithFormat:@"Distance to point %4.0f m.", distance]];
}

-(void)mapView:(MKMapView *)mapView didUpdateUserLocation:(MKUserLocation *)userLocation
{
    if (mapView.selectedAnnotations.count == 0)
        //no annotation is currently selected
        [self updateDistanceToAnnotation:nil];
    else
        //first object in array is currently selected annotation
        [self updateDistanceToAnnotation:[mapView.selectedAnnotations objectAtIndex:0]];
}

- (void)mapView:(MKMapView *)mapView didSelectAnnotationView:(MKAnnotationView *)view
{    
    [self updateDistanceToAnnotation:view.annotation];
}