MKMapKit annotation clustering in Objective-C

CustomAnnotationView.h

#import <MapKit/MapKit.h>

@interface CustomAnnotationView : MKMarkerAnnotationView

- (id) initWithAnnotationWithImage:(id<MKAnnotation>)annotation reuseIdentifier:(NSString *)reuseIdentifier annotationViewImage:(UIImage *)annotViewImage;

@end

CustomAnnotationView.m

#import "CustomAnnotationView.h"

@implementation CustomAnnotationView

- (id) initWithAnnotationWithImage:(id<MKAnnotation>)annotation reuseIdentifier:(NSString *)reuseIdentifier annotationViewImage:(UIImage *)annotViewImage
{
    self = [super initWithAnnotation:annotation reuseIdentifier:reuseIdentifier];
    
    self.image = annotViewImage;
    
    return self;
}

@end

ViewController.h

#import <UIKit/UIKit.h>
#import <MapKit/MapKit.h>

@interface ViewController : UIViewController <MKMapViewDelegate>
@property (weak, nonatomic) IBOutlet MKMapView *mapView;
@end

ViewController.m

#import "ViewController.h"
#import "CustomAnnotationView.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.mapView setDelegate:self];
    [self addDefaultAnnotations];
}

- (void)addDefaultAnnotations {
    MKPointAnnotation *point1 = [[MKPointAnnotation alloc] init];
    CLLocationCoordinate2D c1;
    c1.latitude = 46.469391;
    c1.longitude = 30.740883;
    point1.coordinate = c1;
    point1.title = @"Odessa, Ukraine";
    [self.mapView addAnnotation:point1];
    
    MKPointAnnotation *point2 = [[MKPointAnnotation alloc] init];
    CLLocationCoordinate2D c2;
    c2.latitude = 46.469391;
    c2.longitude = 30.740883;
    point2.coordinate = c2;
    point2.title = @"Odessa, Ukraine";
    [self.mapView addAnnotation:point2];
    
    [_mapView setRegion:MKCoordinateRegionMakeWithDistance(c2, 500, 500) animated:YES];
}

#pragma mark - MKMapViewDelegate

    - (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation {
        if ([annotation isKindOfClass:[MKPointAnnotation class]]) {
            
            CustomAnnotationView *annotationView = (CustomAnnotationView *) [self.mapView dequeueReusableAnnotationViewWithIdentifier:@"Jacky.S"];
            if (annotationView == nil) {
                annotationView = [[CustomAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"Jacky.S"];
                annotationView.enabled = YES;
                annotationView.clusteringIdentifier = @"pins";
                // annotationView.glyphImage = [UIImage imageNamed:@"we can use a nice image instead of the default pins"];
            } else {
                annotationView.annotation = annotation;
                annotationView.clusteringIdentifier = @"pins";
            }
            return annotationView;
        }
        return nil;
    }

@end