グループピンを削除することから始めます
[mapView removeAnnotation:groupAnnotation];
次に、クラスターにピンを追加します
[mapView addAnnotations:clusterAnnotations];
次に、ズームする領域を決定します
CLLocationDegrees minLat = 90;
CLLocationDegrees maxLat = -90;
CLLocationDegress minLong = 180;
CLLocationDegrees maxLong = -180
[clusterAnnotations enumerateUsingBlock:^(id<MKAnnotation> annotation, NSUInteger idx, BOOL *stop) {
CLLocationCoordinate2D coordinate = annotation.coordinate;
minLat = MIN(minLat, coordinate.latitude);
maxLat = MAX(maxLat, coordinate.latitude);
minLong = MIN(minLong, coordinate.longitude);
maxLong = MAX(maxLong, coordinate.longitude);
}
CLLocationCoordinate2D center = CLLocationCoordinate2DMake((minLat + maxLat)/2.f, (minLong + maxLong)/2.f);
MKCoordinateSpan span = MKCoordinateSpanMake((maxLat - minLat)*1.25, (maxLong - minLong)*1.25); //1.25 is for padding
MKCoordinateRegion region = MKCoordinateRegionMake(center, span);
[mapView setRegion:[mapView regionThatFits:region] animated:YES];