// Request permission for notifications
if ([UNUserNotificationCenter class]) {
  UNAuthorizationOptions options = UNAuthorizationOptionAlert | UNAuthorizationOptionBadge | UNAuthorizationOptionSound;
  UNUserNotificationCenter *center = [UNUserNotificationCenter currentNotificationCenter];
  [center requestAuthorizationWithOptions:options completionHandler:^(BOOL granted, NSError * _Nullable error) {
      if (!error) {
          dispatch_async(dispatch_get_main_queue(), ^{
              [[UIApplication sharedApplication] registerForRemoteNotifications];
          });
      }
  }];
} else {
  UIUserNotificationType types = UIUserNotificationTypeAlert | UIUserNotificationTypeBadge | UIUserNotificationTypeSound;
  UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTypes:types categories:nil];
  [[UIApplication sharedApplication] registerUserNotificationSettings:settings];
  [[UIApplication sharedApplication] registerForRemoteNotifications];
}