Create a timer (redundant task) with GCD

GCD_Timer.m

static const NUInteger kFrequency = 15; //seconds

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
if (self.timer) {
    dispatch_source_set_timer(self.timer,
                              dispatch_time(DISPATCH_TIME_NOW, LMConversationHelperPollingFrequency * NSEC_PER_SEC),
                              kFrequency * NSEC_PER_SEC,
                              (1ull * NSEC_PER_SEC) / 10);
    __weak id weakSelf = self;
    dispatch_source_set_event_handler(self.timer, ^{
        // Secret task every 15 seconds
    });
    dispatch_resume(self.timer);
}

- (void)dealloc {
    dispatch_source_cancel(_timer);
}