gcd_timer.m

dispatch_queue_t gcdTimerQueue;
dispatch_source_t gcdTimer;

gcdTimer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, gcdTimerQueue);
if (gcdTimer) {
    uint64_t seconds = 30ull;
    uint64_t interval = seconds * NSEC_PER_SEC;
    uint64_t leeway = 1ull * NSEC_PER_SEC;
    __block typeof(self) _self = self;
    
    dispatch_source_set_timer(gcdTimer, dispatch_walltime(NULL, 0), interval, leeway);
    dispatch_source_set_event_handler(gcdTimer, ^{
		// stuff to run on a periodic basis
    });

    dispatch_resume(gcdTimer);
}