dispatch_queue_t queue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL); self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue); dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, 0); uint64_t interval = (uint64__t)(1.0 * NSEC_PER_SEC); dispatch_source_set_timer(self.timer, start, interval, 0); dispatch_source_set_event_handler(self.timer, ^{ static int i = 0; i++; NSLog(@"currentThread is %@", [NSThread currentThread]); NSLog(@"i is %d", i); if (i == 4) { dispatch_cancel(self.timer); self.timer = nil; } }); dispatch_resume(self.timer);