iOS GCD 定时器

    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);