GCD倒计时

//GCD定时器

dispatch_source_t timer=dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());

dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW, 15ull*NSEC_PER_SEC), DISPATCH_TIME_FOREVER, 1ull*NSEC_PER_SEC);

dispatch_source_set_event_handler(timer, ^{

NSLog(@"wakeup");

dispatch_source_cancel(timer);

});

dispatch_source_set_cancel_handler(timer, ^{

NSLog(@"cancel");

dispatch_release(timer);

});

//启动

dispatch_resume(timer);