dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());
dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 15ull * NSEC_PER_SEC, 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_resume(timer);