倒计时

__block NSInteger second = self.second;

//全局队列    默认优先级

dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

//定时器模式  事件源

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

//NSEC_PER_SEC是秒，＊1是每秒

dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), NSEC_PER_SEC * 1, 0);

//设置响应dispatch源事件的block，在dispatch源指定的队列上运行

dispatch_source_set_event_handler(timer, ^{

//回调主线程，在主线程中操作UI

dispatch_async(dispatch_get_main_queue(), ^{

if (second >= 0) {

self.second --;

self.timeCircleView.second = self.second;            }

else

{

//这句话必须写否则会出问题

dispatch_source_cancel(timer);

}

});

});

//启动源

dispatch_resume(timer);