GCD 开启一个定时器实现倒计时功能

     UIAlertView * alt = [[UIAlertView alloc] initWithTitle:@"提示"
                                                    message:@"操作成功,马上返回继续体验吧"
                                                   delegate:self
                                          cancelButtonTitle:@"留在当前"
                                          otherButtonTitles:@"马上返回", nil];
     [alt show];
     //倒计时时间
     __block int timeout = ;
     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );
     dispatch_source_t source_t= dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, , ,queue);
     //每秒执行
     dispatch_source_set_timer(source_t,dispatch_walltime(NULL, ),1.0*NSEC_PER_SEC, );
     dispatch_source_set_event_handler(source_t, ^
     {
         if(timeout<=)
         {
             //倒计时结束，关闭
             NSString *strTime = [NSString stringWithFormat:@"倒计时,%.2ds后返回", ];
             dispatch_async(dispatch_get_main_queue(), ^
             {
                 //设置界面的按钮显示 根据自己需求设置
                 alt.message = strTime;
                 [alt dismissWithClickedButtonIndex: animated:YES]; // 模拟的没有调用delegate方法
                 dispatch_source_cancel(source_t);
             });
         }
         else
         {
             // int minutes = timeout / 60;   //分
             int seconds = timeout % ; //秒
             NSString *strTime = [NSString stringWithFormat:@"倒计时,%.2ds后返回", seconds];
             dispatch_async(dispatch_get_main_queue(), ^
             {
                 //设置界面的按钮显示 根据自己需求设置
                 alt.message = strTime;
             });
             timeout--;
         }
     });
     dispatch_resume(source_t);

最后，奉上一篇博文 http://blog.csdn.net/chumeng411/article/details/20918895 这一篇博文中简单论述了timer与线程的关系可以参考一下。