GCD 开启一个定时器实现倒计时功能

 1     UIAlertView * alt = [[UIAlertView alloc] initWithTitle:@"提示"
 2                                                    message:@"操作成功,马上返回继续体验吧"
 3                                                   delegate:self
 4                                          cancelButtonTitle:@"留在当前"
 5                                          otherButtonTitles:@"马上返回", nil];
 6     [alt show];
 7     //倒计时时间
 8     __block int timeout = 5;
 9     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
10     dispatch_source_t source_t= dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
11     //每秒执行
12     dispatch_source_set_timer(source_t,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0);
13     dispatch_source_set_event_handler(source_t, ^
14     {
15         if(timeout<=0)
16         {
17             //倒计时结束，关闭
18             NSString *strTime = [NSString stringWithFormat:@"倒计时,%.2ds后返回", 0];
19             dispatch_async(dispatch_get_main_queue(), ^
20             {
21                 //设置界面的按钮显示 根据自己需求设置
22                 alt.message = strTime;
23                 [alt dismissWithClickedButtonIndex:1 animated:YES]; // 模拟的没有调用delegate方法
24                 dispatch_source_cancel(source_t);
25             });
26         }
27         else
28         {
29             // int minutes = timeout / 60;   //分
30             int seconds = timeout % 60; //秒
31             NSString *strTime = [NSString stringWithFormat:@"倒计时,%.2ds后返回", seconds];
32             dispatch_async(dispatch_get_main_queue(), ^
33             {
34                 //设置界面的按钮显示 根据自己需求设置
35                 alt.message = strTime;
36             });
37             timeout--;
38         }
39     });
40     dispatch_resume(source_t);

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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。