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 = 5;
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t source_t= dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
    //每秒执行
    dispatch_source_set_timer(source_t,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0);
    dispatch_source_set_event_handler(source_t, ^
    {
        if(timeout<=0)
        {
            //倒计时结束，关闭
            NSString *strTime = [NSString stringWithFormat:@"倒计时,%.2ds后返回", 0];
            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:1 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 % 60; //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。