GCD实现倒计时

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
dispatch_source_set_event_handler(_timer, ^{
    if(timeout<=0)
    {
        dispatch_source_cancel(_timer); //一定要有取消...
        dispatch_async(dispatch_get_main_queue(), ^{
            blockYes();
        });
    } else
    {
        int seconds = timeout % 60;
        __block NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
        dispatch_async(dispatch_get_main_queue(), ^{
            if([strTime isEqualToString:@"00"])
            {
                strTime = @"60";
            }

// NSLog(@"____%@",strTime);
blockNo(strTime);
});
}
timeout--;
});
dispatch_resume(_timer);