//创建一个全局并非队列
  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
   //创建一个定时器
    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
   //配置定时器，每秒执行
   dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 1.0 * NSEC_PER_SEC, 0);
   //创建事件处理器
   dispatch_source_set_event_handler(_timer, ^{
    timeout --;
     if (timeout <= 0) {
       //定时器结束，关闭
        dispatch_source_cancel(_timer);
       _timer = nil;
       //回到主线程刷新UI
       dispatch_async(dispatch_get_main_queue(), ^{
       [_btn setTitle:@"获取验证码" forState:UIControlStateNormal];
       });
     }
    else
 {
  //回到主线程刷新UI
   dispatch_async(dispatch_get_main_queue(), ^{
     [_btn setTitle:[NSString stringWithFormat:@"%ds后重新获取",timeout] forState:UIControlStateNormal];
     });
     }
    });
  //启动定时器
dispatch_resume(_timer);