利用GCD写了一个倒计时的例子

            __block int timeOut=[endTimeStr intValue]; //倒计时时间

            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){ //倒计时结束，关闭

                    dispatch_source_cancel(_timer);

                    dispatch_async(dispatch_get_main_queue(), ^{

                        //设置界面的按钮显示 根据自己需求设置

                        if (_delegate && [_delegate respondsToSelector:@selector(reloadOrderDetail)]) {

                            [_delegate reloadOrderDetail];

                        }

                    });

                }else{

                    dispatch_async(dispatch_get_main_queue(), ^{

                        //设置界面的按钮显示 根据自己需求设置

                        _countDownLabel.text = [NSString stringWithFormat:@"剩余%d分%d秒", timeOut/60, timeOut%60];

                        timeOut--;

                    });

                }

            });

            dispatch_resume(_timer);