GCD倒计时

            __block NSInteger timeout=time; //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
                        cell.payTimeLabel.text = @"已超时";
                    });
                }else{
                    NSInteger minutes = timeout / 60;
                    NSInteger seconds = timeout % 60;
                    NSString *timeStr = [NSString stringWithFormat:@"%ld:%ld",minutes, seconds];
                    dispatch_async(dispatch_get_main_queue(), ^{
                        //设置界面的按钮显示 根据自己需求设置
                        cell.payTimeLabel.text = timeStr;
    
                    });
                    timeout--;
    
                }
            });
            dispatch_resume(_timer);