Present新视图时旧视图不消失的办法，类似推出UIAlertController

有时候需要些弹出视图，但又不想使用View来添加，这时候可以新建一个controller进行布局，类似退出系统的UIAlertController，具体做法如下：

        RewardViewController *rewawrd = [[RewardViewController alloc] init];
        rewawrd.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
        rewawrd.modalPresentationStyle = UIModalPresentationCustom;
        [self presentViewController:rewawrd animated:YES completion:nil];