第一種：背景透明覆蓋導航欄
NextViewController *vc = [[NextViewController alloc] init];
vc.view.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:.5];
vc.modalPresentationStyle = UIModalPresentationOverFullScreen;
[self presentViewController:vc animated:YES completion:nil];
第二種：背景透明不覆蓋導航欄
NextViewController *vc = [[NextViewController alloc] init];
self.definesPresentationContext = YES;
vc.view.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:.5];
vc.modalPresentationStyle = UIModalPresentationOverCurrentContext;
[self presentViewController:vc animated:YES completion:nil];
第三種：不透明且覆蓋導航欄
NextViewController *vc = [[NextViewController alloc] init];
[self presentViewController:vc animated:YES completion:nil];