iOS 模态弹出，覆盖或不覆盖导航栏的处理

第一种：背景透明覆盖导航栏

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];

第二种：背景透明不覆盖导航栏

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];

第三种：不透明且覆盖导航栏

NextViewController *vc = [[NextViewController alloc] init];
[self presentViewController:vc animated:YES completion:nil];