1.实现父视图半透明效果，子视图不透明，在父视图上设置：
myView.backgroundColor=[[UIColor blackColor] colorWithAlphaComponent:0.5];
2.实现半透明视图覆盖导航栏和状态栏效果（只能将视图添加到Window）：
[[[UIApplication sharedApplication] keyWindow]addSubview:myView];
https://blog.csdn.net/yibuyibulai/article/details/45078275
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];
https://blog.csdn.net/weixin_34237596/article/details/87122124
