从导航栏不透明的vc返回到导航栏透明的vc显示问题

会有透明块，很难看，解决办法如下；
A:导航栏透明vc，B：导航栏不透明vc
在A中写

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];

    [self.navigationController setNavigationBarHidden:YES animated:YES];

}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];

    [self.navigationController setNavigationBarHidden:NO animated:YES];

}