iOS: 在navigation controller里，如何hide navigation bar of first sub controller?

ref link: http://stackoverflow.com/questions/845583/iphone-hide-navigation-bar-only-on-first-page

- (void) viewWillAppear:(BOOL)animated{    [self.navigationController setNavigationBarHidden:YES animated:animated];    [super viewWillAppear:animated];}- (void) viewWillDisappear:(BOOL)animated{    [self.navigationController setNavigationBarHidden:NO animated:animated];    [super viewWillDisappear:animated];}