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//1.UINaivgationController的bar有的需要隐藏，有的需要显示
//这时候可以重写UINavigationControllerDelegate的
- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    if ( viewController ==  [MyViewController class]) {
        [navigationController setNavigationBarHidden:YES animated:animated];
    } else if ( [navigationController isNavigationBarHidden] ) {
        [navigationController setNavigationBarHidden:NO animated:animated];
    }
}
//但是这样push或者pop的时候navigation bar会有白边出现，也就是说bar隐藏或者显示的比较晚
//你可以这样做
//在你需要显示的bar的controller里
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self.navigationController setNavigationBarHidden:YES animated:YES];
}
//隐藏则设置hidden位NO