利用生命周期设置是否隐藏 navigationBar

在页面将要显示或者将要消失时，即viewWillAppear、viewWillDisappear中写navigationBar的显隐，才会在每次进到或退出此页面时，更新显隐！！！

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