iOS 导航栏 设置为透明、恢复

       在viewWillAppear的时候设置
       UIImage *barImage = [[NDImgServer shareInstance] createImageWithColor:[UIColor colorWithRed:255 green:255 blue:255 alpha:0.1ff]];
        [self.navigationController.navigationBar setBackgroundImage:barImage forBarMetrics:UIBarMetricsDefault];
        [self.navigationController.navigationBar setShadowImage:[UIImage new]];


        在viewWillDisappear设置
        [self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
        [self.navigationController.navigationBar setShadowImage:nil];

这样 就不会影响其他controller， 不必隐藏导航栏