UINavigationController

1.initWithRootViewController:方式内部将controller压入栈底 不会调用导航控制器的pushViewController
    
2.导航控制器的childViewControllers与viewControllers都是压栈的控制器 viewControllerske可读可写

3.push与pop的底层最终就是操作viewControllerske

4.导航栏透明的方式
        [self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
        [self.navigationController.navigationBar setShadowImage:[UIImage new]];
5.自定义返回按钮
        UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
        self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:backButton];