整体替换导航栏按钮
@interface CustomNavigationVC : UINavigationController

@end

///////
@implementation CustomNavigationVC

• (void)viewDidLoad {
  [super viewDidLoad];

  [self.navigationBar setShadowImage:[UIImage new]]; //导航栏下方1px横线
  self.interactivePopGestureRecognizer.delegate = (id <UIGestureRecognizerDelegate>)self;
  }

pragma mark - 返回按钮

• (UIBarButtonItem *)createBackButton
  {
  return [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"back"] style:UIBarButtonItemStylePlain target:self action:@selector(popSelf)];
  }

• (void)popSelf
  {
  [self popViewControllerAnimated:YES];
  }

pragma mark - 重写方法

• (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated
  {
  [super pushViewController:viewController animated:animated];

  if (viewController.navigationItem.leftBarButtonItem == nil && self.viewControllers.count > 1) {
  viewController.navigationItem.leftBarButtonItem = [self createBackButton];
  }
  }