自定义 NavigationController 返回按钮

- (void)viewDidLoad {
    [super viewDidLoad];

    UIImage *backButtonBackgroundImage = [UIImage imageNamed:@"ic_back"];
    backButtonBackgroundImage = [backButtonBackgroundImage resizableImageWithCapInsets:UIEdgeInsetsMake(0, backButtonBackgroundImage.size.width - 1, 0, 0)];
    
    id appearance = [UIBarButtonItem appearanceWhenContainedInInstancesOfClasses:@[[WENavigationController class]]];
    [appearance setBackButtonBackgroundImage:backButtonBackgroundImage forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
    
}

- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated {
    [super pushViewController:viewController animated:YES];
    
    viewController.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStylePlain target:nil action:nil];
}