设置为透明
[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
[self.navigationController.navigationBar setShadowImage:[UIImage new]];
恢复为默认样式
[self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
[self.navigationController.navigationBar setShadowImage:nil];