//透明
[self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc] init] forBarMetrics:UIBarMetricsDefault];
//去掉透明后导航栏下边的黑边
[self.navigationController.navigationBar setShadowImage:[[UIImage alloc] init]];
// 重置 变为不透明
[self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
[self.navigationController.navigationBar setShadowImage:nil];