设置 NavigationBar 为透明

#OC

[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];

[self.navigationController.navigationBar setShadowImage:[UIImage new]];

[self.navigationController.navigationBar setTranslucent:YES];

#Swift

self.navigationController?.navigationBar.setBackgroundImage(UIImage(),forBarMetrics: UIBarMetrics.Default)

self.navigationController?.navigationBar.shadowImage = UIImage()

self.navigationController?.navigationBar.translucent = true