iOS navigationBar设置透明和恢复

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];