这样子就透明了

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

Swift：

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self.navigationController!.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: UIBarMetrics.Default)
self.navigationController!.navigationBar.shadowImage = UIImage()