对齐UIBarButtonItem后退按钮

Question

以下是我在viewDidLoad方法中的内容：

self.navigationController?.navigationBar.backIndicatorImage = UIImage(named: "back")?.imageWithRenderingMode(.AlwaysOriginal)
self.navigationController?.navigationBar.backIndicatorTransitionMaskImage = UIImage(named: "back")?.imageWithRenderingMode(.AlwaysOriginal)
self.navigationItem.backBarButtonItem = UIBarButtonItem(title: "", style: UIBarButtonItemStyle.Plain, target: nil, action: nil)

这会显示一个自定义图像，该图像在单击按钮时有效，但按钮会在状态栏上向上推。 如何降低按钮？