迅捷：UIAlertView背景

Question

我有UIAlertView。

let alertController = UIAlertController(title: "11", message: "1", preferredStyle: .alert)

let okAction = UIAlertAction(title: "1", style: .default) {
    UIAlertAction in

    }

let cancelAction = UIAlertAction(title: "1", style: UIAlertActionStyle.cancel) {
    UIAlertAction in

    }

alertController.addAction(okAction)
alertController.addAction(cancelAction)

self.present(alertController, animated: true, completion: nil)

我想更改UIAlertView和UIAlertView的背景图像。和按钮的背景图像。怎么做？