覆盖hitTest以忽略透明度

Question

我有显示蒙版图像的简单代码。

@IBOutlet weak var mainImage: UIImageView!

override func viewDidLoad() {
    super.viewDidLoad()

    let maskView = UIImageView()
    maskView.image = ... //mask image
    maskView.frame = mainImage.bounds

    mainImage.mask = maskView

    mainImage.isUserInteractionEnabled = true
}

如何覆盖hitTest以忽略遮罩（透明区域）之外的触摸？