有一个超级简单的方法。我在这里找到的。

view.clipsToBounds = true
view.layer.cornerRadius = 24
view.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]

它在视图的CALayer上使用stock cornerRadius属性。你只需要定义这些角。layerMinXMinYCorner是左上layerMaxXMinYCorner是右上。

在Swift 4.2中，通过@IBDesignable创建它，如下所示:

@IBDesignable

class DesignableViewCustomCorner: UIView {

    @IBInspectable var cornerRadious: CGFloat = 0 {
        didSet {
            let path = UIBezierPath(roundedRect: self.bounds, byRoundingCorners: [.topLeft, .topRight], cornerRadii: CGSize(width: cornerRadious, height: cornerRadious))
            let mask = CAShapeLayer()
            mask.path = path.cgPath
            self.layer.mask = mask
        }
    }

}

    // Create the path (with only the top-left corner rounded)
UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:view.bounds 
                           byRoundingCorners:(UIRectCornerBottomLeft | UIRectCornerBottomRight) 
                           cornerRadii:CGSizeMake(7.0, 7.0)];

// Create the shape layer and set its path
CAShapeLayer *maskLayer = [CAShapeLayer layer];
maskLayer.frame = cell.stripBlackImnageView.bounds;
maskLayer.path = maskPath.CGPath; 
// Set the newly created shapelayer as the mask for the image view's layer
view.layer.mask = maskLayer;

如果你正在寻找一个界面构建器的唯一解决方案，有一个适用于iOS 11或更高版本。在这里可以看到我的答案:https://stackoverflow.com/a/58626264

使用这个扩展，它将涵盖一切。

extension UIView {

   func roundTopCorners(radius: CGFloat = 10) {
    
       self.clipsToBounds = true
       self.layer.cornerRadius = radius
       if #available(iOS 11.0, *) {
           self.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]
       } else {
           self.roundCorners(corners: [.topLeft, .topRight], radius: radius)
       }
   }

   func roundBottomCorners(radius: CGFloat = 10) {
    
       self.clipsToBounds = true
       self.layer.cornerRadius = radius
       if #available(iOS 11.0, *) {
           self.layer.maskedCorners = [.layerMinXMaxYCorner, .layerMaxXMaxYCorner]
       } else {
           self.roundCorners(corners: [.bottomLeft, .bottomRight], radius: radius)
       }
   }

   private func roundCorners(corners: UIRectCorner, radius: CGFloat) {
    
        let path = UIBezierPath(roundedRect: bounds, byRoundingCorners: corners, cornerRadii: CGSize(width: radius, height: radius))
        let mask = CAShapeLayer()
        mask.path = path.cgPath
        layer.mask = mask
    }
}

然后像这样使用它:-

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
            
    self.yourView.roundTopCorners()
}

注意:-我建议你不要把这段代码放在viewDidLayoutSubviews()中，因为每当视图更新时，你都会在里面得到调用。所以使用viewDidAppear()，它会像一个咒语一样工作。

在Swift 4.1和Xcode 9.4.1中

在iOS 11中，这一行就足够了:

detailsSubView.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]//Set your view here

查看完整代码:

//In viewDidLoad
if #available(iOS 11.0, *) {
        detailsSubView.clipsToBounds = false
        detailsSubView.layer.cornerRadius = 10
        detailsSubView.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]
} else {
      //For lower versions
}

但是对于低版本

let rectShape = CAShapeLayer()
    rectShape.bounds = detailsSubView.frame
    rectShape.position = detailsSubView.center
    rectShape.path = UIBezierPath(roundedRect: detailsSubView.bounds,    byRoundingCorners: [.topLeft , .topRight], cornerRadii: CGSize(width: 20, height: 20)).cgPath
    detailsSubView.layer.mask = rectShape

完整的代码是。

if #available(iOS 11.0, *) {
    detailsSubView.clipsToBounds = false
    detailsSubView.layer.cornerRadius = 10
    detailsSubView.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]
} else {
    let rectShape = CAShapeLayer()
    rectShape.bounds = detailsSubView.frame
    rectShape.position = detailsSubView.center
    rectShape.path = UIBezierPath(roundedRect: detailsSubView.bounds,    byRoundingCorners: [.topLeft , .topRight], cornerRadii: CGSize(width: 20, height: 20)).cgPath
    detailsSubView.layer.mask = rectShape
}

如果你在故事板中使用AutoResizing，请在viewDidLayoutSubviews()中编写以下代码。

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    if #available(iOS 11.0, *) {
        detailsSubView.clipsToBounds = false
        detailsSubView.layer.cornerRadius = 10
        detailsSubView.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]
    } else {
        let rectShape = CAShapeLayer()
        rectShape.bounds = detailsSubView.frame
        rectShape.position = detailsSubView.center
        rectShape.path = UIBezierPath(roundedRect: detailsSubView.bounds,    byRoundingCorners: [.topLeft , .topRight], cornerRadii: CGSize(width: 20, height: 20)).cgPath
        detailsSubView.layer.mask = rectShape
    }
}