注意，如果你有布局约束附加到它，你必须在你的UIView子类中按如下方式刷新它:

override func layoutSubviews() {
    super.layoutSubviews()
    roundCorners(corners: [.topLeft, .topRight], radius: 3.0)
}

如果你不这样做，它就不会出现。

和圆角，使用扩展:

extension UIView {
   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
    }
}

附加的视图控制器情况:无论你不能或不想子类化一个视图，你仍然可以舍入一个视图。通过覆盖viewWillLayoutSubviews()函数，从它的视图控制器中完成，如下所示:

class MyVC: UIViewController {
    /// The view to round the top-left and top-right hand corners
    let theView: UIView = {
        let v = UIView(frame: CGRect(x: 10, y: 10, width: 200, height: 200))
        v.backgroundColor = .red
        return v
    }()
    
    override func loadView() {
        super.loadView()
        view.addSubview(theView)
    }
    
    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()

        // Call the roundCorners() func right there.
        theView.roundCorners(corners: [.topLeft, .topRight], radius: 30)
    }
}

所有已经给出的答案都是非常好的和有效的(特别是尤努斯使用蒙版属性的想法)。

然而，我需要一些更复杂的东西，因为我的层可以经常改变大小，这意味着我需要每次都调用掩蔽逻辑，这有点烦人。

我使用了swift扩展和计算属性来构建一个真正的cornerRadii属性，它负责在图层布局时自动更新蒙版。

这是使用Peter Steinberg伟大的Aspects库实现的。

完整代码在这里:

extension CALayer {
  // This will hold the keys for the runtime property associations
  private struct AssociationKey {
    static var CornerRect:Int8 = 1    // for the UIRectCorner argument
    static var CornerRadius:Int8 = 2  // for the radius argument
  }

  // new computed property on CALayer
  // You send the corners you want to round (ex. [.TopLeft, .BottomLeft])
  // and the radius at which you want the corners to be round
  var cornerRadii:(corners: UIRectCorner, radius:CGFloat) {
    get {
      let number = objc_getAssociatedObject(self, &AssociationKey.CornerRect)  as? NSNumber ?? 0
      let radius = objc_getAssociatedObject(self, &AssociationKey.CornerRadius)  as? NSNumber ?? 0
      return (corners: UIRectCorner(rawValue: number.unsignedLongValue), radius: CGFloat(radius.floatValue))
    }
    set (v) {
      let radius = v.radius
      let closure:((Void)->Void) = {
        let path = UIBezierPath(roundedRect: self.bounds, byRoundingCorners: v.corners, cornerRadii: CGSize(width: radius, height: radius))
        let mask = CAShapeLayer()
        mask.path = path.CGPath
        self.mask = mask
      }
      let block: @convention(block) Void -> Void = closure
      let objectBlock = unsafeBitCast(block, AnyObject.self)
      objc_setAssociatedObject(self, &AssociationKey.CornerRect, NSNumber(unsignedLong: v.corners.rawValue), .OBJC_ASSOCIATION_RETAIN)
      objc_setAssociatedObject(self, &AssociationKey.CornerRadius, NSNumber(float: Float(v.radius)), .OBJC_ASSOCIATION_RETAIN)
      do { try aspect_hookSelector("layoutSublayers", withOptions: .PositionAfter, usingBlock: objectBlock) }
      catch _ { }
    }
  }
}

我写了一篇简单的博客来解释这一点。

我不知道为什么你的解决方案没有工作，但下面的代码是为我工作。创建一个贝塞尔蒙版，并将其应用到您的视图。在下面的代码中，我将_backgroundView的底角圆角化为3个像素。self是一个自定义的UITableViewCell:

UIBezierPath *maskPath = [UIBezierPath
    bezierPathWithRoundedRect:self.backgroundImageView.bounds
    byRoundingCorners:(UIRectCornerBottomLeft | UIRectCornerBottomRight)
    cornerRadii:CGSizeMake(20, 20)
];

CAShapeLayer *maskLayer = [CAShapeLayer layer];

maskLayer.frame = self.bounds;
maskLayer.path = maskPath.CGPath;

self.backgroundImageView.layer.mask = maskLayer;

Swift 2.0版本与一些改进:

let path = UIBezierPath(roundedRect:viewToRound.bounds, byRoundingCorners:[.TopRight, .BottomLeft], cornerRadii: CGSizeMake(20, 20))
let maskLayer = CAShapeLayer()

maskLayer.path = path.CGPath
viewToRound.layer.mask = maskLayer

Swift 3.0版本:

let path = UIBezierPath(roundedRect:viewToRound.bounds,
                        byRoundingCorners:[.topRight, .bottomLeft],
                        cornerRadii: CGSize(width: 20, height:  20))

let maskLayer = CAShapeLayer()

maskLayer.path = path.cgPath
viewToRound.layer.mask = maskLayer

这里的快速扩展

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
    }
}

这是如何在c#中使用Xamarin为按钮的每个角落设置角半径:

var maskPath = UIBezierPath.FromRoundedRect(MyButton.Bounds, UIRectCorner.BottomLeft | UIRectCorner.BottomRight,
    new CGSize(10.0, 10.0));
var maskLayer = new CAShapeLayer
{
    Frame = MyButton.Bounds,
    Path = maskPath.CGPath
};
MyButton.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