在swift 4.3中，修改代码后@apinho工作正常

extension UIView {
func roundCornersWithLayerMask(cornerRadii: CGFloat, corners: UIRectCorner) {
    let path = UIBezierPath(roundedRect: bounds,
                            byRoundingCorners: corners,
                            cornerRadii: CGSize(width: cornerRadii, height: cornerRadii))
    let maskLayer = CAShapeLayer()
    maskLayer.path = path.cgPath
    layer.mask = maskLayer
  }
}

为您的视图使用此函数

YourViewName. roundCornersWithLayerMask(cornerRadii: 20,corners: [.topLeft,.topRight])

Emma: . topright和。bottom right对你不起作用，可能是因为视图调用。roundCorners是在最终视图边界计算之前完成的。注意，贝塞尔路径在被调用时起源于视图边界。例如，如果自动布局将缩小视图，右侧的圆角可能在视图之外。 尝试在viewDidLayoutSubviews中调用它，其中视图的边界是final。

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。

Stephane回答的另一个版本。

import UIKit

    class RoundCornerView: UIView {
    var corners : UIRectCorner = [.topLeft,.topRight,.bottomLeft,.bottomRight]
        var roundCornerRadius : CGFloat = 0.0
        override func layoutSubviews() {
            super.layoutSubviews()
            if corners.rawValue > 0 && roundCornerRadius > 0.0 {
                self.roundCorners(corners: corners, radius: roundCornerRadius)
            }
        }
        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
        }

    }

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
    }
}

在swift 4.3中，修改代码后@apinho工作正常

extension UIView {
func roundCornersWithLayerMask(cornerRadii: CGFloat, corners: UIRectCorner) {
    let path = UIBezierPath(roundedRect: bounds,
                            byRoundingCorners: corners,
                            cornerRadii: CGSize(width: cornerRadii, height: cornerRadii))
    let maskLayer = CAShapeLayer()
    maskLayer.path = path.cgPath
    layer.mask = maskLayer
  }
}

为您的视图使用此函数

YourViewName. roundCornersWithLayerMask(cornerRadii: 20,corners: [.topLeft,.topRight])