参考までに


///利用側
func _() {
    hogeView.addOneSideDashBorder(position: .bottom, color: .red, lineDashPattern: [5, 5])
}

extension UIView {
    /// 一辺に点線を引く
    @discardableResult
    func addOneSideDashBorder(position: BorderPosition, color: UIColor, lineDashPattern: [NSNumber]?) -> CALayer {
        layoutIfNeeded()

        let shapeLayer = CAShapeLayer()
        shapeLayer.lineDashPattern = lineDashPattern
        shapeLayer.strokeColor = color.cgColor
        shapeLayer.frame = bounds

        let line = UIBezierPath()
        switch position {
        case .top:
            line.move(to: bounds.leftTop)
            line.addLine(to: bounds.rightTop)
        case .left:
            line.move(to: bounds.leftTop)
            line.addLine(to: bounds.leftBottom)
        case .right:
            line.move(to: bounds.rightTop)
            line.addLine(to: bounds.rightBottom)
        case .bottom:
            line.move(to: bounds.leftBottom)
            line.addLine(to: bounds.rightBottom)
        }

        shapeLayer.path = line.cgPath
        layer.addSublayer(shapeLayer)

        return shapeLayer
    }
}

/// 上下左右
enum BorderPosition {
    case top
    case left
    case right
    case bottom
}

/// 
extension CGRect {
    var leftTop: CGPoint {
        return CGPoint(x: 0, y: 0)
    }
    var rightTop: CGPoint {
        return CGPoint(x: width-1, y: 0)
    }
    var leftBottom: CGPoint {
        return CGPoint(x: 0, y: height-1)
    }
    var rightBottom: CGPoint {
        return CGPoint(x: width-1, y: height-1)
    }
}

結果

Simulator Screen Shot - iPhone X - 2020-01-01 at 18.02.47.png

考えたら実線もこっちでいいのではないでしょうか？
borderを使う方法のほうが一般的ですが、ソッチのほうがいい理由はあるのか（ちょっとわかりません）


extension UIView {
    /// 一辺に実践を引く
    @discardableResult
    func addOneSideBorder(position: BorderPosition, color: UIColor) -> CALayer {
        layoutIfNeeded()

        let shapeLayer = CAShapeLayer()
        shapeLayer.strokeColor = color.cgColor
        shapeLayer.lineWidth = 1
        shapeLayer.frame = bounds

        let line = UIBezierPath()
        switch position {
        case .top:
            line.move(to: bounds.leftTop)
            line.addLine(to: bounds.rightTop)
        case .left:
            line.move(to: bounds.leftTop)
            line.addLine(to: bounds.leftBottom)
        case .right:
            line.move(to: bounds.rightTop)
            line.addLine(to: bounds.rightBottom)
        case .bottom:
            line.move(to: bounds.leftBottom)
            line.addLine(to: bounds.rightBottom)
        }

        shapeLayer.path = line.cgPath
        layer.addSublayer(shapeLayer)

        return shapeLayer
    }
}

追記：欠点見つけました。borderと違い、Viewのframeに連動してくれないので付け替えが面倒ですね。