ShadowDemo.swift

import UIKit
import PlaygroundSupport

public class ShadowView: UIView {
  var offset: CGSize = CGSize(width: 0, height: 0)
  var radius: CGFloat = 10
  var opacity: Float = 0.5
  var color: UIColor = UIColor.black
  var cornerRadius: Float = 0.0
  var isCircle: Bool = false
  var showOnlyOutsideBounds: Bool = true
  
  override public func layoutSubviews() {
    super.layoutSubviews()
    
    var cornerRadius = self.cornerRadius
    if isCircle {
      cornerRadius = Float(min(frame.height, frame.width) / 2.0)
    }
    
    if showOnlyOutsideBounds {
      let maskLayer = CAShapeLayer()
      let path = CGMutablePath()
      path.addPath(UIBezierPath(roundedRect: bounds.inset(by: UIEdgeInsets.zero), cornerRadius: CGFloat(cornerRadius)).cgPath)
      path.addPath(UIBezierPath(roundedRect: bounds.inset(by: UIEdgeInsets(top: -offset.height - radius*2, left: -offset.width - radius*2, bottom: -offset.height - radius*2, right: -offset.width - radius*2)), cornerRadius: CGFloat(cornerRadius)).cgPath)
      maskLayer.backgroundColor = UIColor.black.cgColor
      maskLayer.path = path;
      maskLayer.fillRule = .evenOdd
      self.layer.mask = maskLayer
    } else {
      self.layer.masksToBounds = false
    }
    
    self.layer.shadowOffset = self.offset
    self.layer.shadowRadius = self.radius
    self.layer.shadowOpacity = self.opacity
    self.layer.shadowColor = self.color.cgColor
    self.layer.shadowPath = UIBezierPath(roundedRect: bounds, cornerRadius: CGFloat(cornerRadius)).cgPath
  }
}

class DemoView : UIView {
  override init(frame: CGRect) {
    super.init(frame: frame)
    
    let gradient = CAGradientLayer()
    gradient.colors = [
      UIColor(red: 87.0/255.0, green: 247.0/255.0, blue: 141.0/255.0, alpha: 1.0).cgColor,
      UIColor(red: 31/255.0, green: 148.0/255.0, blue: 151.0/255.0, alpha: 1.0).cgColor
    ]
    gradient.frame = self.bounds
    
    self.layer.addSublayer(gradient)
    
    let label = UILabel()
    label.frame = CGRect(x: 100, y: 130, width: 100, height: 40)
    label.text = "Hello World!"
    label.textColor = .black
    label.textAlignment = .center
    self.addSubview(label)

    let shadow = ShadowView(frame: label.frame)
    self.addSubview(shadow)
  }
  
  required init?(coder aDecoder: NSCoder) { return nil }
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = DemoView(frame: CGRect(x: 0, y: 0, width: 300, height: 300))