From Ray Wenderlich: https://www.raywenderlich.com/216251-core-graphics-how-to-make-a-glossy-button

CoolButton.swift

/// Copyright (c) 2019 Razeware LLC
///
/// Permission is hereby granted, free of charge, to any person obtaining a copy
/// of this software and associated documentation files (the "Software"), to deal
/// in the Software without restriction, including without limitation the rights
/// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
/// copies of the Software, and to permit persons to whom the Software is
/// furnished to do so, subject to the following conditions:
///
/// The above copyright notice and this permission notice shall be included in
/// all copies or substantial portions of the Software.
///
/// Notwithstanding the foregoing, you may not use, copy, modify, merge, publish,
/// distribute, sublicense, create a derivative work, and/or sell copies of the
/// Software in any work that is designed, intended, or marketed for pedagogical or
/// instructional purposes related to programming, coding, application development,
/// or information technology.  Permission for such use, copying, modification,
/// merger, publication, distribution, sublicensing, creation of derivative works,
/// or sale is expressly withheld.
///
/// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
/// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
/// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
/// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
/// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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/// THE SOFTWARE.

import UIKit

class CoolButton: UIButton {
  var hue: CGFloat {
    didSet {
      setNeedsDisplay()
    }
  }
  
  var saturation: CGFloat {
    didSet {
      setNeedsDisplay()
    }
  }
  
  var brightness: CGFloat {
    didSet {
      setNeedsDisplay()
    }
  }
  
  required init?(coder aDecoder: NSCoder) {
    self.hue = 0.5
    self.saturation = 0.5
    self.brightness = 0.5
    
    super.init(coder: aDecoder)
    
    self.isOpaque = false
    self.backgroundColor = .clear
  }
  
  override func draw(_ rect: CGRect) {
    guard let context = UIGraphicsGetCurrentContext() else {
      return
    }
    
    var actualBrightness = brightness
    
    if state == .highlighted {
      actualBrightness -= 0.1
    }
    
    let outerColor = UIColor(hue: hue, saturation: saturation, brightness: actualBrightness, alpha: 1.0)
    let shadowColor = UIColor(red: 0.2, green: 0.2, blue: 0.2, alpha: 0.5)
    
    let outerMargin: CGFloat = 5.0
    let outerRect = rect.insetBy(dx: outerMargin, dy: outerMargin)
    let outerPath = createRoundedRectPath(for: outerRect, radius: 6.0)
    
    if state != .highlighted {
      context.saveGState()
      context.setFillColor(outerColor.cgColor)
      context.setShadow(offset: CGSize(width: 0, height: 2), blur: 3.0, color: shadowColor.cgColor)
      context.addPath(outerPath)
      context.fillPath()
      context.restoreGState()
    }
    
    // Outer Path Gloss & Gradient
    let outerTop = UIColor(hue: hue, saturation: saturation, brightness: actualBrightness, alpha: 1.0)
    let outerBottom = UIColor(hue: hue, saturation: saturation, brightness: actualBrightness * 0.8, alpha: 1.0)
    
    context.saveGState()
    context.addPath(outerPath)
    context.clip()
    drawGlossAndGradient(context: context, rect: outerRect, startColor: outerTop.cgColor, endColor: outerBottom.cgColor)
    context.restoreGState()
    
    // Inner Path Gloss & Gradient
    let innerTop = UIColor(hue: hue, saturation: saturation, brightness: actualBrightness * 0.9, alpha: 1.0)
    let innerBottom = UIColor(hue: hue, saturation: saturation, brightness: actualBrightness * 0.7, alpha: 1.0)
    
    let innerMargin: CGFloat = 3.0
    let innerRect = outerRect.insetBy(dx: innerMargin, dy: innerMargin)
    let innerPath = createRoundedRectPath(for: innerRect, radius: 6.0)
    
    context.saveGState()
    context.addPath(innerPath)
    context.clip()
    drawGlossAndGradient(context: context, rect: innerRect, startColor: innerTop.cgColor, endColor: innerBottom.cgColor)
    context.restoreGState()
  }
  
  @objc func hesitateUpdate() {
    setNeedsDisplay()
  }
  
  override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesBegan(touches, with: event)
    setNeedsDisplay()
  }
  
  override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesMoved(touches, with: event)
    setNeedsDisplay()
  }
  
  override func touchesCancelled(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesCancelled(touches, with: event)
    setNeedsDisplay()
    
    perform(#selector(hesitateUpdate), with: nil, afterDelay: 0.1)
  }
  
  override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesEnded(touches, with: event)
    setNeedsDisplay()
    
    perform(#selector(hesitateUpdate), with: nil, afterDelay: 0.1)
  }
}