ColorPickerView.swift

//: A UIKit based Playground for presenting user interface

import UIKit
import PlaygroundSupport

enum ColorSpace {
  case rainbow
  case gray

  var colors: [CGColor] {
    switch self {
    case .rainbow:
      return [UIColor.white.cgColor, UIColor.blue.cgColor, UIColor.red.cgColor, UIColor.purple.cgColor]
    case .gray:
      return [UIColor.white.cgColor, UIColor.gray.cgColor, UIColor.black.cgColor]
    }
  }
}

extension UIView {
  func colorAt(point: CGPoint) -> UIColor {
    let rgbMax: CGFloat = 255.0
    let colorSpace: CGColorSpace = CGColorSpaceCreateDeviceRGB()
    let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue)

    var pixelData: [UInt8] = [0, 0, 0, 0]

    guard let context = CGContext(
      data: &pixelData,
      width: 1,
      height: 1,
      bitsPerComponent: 8,
      bytesPerRow: 4,
      space: colorSpace,
      bitmapInfo: bitmapInfo.rawValue
    ) else {
      return UIColor.white
    }

    context.translateBy(x: -point.x, y: -point.y)

    layer.render(in: context)

    let red: CGFloat = CGFloat(pixelData[0]) / rgbMax
    let green: CGFloat = CGFloat(pixelData[1]) / rgbMax
    let blue: CGFloat = CGFloat(pixelData[2]) / rgbMax
    let alpha: CGFloat = CGFloat(pixelData[3]) / rgbMax

    return UIColor(red: red, green: green, blue: blue, alpha: alpha)
  }
}

class ColorPickerView: UIView {
  let gradient = CAGradientLayer()
  let pickerIndicator = UIView()
  var selectedColor = UIColor.black

  convenience init() {
    self.init(frame:.zero)
  }

  override init(frame: CGRect) {
    super.init(frame: frame)
    setup()
  }

  required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    setup()
  }

  private func setup() {
    pickerIndicator.backgroundColor = selectedColor
    pickerIndicator.layer.borderColor = UIColor.white.cgColor
    pickerIndicator.layer.borderWidth = 2
    addSubview(pickerIndicator)
  }

  override func layoutSubviews() {
    super.layoutSubviews()
    let indicatorSize = frame.width / 2
    pickerIndicator.frame.size = CGSize(width: indicatorSize, height: indicatorSize)
    print(indicatorSize)
    pickerIndicator.layer.cornerRadius = indicatorSize / 2
    gradient.frame = frame
    bringSubviewToFront(pickerIndicator)
  }

  override func draw(_ rect: CGRect) {
    super.draw(rect)
    gradient.frame = rect
    gradient.colors = ColorSpace.rainbow.colors
    layer.addSublayer(gradient)
  }
}

class MyViewController : UIViewController {
  var picker: ColorPickerView?
  override func loadView() {
    let view = UIView()
    view.backgroundColor = .white
    self.view = view
  }

  override func viewDidLoad() {
    super.viewDidLoad()
    let colorPicker = ColorPickerView()
    colorPicker.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(colorPicker)

    NSLayoutConstraint.activate([
      colorPicker.leadingAnchor.constraint(equalTo: view.leadingAnchor),
      colorPicker.trailingAnchor.constraint(equalTo: view.trailingAnchor),
      colorPicker.topAnchor.constraint(equalTo: view.topAnchor),
      colorPicker.bottomAnchor.constraint(equalTo: view.bottomAnchor)
    ])
    picker = colorPicker
  }

  override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    print(picker!.colorAt(point: CGPoint(x: 50, y: 150)))
  }
  
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()