ButtonPanelView.swift

class ButtonPanelView: UIView {

  // ...

  override init(frame: CGRect) {
    super.init(frame: frame)
    backgroundColor = UIColor(red: 81/255, green: 166/255, blue: 219/255, alpha: 1)

    layer.cornerRadius = buttonSize / 2
    layer.shadowColor = UIColor.lightGray.cgColor
    layer.shadowOpacity = shadowOpacity

    layer.shadowOffset = .zero

    addSubview(containerStackView)
    setConstraints()
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  private func setConstraints() {
    // Main button
    menuButton.translatesAutoresizingMaskIntoConstraints = false
    menuButton.widthAnchor.constraint(equalToConstant: buttonSize).isActive = true
    menuButton.heightAnchor.constraint(equalToConstant: buttonSize).isActive = true

    // Dog button
    dogButton.translatesAutoresizingMaskIntoConstraints = false
    dogButton.widthAnchor.constraint(equalToConstant: buttonSize).isActive = true
    dogButton.heightAnchor.constraint(equalToConstant: buttonSize).isActive = true

    // Cat button
    catButton.translatesAutoresizingMaskIntoConstraints = false
    catButton.widthAnchor.constraint(equalToConstant: buttonSize).isActive = true
    catButton.heightAnchor.constraint(equalToConstant: buttonSize).isActive = true

    // Container stack view
    containerStackView.translatesAutoresizingMaskIntoConstraints = false
    containerStackView.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true
    containerStackView.centerYAnchor.constraint(equalTo: self.centerYAnchor).isActive = true

    translatesAutoresizingMaskIntoConstraints = false
    self.widthAnchor.constraint(equalTo: containerStackView.widthAnchor).isActive = true
    self.heightAnchor.constraint(equalTo: containerStackView.heightAnchor).isActive = true
  }
  
  // ...
}