IBDesignable element misplaced in interface builder, needs to update frames manually

Question

I'm working on a Radio Button View, and already uploaded a test project on github at https://github.com/paulicelli/RadioButton

This is basically a subclass of a UIStackView, with a IBInspectable number of buttons to create some radio buttons.

Working on the device, but I always see a misplacement of the buttons when I open Interface Builder, until I change the number of buttons or the spacing. You can see the problem in view1, then the correct view in view2 attached.

QUESTION: How can I tell Interface builder to properly position the buttons as soon as I open it?

here is the code:

import UIKit

// ROUNDBUTTON
final class RoundButton: UIButton {

  var borderWidth: CGFloat = 2.0
  var primaryColor: UIColor = UIColor.blue

  var primaryColorWithAlpha: UIColor {
    return primaryColor.withAlphaComponent(0.25)
  }

  override public var buttonType: UIButtonType {
    return .custom
  }

  override public func layoutSubviews() {
    super.layoutSubviews()
    self.layer.cornerRadius = bounds.size.width * 0.5
    self.clipsToBounds = true
    setupColors()
  }

  func setupColors() {
    switch self.state {
    case UIControlState.normal:
      super.backgroundColor = .white
      self.setTitleColor(primaryColor, for: state)
      self.layer.borderColor = primaryColor.cgColor
      self.layer.borderWidth = borderWidth
    default:
      super.backgroundColor = primaryColorWithAlpha
      self.setTitleColor(.white, for: state)
    }
  }

}
// END OF ROUNDBUTTON

// PROTOCOL
@objc protocol RadioButtonsViewDelegate {
  @objc optional func didSelectButton(_ aButton: RoundButton?)
}
// END OF PROTOCOL

// RADIOBUTTONSVIEW
@IBDesignable
class RadioButtonsView: UIStackView {

  var shouldLetDeselect = true

  var buttons = [RoundButton]()

  weak var delegate : RadioButtonsViewDelegate? = nil

  private(set) weak var currentSelectedButton: RoundButton? = nil

  @IBInspectable
  var number: Int = 2 {
    didSet {
      number = max(number, 2)
      setupButtons()
    }
  }

  func setupButtons() {
    if !buttons.isEmpty {
      buttons.forEach { self.removeArrangedSubview($0) }
      buttons.removeAll()
    }

    for i in 0..<self.number {
      let button = RoundButton()
      button.setTitle("\(i)", for: .normal)
      button.addTarget(self, action: #selector(pressed(_:)),
                       for: UIControlEvents.touchUpInside)
      buttons.append(button)
    }
  }

  func pressed(_ sender: RoundButton) {
    if (sender.isSelected) {
      if shouldLetDeselect {
        sender.isSelected = false
        currentSelectedButton = nil
      }
    }else {
      buttons.forEach { $0.isSelected = false }
      sender.isSelected = true
      currentSelectedButton = sender
    }
    delegate?.didSelectButton?(currentSelectedButton)
  }

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

  required init(coder: NSCoder) {
    super.init(coder: coder)
    setupButtons()
  }

  override var intrinsicContentSize: CGSize {
    return CGSize(width: self.bounds.width,
                  height: UIViewNoIntrinsicMetric)
  }

  override func layoutSubviews() {
    self.translatesAutoresizingMaskIntoConstraints = false

    buttons.forEach { self.addArrangedSubview($0) }

    let constraints: [NSLayoutConstraint] = {
      var constraints = [NSLayoutConstraint]()
      constraints.append(NSLayoutConstraint(item: buttons[0],
                                                 attribute: NSLayoutAttribute.width,
                                                 relatedBy: NSLayoutRelation.equal,
                                                 toItem: self,
                                                 attribute: NSLayoutAttribute.height,
                                                 multiplier: 1,
                                                 constant: 0)
        )
      for i in 1..<buttons.count {
        constraints.append(NSLayoutConstraint(item: buttons[i],
                                            attribute: NSLayoutAttribute.width,
                                            relatedBy: NSLayoutRelation.equal,
                                            toItem: buttons[i - 1],
                                            attribute: NSLayoutAttribute.width,
                                            multiplier: 1,
                                            constant: 0)
        )
      }
      return constraints
    }()

    self.addConstraints(constraints)
  }

}
// END OF RADIOBUTTONSVIEW

EDIT: As requested by @dfd I also tried:

@IBInspectable
var number: Int {
  get {
    setupButtons()
    return self.number
  }
  set {
    self.number = max(newValue, 2)
    setupButtons()
  }
}

and I got 2 errors while loading Interface Builder:

1. error: IB Designables: Failed to render and update auto layout status for ViewController (BYZ-38-t0r): The agent crashed
2. error: IB Designables: Failed to update auto layout status: The agent crashed