Locates the distance of the center of all subviews from the tap location of a UIControl, and returns the subview with the closest center to tap location.

UIControl+CenterTapLocation.swift

class SomeControl: UIControl {
  private var tapGestureRecognizer: UITapGestureRecognizer!
  
  func setupGestureRecognizer() {
    tapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(tapped(_:)))
    addGestureRecognizer(tapGestureRecognizer)
  }
  
  @objc func tapped(_ gestureRecognizer: UITapGestureRecognizer) {
    let location = gestureRecognizer.location(in: self)
    selectedIndex = closestIndex(toPoint: location)
  }
  
  func closestIndex(toPoint point: CGPoint) -> Int {
    let distances = stackView.subviews.map { abs(point.x - $0.center.x) }
    return Int(distances.firstIndex(of: distances.min()!)!)
  }
}