UIPageViewControllerWithScrollProgress.swift

class ViewController: UIViewController {
    @IBOutlet weak var stackView: UIStackView!
    @IBOutlet weak var stackViewIndicatorLeadingConstraint: NSLayoutConstraint!
    fileprivate var previousStackViewIndicatorLeadingConstraintConstant: CGFloat = 0
    fileprivate var selectedButton: UIButton?
    var pageViewController: UIPageViewController?

    override func viewDidLoad() {
      super.viewDidLoad()

      // ......

      if let controller = viewControllerForPage(at: 1) {
          pageViewController?.setViewControllers([controller], direction: .forward, animated: false) { (_) in
              self.selectedButton = self.stackView.arrangedSubviews[1] as? UIButton
              self.stackViewIndicatorLeadingConstraint.constant = 84 // 初始值
              self.previousStackViewIndicatorLeadingConstraintConstant = self.stackViewIndicatorLeadingConstraint.constant
          }
       }

       if let scrollView = pageViewController?.view.subviews.filter({ $0 is UIScrollView }).first as? UIScrollView {
          scrollView.delegate = self
       }
    }
}

extension ViewController: UIScrollViewDelegate {
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let offsetX = scrollView.contentOffset.x
        var percentage: CGFloat = 0
        if offsetX == scrollView.frame.width {
            // Page ended
            previousStackViewIndicatorLeadingConstraintConstant = self.stackViewIndicatorLeadingConstraint.constant // 重置
        } else {
            // Paging
            percentage = (offsetX - scrollView.frame.width) / scrollView.frame.width
            if percentage > 0 {
                // Next paging
            } else {
                // Previous paging
            }
            self.stackViewIndicatorLeadingConstraint.constant = previousStackViewIndicatorLeadingConstraintConstant + (84 - 29) * percentage
            print(percentage)
            self.stackView.layoutIfNeeded()
        }
    }
}

extension ViewController: UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, willTransitionTo pendingViewControllers: [UIViewController]) { }
    
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        guard let currentController = pageViewController.viewControllers?.first else { return }
        let index = indexForPage(of: currentController)
        selectedButton?.isSelected = false
        
        let pendingButton = stackView.arrangedSubviews[index] as! UIButton
        pendingButton.isSelected = true
        selectedButton = pendingButton
        
        self.stackViewIndicatorLeadingConstraint.constant = selectedButton?.tag == 0 ? 29 : 84 // 动画完成，确定位置
        self.stackView.layoutIfNeeded()
        UIView.animate(withDuration: 0.25) {
            self.navigationView.layoutIfNeeded()
        }
    }
}