GestureRecognizer.swift

private lazy var gestureRecognizer: UIPanGestureRecognizer = {
    let recognizer = UIPanGestureRecognizer(target: self, action: #selector(updateProgress(_:)))
    recognizer.maximumNumberOfTouches = 1
    return recognizer
}()
    
@objc private func updateProgress(_ recognizer: UIPanGestureRecognizer) {
    if animator == nil {
        animator = makeAnimator()
    }
    guard let animator = animator else { return }
    switch recognizer.state {
    case .began:
        animator.pauseAnimation()
        fractionCompletedStart = animator.fractionComplete
        dragStartPosition = recognizer.location(in: view)
    case .changed:
        animator.pauseAnimation()
        let delta = recognizer.location(in: view).x - dragStartPosition.x
        animator.fractionComplete = max(0.0, min(1.0, fractionCompletedStart + delta / 300.0))
    case .ended:
        animator.startAnimation()
    default:
        break
    }
}