Onboarding with scaling image.

OnboardingViewController+UIScrollViewDelegate.swift

typealias Interval = (a: CGFloat, b: CGFloat)

extension OnboardingViewController: UIScrollViewDelegate {
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let pageIndex = round(scrollView.contentOffset.x / scrollView.frame.width)
        pageControl.currentPage = Int(pageIndex)

        let maximumHorizontalOffset: CGFloat = scrollView.contentSize.width - scrollView.frame.width
        let currentHorizontalOffset: CGFloat = scrollView.contentOffset.x
        let percentageHorizontalOffset: CGFloat = currentHorizontalOffset / maximumHorizontalOffset

        let percentOffset: CGPoint = CGPoint(x: percentageHorizontalOffset, y: 0.0)
        let intervals = makeIntervals(split: 1.0, by: pageControl.numberOfPages - 1)

        guard let interval = intervals.enumerated().first(where: { percentOffset.x > $0.element.a && percentOffset.x <= $0.element.b }) else {
            return
        }
        guard percentOffset.x > 0.0 && percentOffset.x <= 1, (interval.offset + 1) < onboardingViews.count else {
            return
        }

        let scaleFactor = 1.0 / CGFloat(pageControl.numberOfPages - 1)
        let scale0 = (interval.element.b - percentOffset.x) / scaleFactor
        let scale1 = percentOffset.x / interval.element.b

        onboardingViews[interval.offset].scaleImage(scale: scale0)
        onboardingViews[interval.offset + 1].scaleImage(scale: scale1)
    }

    // MARK: - Utilities

    private func makeIntervals(split whole: CGFloat, by n: Int) -> [Interval] {
        let step: CGFloat = whole / CGFloat(n)
        let iterations: [CGFloat] = Array.init(repeating: step, count: n)

        return iterations
            .enumerated()
            .map { Interval(a: $0.element * CGFloat($0.offset), b: $0.element * CGFloat($0.offset + 1)) }
    }
}

Great. I am wondering where is scaleImage method?