https://stackoverflow.com/questions/1316451/center-content-of-uiscrollview-when-smaller

UIStackView+ContentCentralization.swift

import UIKit.UIScrollView

// MARK: - CONTENT CENTRALIZATION

extension UIScrollView {
    /// Call this method on your `UIScrollView` instance
    /// when you need to center its content
    /// relative to the width and height of the `UIScrollView`.
    ///
    /// The function first attempts to centralize along the Y-axis
    /// if the content height is less than the height of your instance.
    ///
    /// Then, it attempts to centralize along the X-axis
    /// if the content width is less than the width of your instance.
    ///
    /// - Warning: The only content of your `UIScrollView`
    /// should be a `contentView` (`UIView`), inside which
    /// you place all the content.
    
    func centralizeContentIfAvailableSpaceExists() {
        self.centralizeContentByYIfAvailableSpaceExists()
        self.centralizeContentByXIfAvailableSpaceExists()
    }
    
    /// Call this method on your `UIScrollView` instance
    /// when you need to center the content
    /// along the Y-axis (vertically) relative to the height of `UIScrollView`.
    ///
    /// This function is appropriate when three conditions are met:
    ///
    /// **1.** The content height of your instance is less
    /// than the height of your `UIScrollView`.
    ///
    /// **2.** You need to place the content inside your
    /// `UIScrollView` centered along the Y-axis relative to `UIScrollView`.
    ///
    /// **3.** The only content of your `UIScrollView`
    /// is a certain `contentView` (`UIView`).
    
    func centralizeContentByYIfAvailableSpaceExists() {
        guard let subview = self.subviews.first else {
            return
        }
        let subviewHeight = subview.bounds.size.height
        let scrollViewHeight = self.bounds.size.height
        let availableYSpaceForSubview = scrollViewHeight - subviewHeight
        guard availableYSpaceForSubview > .zero else {
            return
        }
        let availableSpaceByTopForSubview = availableYSpaceForSubview / 2
        let offsetByY = scrollViewHeight / 2
        let updatedY = offsetByY + availableSpaceByTopForSubview
        let oldX = self.center.x
        let updatedCenter = CGPoint(x: oldX, y: updatedY)
        self.center = updatedCenter
    }
    
    /// Call this method on your `UIScrollView` instance
    /// when you need to center the content
    /// along the X-axis (horizontally) relative to the width of `UIScrollView`.
    ///
    /// This function is appropriate when three conditions are met:
    ///
    /// **1.** The content width of your instance is less
    /// than the width of your `UIScrollView`.
    ///
    /// **2.** You need to place the content inside your
    /// `UIScrollView` centered along the X-axis relative to `UIScrollView`.
    ///
    /// **3.** The only content of your `UIScrollView`
    /// is a certain `contentView` (`UIView`).
    
    func centralizeContentByXIfAvailableSpaceExists() {
        guard let subview = self.subviews.first else {
            return
        }
        let subviewWidth = subview.bounds.size.width
        let scrollViewWidth = self.bounds.size.width
        let availableXSpaceForSubview = scrollViewWidth - subviewWidth
        guard availableXSpaceForSubview > .zero else {
            return
        }
        let availableSpaceByLeftForSubview = availableXSpaceForSubview / 2
        let offsetByX = scrollViewWidth / 2
        let updatedX = offsetByX + availableSpaceByLeftForSubview
        let oldY = self.center.y
        let updatedCenter = CGPoint(x: updatedX, y: oldY)
        self.center = updatedCenter
    }
}