FrameBasedIceCream.swift

import UIKit

class ViewController: UIPageViewController, UIPageViewControllerDataSource {
    var redBox: UIView!
    
    init() {
        super.init(transitionStyle: .scroll, navigationOrientation: .horizontal)
        
        dataSource = self
    }
    
    required init?(coder aDecoder: NSCoder) { fatalError("\(#file) does not implement coder.") }
    
    override func viewDidLoad() {
        super.viewDidLoad()
     
        redBox = UIView()
        redBox.backgroundColor = .systemRed
        view.addSubview(redBox)
        
        let vcB = ViewControllerB(holderViewController: self)
        setViewControllers([vcB], direction: .forward, animated: false)
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        let sideInset = 50.0
        let height = 50.0
        redBox.frame = CGRect(x: sideInset, y: view.bounds.height - view.safeAreaInsets.bottom - height, width: view.bounds.width - sideInset * 2.0, height: height)
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        return ViewControllerB(holderViewController: self)
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        return ViewControllerB(holderViewController: self)
    }
}

class ViewControllerB: UIViewController {
    var greenBox: UIView!
    
    private(set) weak var holderViewController: ViewController?
    
    init(holderViewController: ViewController) {
        self.holderViewController = holderViewController
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder aDecoder: NSCoder) { fatalError("\(#file) does not implement coder.") }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let randomWhite = CGFloat((0 ... 100).randomElement()!) / 100.0
        view.backgroundColor = UIColor(white: randomWhite, alpha: 1.0)
        
        greenBox = UIView()
        greenBox.backgroundColor = .systemGreen
        view.addSubview(greenBox)
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        let sideInset = 50.0
        let height = 50.0
        greenBox.frame = CGRect(x: sideInset, y: holderViewController!.redBox.frame.origin.y - height, width: view.bounds.width - sideInset * 2.0, height: height)
    }
}