Squares.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()
     
        // Trying to migrate some old frame based code to Auto Layout
        redBox = UIView()
        redBox.backgroundColor = .systemRed
        redBox.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(redBox)
        
        NSLayoutConstraint.activate([
            redBox.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50.0),
            redBox.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50.0),
            redBox.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
            redBox.heightAnchor.constraint(equalToConstant: 50.0)
        ])
        
        let vcB = ViewControllerB()
        setViewControllers([vcB], direction: .forward, animated: false)
        
//        NSLayoutConstraint.activate([
//            vcB.greenBox.bottomAnchor.constraint(equalTo: redBox.topAnchor)
//        ])
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        return ViewControllerB()
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        return ViewControllerB()
    }
}

class ViewControllerB: UIViewController {
    var greenBox: UIView!
    
    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
        greenBox.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(greenBox)
        
        NSLayoutConstraint.activate([
            greenBox.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 50.0),
            greenBox.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -50.0),
            greenBox.heightAnchor.constraint(equalToConstant: 50.0)
        ])
    }
}