ChildParentAutoLayout.swift

import UIKit

class ViewController: UIViewController {
    private(set) var bottomBarLayoutGuide = UILayoutGuide()

    private var redBox: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addLayoutGuide(bottomBarLayoutGuide)

        // 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),
            bottomBarLayoutGuide.bottomAnchor.constraint(equalTo: redBox.topAnchor)
        ])

        let childViewController = ViewControllerB()
        childViewController.view.frame = view.bounds
        addChild(childViewController)
        view.addSubview(childViewController.view)
        childViewController.didMove(toParent: self)

        view.backgroundColor = .systemBackground
    }
}

class ViewControllerB: UIViewController {

    init() {
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) { fatalError("\(#file) does not implement coder.") }

    private lazy var greenBox: UIView = {
        let greenBox = UIView()
        greenBox.backgroundColor = .systemGreen
        greenBox.translatesAutoresizingMaskIntoConstraints = false
        return greenBox
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .clear

        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)
        ])
    }

    private var greenBoxBottomConstraint: NSLayoutConstraint?

    override func didMove(toParent parent: UIViewController?) {
        super.didMove(toParent: parent)
        resetGreenBoxBottomConstraint(parent: parent)
    }

    func resetGreenBoxBottomConstraint(parent: UIViewController?) {
        greenBoxBottomConstraint?.isActive = false

        if let layoutGuide = (parent as? ViewController)?.bottomBarLayoutGuide {
            greenBoxBottomConstraint = greenBox.bottomAnchor.constraint(equalTo: layoutGuide.topAnchor)
        } else {
            greenBoxBottomConstraint = greenBox.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor)
        }

        greenBoxBottomConstraint?.isActive = true
    }
}