ContainerViewController.swift

import UIKit

class ContainerViewController: UIViewController {
    
    convenience init(contentViewController: UIViewController) {
        self.init()
        self.contentViewController = contentViewController
    }
    
    var contentViewController: UIViewController? {
        didSet {
            if let oldViewController = oldValue {
                hideContent(viewController: oldViewController)
            }
            if let newViewController = contentViewController {
                displayContent(viewController: newViewController)
            }
        }
    }
    
    private func displayContent(viewController: UIViewController) {
        self.addChild(viewController)
        self.view.addSubview(viewController.view)
        viewController.view.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            viewController.view.topAnchor.constraint(equalTo: self.view.topAnchor),
            viewController.view.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),
            viewController.view.leadingAnchor.constraint(equalTo: self.view.leadingAnchor),
            viewController.view.trailingAnchor.constraint(equalTo: self.view.trailingAnchor)
        ])
        viewController.didMove(toParent: self)
    }
    
    private func hideContent(viewController: UIViewController) {
        viewController.willMove(toParent: nil)
        viewController.view.removeFromSuperview()
        viewController.removeFromParent()
    }
}