A protocol for view controllers with a replaceable content view controller

ContentViewEmbedding.swift

import UIKit

/// For a view controller that has no content of its own, implement like so:
///
///     extension RootViewController: ContentViewEmbedding {
///       var contentView: UIView! {
///         return view
///       }
///     }
protocol ContentViewEmbedding {
    var contentView: UIView! { get }
}

extension ContentViewEmbedding where Self: UIViewController {
    func removeContentViewController() {
        guard let vc = childViewControllers.first(where: { $0.view.isDescendant(of: contentView) }) else { return }
        vc.willMove(toParentViewController: nil)
        vc.view.removeFromSuperview()
        vc.removeFromParentViewController()
    }

    func replaceContentViewController(with vc: UIViewController, animated: Bool) {
        let duration = animated ? 0.2 : 0

        if let currentChild = childViewControllers.first {
            currentChild.willMove(toParentViewController: nil)
            addChildViewController(vc)
            contentView.embedSubview(vc.view)

            UIView.transition(
                with: contentView,
                duration: duration,
                options: [.transitionCrossDissolve],
                animations: setNeedsStatusBarAppearanceUpdate,
                completion: { _ in
                    currentChild.view.removeFromSuperview()
                    currentChild.removeFromParentViewController()
                    vc.didMove(toParentViewController: self)
                }
            )
        } else {
            addChildViewController(vc)
            contentView.embedSubview(vc.view)
            vc.didMove(toParentViewController: self)
        }
    }
}

extension UIViewController {
    @available(*, unavailable, message: "Please conform to 'ContentViewEmbedding'.")
    func removeContentViewController() {}

    @available(*, unavailable, message: "Please conform to 'ContentViewEmbedding'.")
    func replaceContentViewController(with _: UIViewController, animated _: Bool) {}
}