Experimenting altering an iOS app size class to be responsive to the Dynamic Text size.

AdaptiveTraitsContainer.swift

import UIKit

/// A wrapper view controller that makes the horizontal size class
/// be based on both the Dynamic Text size and the width available.
class AdaptiveTraitsContainer: UIViewController {
    let wrappedViewController: UIViewController

    init(wrappedViewController: UIViewController) {
        self.wrappedViewController = wrappedViewController
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        addChild(wrappedViewController)
        view.addSubview(wrappedViewController.view)
        NSLayoutConstraint.activate([
            wrappedViewController.view.topAnchor.constraint(equalTo: view.topAnchor),
            wrappedViewController.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            view.trailingAnchor.constraint(equalTo: wrappedViewController.view.trailingAnchor),
            view.bottomAnchor.constraint(equalTo: wrappedViewController.view.bottomAnchor),
        ])
        wrappedViewController.didMove(toParent: self)
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        NotificationCenter.default.addObserver(self, selector: #selector(updateTraitCollection_), name: UIContentSizeCategory.didChangeNotification, object: nil)
    }

    override func viewDidDisappear(_ animated: Bool) {
        NotificationCenter.default.removeObserver(self, name: UIContentSizeCategory.didChangeNotification, object: nil)

        super.viewDidDisappear(animated)
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)

        updateTraitCollection(withSize: size)
    }

    @objc private func updateTraitCollection_() {
        updateTraitCollection(withSize: view.bounds.size)
    }

    private func updateTraitCollection(withSize size: CGSize) {
        let widthThreshold = 30 * UIFontDescriptor.preferredFontDescriptor(withTextStyle: .body).pointSize

        let sizeClass: UIUserInterfaceSizeClass = size.width < widthThreshold ? .compact : .regular
        setOverrideTraitCollection(UITraitCollection(horizontalSizeClass: sizeClass), forChild: wrappedViewController)
    }
}