ContentView.swift

import Foundation
import UIKit

final class ContentView: UIView {
    private lazy var stackView: UIStackView = {
        let stackView = UIStackView()
        stackView.spacing = 16.0
        stackView.distribution = .equalSpacing
        stackView.translatesAutoresizingMaskIntoConstraints = false
        return stackView
    }()
    
    private lazy var label: UILabel = {
        let label = UILabel()
        label.text = "Lorem ipsum dolor"
        label.font = UIFont.preferredFont(forTextStyle: .body)
        label.adjustsFontForContentSizeCategory = true
        return label
    }()
    
    private lazy var button: UIButton = {
        let button = UIButton(type: .system)
        button.setTitle("Action", for: .normal)
        button.titleLabel?.font = UIFont.preferredFont(forTextStyle: .body)
        button.titleLabel?.adjustsFontForContentSizeCategory = true
        return button
    }()
    
    init() {
        super.init(frame: .zero)
        
        stackView.addArrangedSubview(label)
        stackView.addArrangedSubview(button)
        addSubview(stackView)
        
        stackView.pinEdges(to: self)
    }
    
    @available(*, unavailable)
    required init(coder _: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        updateStackViewAxis()
    }
    
    private func updateStackViewAxis() {
        let maxWidth = bounds.width
        let labelWidth = label.sizeThatFits(frame.size).width
        let buttonWidth = button.sizeThatFits(frame.size).width
        let spacing = stackView.spacing

        let isHorizontal = maxWidth > labelWidth + buttonWidth + spacing

        stackView.axis = isHorizontal ? .horizontal : .vertical
        stackView.alignment = isHorizontal ? .firstBaseline : .leading
        stackView.invalidateIntrinsicContentSize()
    }
}

private extension UIView {
    func pinEdges(to other: UIView) {
        leadingAnchor.constraint(equalTo: other.leadingAnchor).isActive = true
        trailingAnchor.constraint(equalTo: other.trailingAnchor).isActive = true
        topAnchor.constraint(equalTo: other.topAnchor).isActive = true
        bottomAnchor.constraint(equalTo: other.bottomAnchor).isActive = true
    }
}

ViewController.swift

import UIKit

class ViewController: UIViewController {
    private let contentView = ContentView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        contentView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(contentView)
        
        contentView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
        contentView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 16.0).isActive = true
        view.trailingAnchor.constraint(equalTo: contentView.trailingAnchor, constant: 16.0).isActive = true
    }
}