ltr-rtl.swift

import Foundation
import PlaygroundSupport
import UIKit

class VC: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()

    self.view.backgroundColor = .white
    self.view.translatesAutoresizingMaskIntoConstraints = false

    let stackView = UIStackView()
    stackView.distribution = .fillEqually
    stackView.translatesAutoresizingMaskIntoConstraints = false
    stackView.axis = .horizontal

    let label1 = UILabel()
    label1.text = "1"
    label1.textColor = .black
    label1.backgroundColor = .blue
    stackView.addArrangedSubview(label1)

    let label2 = UILabel()
    label2.text = "2"
    label2.backgroundColor = .red
    label2.textColor = .black
    stackView.addArrangedSubview(label2)

    self.view.addSubview(stackView)
    NSLayoutConstraint.activate([
      stackView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor),
      stackView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor),
      stackView.topAnchor.constraint(equalTo: self.view.topAnchor),
      stackView.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),
      ])
  }
}

let child = VC()
let container = UIViewController()

container.view.addSubview(child.view)
container.addChild(child)
container.setOverrideTraitCollection(
  UITraitCollection(layoutDirection: .rightToLeft),
//  UITraitCollection(layoutDirection: .leftToRight),
  forChild: child
)

PlaygroundPage.current.liveView = container