iOS Updating Auto Layout with Trait Collection Changes


Making an app adaptive – that is, responding to size class changes by changing your layout – often involves lots of help from the Auto Layout system. One of the primary ways apps become adaptive is by updating the active Auto Layout constraints when a view's size class changes.

For example, consider an app that uses a UIStackView to arrange two UILabels. We might want these labels to stack on top of each other in horizontally compact environments, but sit next to each other when we have a little more room in horizontally regular environments.

class ViewController: UIViewController {
    var stackView: UIStackView!
    override func viewDidLoad() {
        stackView = UIStackView()
        for text in ["foo", "bar"] {
            let label = UILabel()
            label.translatesAutoresizingMaskIntoConstraints = false
            label.text = text
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    override func viewDidAppear(_ animated: Bool) {
        updateAxis(forTraitCollection: traitCollection)
    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        updateAxis(forTraitCollection: traitCollection)
    private func updateAxis(forTraitCollection traitCollection: UITraitCollection) {
        switch traitCollection.horizontalSizeClass {
        case .regular:
            stackView.axis = .horizontal
        case .compact:
            stackView.axis = .vertical
        case .unspecified:
            print("Unspecified size class!")
            stackView.axis = .horizontal