Blog post / Dynamic Type - Shows how to listen to content size category changes above and under iOS 10

ViewController.swift

private func adjustForContentSizeCategoryChanges() {
    if #available(iOS 10.0, *) {
        label.adjustsFontForContentSizeCategory = true
    } else {
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(self.didChangeContentSizeCategory),
                                               name: UIContentSizeCategory.didChangeNotification,
                                               object: nil)
    }
}

@objc func didChangeContentSizeCategory() {
    label.font = UIFont.preferredFont(forTextStyle: .body)
}