public enum TextStyle {

  case Title1
  case Title2
  case Headline
  case Subheadline
  case Body
  case Caption1
  case Caption2
  case Footnote
  case Callout
}

import TextStyle

class ViewController: UIViewController {
  let label = UILabel()

  private func viewDidLoad() {
    super.viewDidLoad()
    label.font = TextStyle.Body.font
    TextStyle.observeFontChanged(self, selector: #selector(ViewController.fontChanged))
  }

  private func fontChanged() {
    label.font = TextStyle.Body.font
  }

  deinit {
    TextStyle.removeObserver(self)
  }
}

import TextStyle
import RxSwift

class ViewController: UIViewController {
  let label = UILabel()
  let disposeBag = DisposeBag()

  private func viewDidLoad() {
    super.viewDidLoad()
    TextStyle.Body.rx_font
      .bindTo(label.rx_font)
      .addDisposableTo(disposeBag)
  }
}