Get the UIFont.TextStyle that a font was created with

UIFont+TextStyle.swift

// Fonts can be created with UIFont.preferredFont(forTextStyle:), but there's
// no direct way to find out what textStyle a font was created with.
//
// Requires: Swift 4.2, iOS 11
extension UIFont {
  // Can be nil if the font was created using another method other than UIFont.preferredFont(forTextStyle:)
  var textStyle: TextStyle? {
    guard let styleName = fontDescriptor.fontAttributes[.textStyle] as? String else {
      return nil
    }

    let textStyle = TextStyle(rawValue: styleName)
    return TextStyle.allCases.contains(textStyle) ? textStyle : nil
  }
}

extension UIFont.TextStyle: CaseIterable {
  public static var allCases: [UIFont.TextStyle] {
    return [
      .largeTitle,
      .title1,
      .title2,
      .title3,
      .headline,
      .subheadline,
      .body,
      .callout,
      .footnote,
      .caption1,
      .caption2
    ]
  }
}