Using iOS FontStyles with different weights and scales

font.swift

import UIKit
import PlaygroundSupport

extension UIFont {

  static func font(for style: UIFontTextStyle,
                   atScale scale: CGFloat = 1,
                   usingFont fontInSize: ((CGFloat) -> UIFont)? = nil) -> UIFont {
    switch (fontInSize, scale) {
      // in case we provide a way to deliver a different UIFont for a size
    // we calculate the preferredFont
    case let (fontInSize?, scale):
      let size = UIFont.preferredFont(forTextStyle: style).pointSize * scale
      return fontInSize(size)
    // scale only
    case let (_, scale) where scale != 1:
      let size = UIFont.preferredFont(forTextStyle: style).pointSize * scale
      return UIFont.systemFont(ofSize: size)
    // as default we use preferredFont
    default:
      return UIFont.preferredFont(forTextStyle: style)

    }
  }
}

protocol StyleableText: class {
  func styleFont(_ font: UIFont)
}

extension StyleableText {
  /// Shortcut for setting the UIFont in UIFontTextStyle.
  /// When providing a `usingFont` Closure, the font can be changed.
  ///
  ///     // setting default
  ///     label.setTextStyle(.title1)
  ///
  ///     // setting scale and font
  ///     label.setTextStyle(.title1, atScale: 1.5, usingFont: UIFont.boldSystemFont)
  ///
  /// - parameter style: UIFontTextStyle
  /// - parameter atScale: CGFloat, default is 1
  /// - parameter usingFont: ((CGFloat) -> UIFont)?, default is nil
  /// - returns: Void
  func setTextStyle(_ style: UIFontTextStyle,
                    atScale scale: CGFloat = 1,
                    usingFont fontInSize: ((CGFloat) -> UIFont)? = nil) {
    styleFont(UIFont.font(for: style, atScale: scale, usingFont: fontInSize))
  }

}

extension UILabel: StyleableText {
  func styleFont(_ font: UIFont) {
    self.font = font
  }
}

extension UITextView: StyleableText {
  func styleFont(_ font: UIFont) {
    self.font = font
  }
}


let label = UILabel(frame: CGRect(x: 0, y: 0, width: 100, height: 30))
label.setTextStyle(.title1, atScale: 1, usingFont: UIFont.boldSystemFont)
label.text = "Test"
label.backgroundColor = .white
PlaygroundPage.current.liveView = label