Dynamic Type: Scaling Custom Fonts

.swiftlint.yml

# https://github.com/realm/SwiftLint#defining-custom-rules
custom_rules:
  use_scaled_fonts:
    name: "Use scaled fonts: UIFont.scaledFont(style:)"
    regex: '\.(?!scaled)[a-zA-Z]*[fF]ont\('
    severity: error

TextStyle.swift

struct TextStyle {
	let size: CGFloat
	let emphasis: Emphasis
	let name: Name
	
	init(size: CGFloat, emphasis: Emphasis = .none, name: Name = .ptSans) {
		self.size = size
		self.emphasis = emphasis
		self.name = name
	}
}

extension TextStyle {
	typealias Name = String

	enum Emphasis {
		case none
		case bold
		case italic

		var symbolicTraits: UIFontDescriptor.SymbolicTraits? {
			switch self {
			case .none:
				return nil
			case .bold:
				return .traitBold
			case .italic:
				return .traitItalic
			}
		}
	}
}

extension TextStyle.Name {
	static let montserrat: TextStyle.Name = "Montserrat-Regular"
	static let ptSans: TextStyle.Name = "PTSans-Regular"
}

extension TextStyle {
	// MARK: - Headings
	
	static let h1 = TextStyle(size: 24, emphasis: .bold, name: .montserrat)
	static let h2 = TextStyle(size: 20, emphasis: .bold, name: .montserrat)
	static let h3 = TextStyle(size: 18, emphasis: .bold, name: .montserrat)
	static let h4 = TextStyle(size: 16, emphasis: .bold, name: .montserrat)
	static let h5 = TextStyle(size: 14, emphasis: .bold, name: .montserrat)
	static let h6 = TextStyle(size: 13, emphasis: .bold, name: .montserrat)

	// MARK: - Text buttons
	static func buttonLarge(emphasis: Emphasis) -> TextStyle {
		TextStyle(size: 16, emphasis: emphasis)
	}

	static func buttonSmall(emphasis: Emphasis) -> TextStyle {
		TextStyle(size: 14, emphasis: emphasis)
	}

	// MARK: - Texts
	static let text18 = TextStyle(size: 18)
	static let text16 = TextStyle(size: 16)
	static let text13 = TextStyle(size: 13)
	static let text14 = TextStyle(size: 14)

	// MARK: - Text highlights
	static let textHighlight24 = TextStyle(size: 24, emphasis: .bold)
	static let textHighlight20 = TextStyle(size: 20, emphasis: .bold)
}

TextStyleAdjustable.swift

protocol TextStyleAdjustable: UIContentSizeCategoryAdjusting {
	func setFont(_ font: UIFont)
	func apply(textStyle: TextStyle) -> Self
}

extension TextStyleAdjustable {
	@discardableResult
	func apply(textStyle: TextStyle) -> Self {
		setFont(.scaledFont(style: textStyle))
		adjustsFontForContentSizeCategory = true

		return self
	}
}

extension UILabel: TextStyleAdjustable {
	func setFont(_ font: UIFont) {
		self.font = font
	}
}

extension UITextView: TextStyleAdjustable {
	func setFont(_ font: UIFont) {
		self.font = font
	}
}

extension UITextField: TextStyleAdjustable {
	func setFont(_ font: UIFont) {
		self.font = font
	}
}

extension UIButton: TextStyleAdjustable {
	func setFont(_ font: UIFont) {
		titleLabel?.font = font
	}

	public var adjustsFontForContentSizeCategory: Bool {
		get {
			titleLabel?.adjustsFontForContentSizeCategory ?? false
		}
		set {
			titleLabel?.adjustsFontForContentSizeCategory = newValue
		}
	}
}

UIFont+Extensions.swift

extension UIFont {
	/// Returns a font matching `style`
	static func font(style: <ModuleName>.TextStyle) -> UIFont {
		var descriptor = UIFontDescriptor(name: style.name, size: style.size)

		if let symbolicTraits = style.emphasis.symbolicTraits {
			descriptor = descriptor.withSymbolicTraits(symbolicTraits)!
		}

		return UIFont(descriptor: descriptor, size: style.size)
	}

	/// Returns a font matching `style` scaled to the current Content Size Category.
	static func scaledFont(style: <ModuleName>.TextStyle) -> UIFont {
		UIFontMetrics.default.scaledFont(for: font(style: style))
	}
}

UsageExample.swift

label.apply(textStyle: .h1)
textField.apply(textStyle: .text16)
textView.apply(textStyle: .text16)
button.apply(textStyle: .buttonLarge(emphasis: .bold))

// or

label.font = .scaledFont(style: .h1)
label.adjustsFontForContentSizeCategory = true

textField.font = .scaledFont(style: .text16)
textField.adjustsFontForContentSizeCategory = true

textView.font = .scaledFont(style: .text16)
textView.adjustsFontForContentSizeCategory = true

button.titleLabel?.font = .scaledFont(style: .buttonLarge(emphasis: .bold))
button.titleLabel?.adjustsFontForContentSizeCategory = true