TextStyle.swift

import UIKit

public struct TextStyle {
    let font: UIFont
    let color: UIColor
    let kerning: CGFloat?
    let lineHeight: CGFloat?
    
}

extension TextStyle {
    init(fontName: String, pointSize: CGFloat, color: UIColor,
         kerning: CGFloat, lineHeight: CGFloat) {
        let fontDescriptor = UIFontDescriptor(name: fontName, size: pointSize)
        let font = UIFont(descriptor: fontDescriptor, size: pointSize)
        self.init(font: font, color: color, kerning: kerning, lineHeight: lineHeight)
    }

    init(font: UIFont, color: UIColor) {
        self.init(font: font, color: color, kerning: nil, lineHeight: nil)
    }
}


// Then concrete TextStyles could be described by extension like fonts/colors.

extension TextStyle {
    static let visibleButton = TextStyle(font: .visibleButton, color: .orangePink)
}


// TextStyle could be used for NSMutableString configuration

extension NSAttributedString {

    private static func attributes(for textStyle: TextStyle) -> [NSAttributedStringKey: Any] {
        var attributes: [NSAttributedStringKey: Any] = [
            .font: textStyle.font,
            .foregroundColor: textStyle.color,
        ]

        if let lineHeight = textStyle.lineHeight {
            let paragraphStyle = NSMutableParagraphStyle()
            paragraphStyle.maximumLineHeight = lineHeight
            attributes[.paragraphStyle] = paragraphStyle
        }

        if let kerning = textStyle.kerning {
            attributes[.kern] = kerning
        }

        return attributes
    }
    
    public convenience init(string: String, textStyle: TextStyle) {
        let attributes = NSAttributedString.attributes(for: textStyle)
        self.init(string: string, attributes: attributes)
    }
}



// TextStyle could be applied directly to text-related entities

protocol TextStyleApplicable {
    func applyPlainStyle(_ textStyle: TextStyle)
}

extension UILabel: TextStyleApplicable {
    func applyPlainStyle(_ textStyle: TextStyle) {
        font = textStyle.font
        textColor = textStyle.color
    }
}

extension UITextField: TextStyleApplicable {
    func applyPlainStyle(_ textStyle: TextStyle) {
        font = textStyle.font
        textColor = textStyle.color
    }
}

extension UITextView: TextStyleApplicable {
    func applyPlainStyle(_ textStyle: TextStyle) {
        font = textStyle.font
        textColor = textStyle.color
    }
}