Some extension functions of UILabel that helps to format the text of the label

UILabel+Utils.swift

//
//  UILabel+Utils.swift
//
//  Created by Jesus Martin Alonso on 21/11/2018.
//

import Foundation
import UIKit

extension UILabel {

    /// Set line spacing of attributedText keeping the previous attributes settings
    ///
    /// - Parameter lineSpacing: CGFloat with the desired line spacing
    func setLineSpacing(_ lineSpacing: CGFloat) {

        guard let labelText = text, labelText.count > 0 else {
            return
        }

        var attributes = attributedText?.attributes(at: 0, effectiveRange: nil) ?? [:]

        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.lineSpacing = lineSpacing // Line Spacing
        paragraphStyle.alignment = .center

        attributes[.paragraphStyle] = paragraphStyle

        attributedText = NSAttributedString(string: labelText, attributes: attributes)
    }

    /// Underline text of  attributedText keeping the previous attributes settings
    func underline() {

        guard let labelText = text, labelText.count > 0 else {
            return
        }

        var attributes = attributedText?.attributes(at: 0, effectiveRange: nil) ?? [:]

        attributes[.underlineStyle] = NSUnderlineStyle.single.rawValue

        attributedText =   NSAttributedString(string: labelText,
                                              attributes: attributes)

    }

    /// Set letter spacing of attributedText keeping the previous attributes settings
    ///
    /// - Parameter kernValue: Double with the kern value of the desired letter spacing
    func setLetterSpacing(kernValue: Double) {

        guard let labelText = text, labelText.count > 0 else {
            return
        }

        var attributes = attributedText?.attributes(at: 0, effectiveRange: nil) ?? [:]

        attributes[.kern] = kernValue

        attributedText =   NSAttributedString(string: labelText,
                                              attributes: attributes)

    }

    /// Adjust the font size of the label to fit the text in the frame of the label
    func adjustFontToFitText() {

        guard let text = text else {
            print("Text is nil")
            return
        }
        self.adjustsFontSizeToFitWidth = false
        self.font = maxFontToFitStringInFrame(font: font, string: text, frame: self.frame)
    }

    /// Override localize function to localize text of the label
    public override func localize() {
        super.localize()
        localize(text) { text = $0 }
    }

    /// Function that returns the font with its max size to fit a given string in a given width
    ///
    /// The function reduces the given font size until the given string fits in the given width
    /// - Parameters:
    ///   - font: The font of the string.
    ///   - string: The string to fit in the frame
    ///   - frame: The frame
    /// - Returns: The new UIFont that fits the given width with the given string
    fileprivate func maxFontToFitStringInFrame(font: UIFont, string: String, frame: CGRect) -> UIFont {

        var newFont = font
        print("text of label: \(string)")
        print("Original font: \(font)")
        let myString: NSString = string as NSString
        var widthOfText = myString.size(withAttributes: [NSAttributedString.Key.font: newFont]).width
        var heightOfText = myString.size(withAttributes: [NSAttributedString.Key.font: newFont]).height
        while frame.width < widthOfText ||  frame.height < heightOfText {
            let fontSize = newFont.pointSize
            newFont = newFont.withSize(fontSize - 0.5)
            widthOfText = myString.size(withAttributes: [NSAttributedString.Key.font: newFont]).width
            heightOfText = myString.size(withAttributes: [NSAttributedString.Key.font: newFont]).height
        }
        print("New font: \(newFont)")

        return newFont
    }
}