Make the first line of a UILabel (any multi line text really) bold

MakeFirstLineBold.swift

import UIKit
import PlaygroundSupport

extension UILabel {

    /// Makes the first line (of a multiline label) bold.
    /// In case the label is a "single line" one, the function does nothing.
    /// In case there is no bold version of the current font, the function does nothing.
    public func makeFirstLineBold() {

        // Current text.
        guard let text = text else { return }

        // First line.
        let components = text.components(separatedBy: .newlines)
        guard components.count > 1, let firstLine = components.first else { return }

        // Bold version of the current font.
        guard let boldFont = font.bold() else { return }

        // Attributed bold.
        let boldFontAttribute: [NSAttributedStringKey: Any] = [.font: boldFont]
        let boldRange = (text as NSString).range(of: firstLine)

        // Attributed text.
        let attributedText = NSMutableAttributedString(string: text, attributes: [.font: font])
        attributedText.addAttributes(boldFontAttribute, range: boldRange)

        // Updated label's attributed text.
        self.attributedText = attributedText
    }
}

extension UIFont {
    /// Returns the bold version of the `self`. Or `nil` if it could not be found.
    func bold() -> UIFont? {
        let fontDescriptorSymbolicTraits: UIFontDescriptorSymbolicTraits = [fontDescriptor.symbolicTraits, .traitBold]
        let bondFontDescriptor = fontDescriptor.withSymbolicTraits(fontDescriptorSymbolicTraits)
        return bondFontDescriptor.flatMap { UIFont(descriptor: $0, size: pointSize) }
    }
}

class LabelViewController : UIViewController {

    override func loadView() {

        let text = """
You’ll get your last delivery on Mon, Mar 26

Thanks for trying us out! We'd love to hear what you think about your experience.
"""

        let view = UIView()
        view.backgroundColor = .white

        let label = UILabel()
        label.text = text
        label.textAlignment = .center
        label.numberOfLines = 0
        label.makeFirstLineBold()

        view.addSubview(label)

        // Layout

        label.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            label.topAnchor.constraint(equalTo: view.topAnchor, constant: 0),
            label.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: 0),
            label.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0),
            label.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: 0)
        ])
        self.view = view
    }
}

PlaygroundPage.current.liveView = LabelViewController()