How to display non-slashed zero character in Swift for the system font SF Mono

The system font SF Mono has two variations for the number zero (0) and number one (1).

By default, a slashed zero character is displayed for text with system font SF Mono.

In the MacOS Font Book, both a slashed zero and non-slashed zero have the code U+0030.

How can I specify a non-slashed zero on a UIButton title in Swift?

Thanks!

Example of a non-slashed zero character:

This answer uses the Stylistic Alternatives feature in CoreText.

Source:

This link describes the set of currently defined typographic and layout features available from applications using CoreText.

https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html#Type35

Try this code:

import UIKit

class ViewController: UIViewController {
    
    // Set the status bar of the view to hidden
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        self.setNeedsStatusBarAppearanceUpdate()
    }
    override var prefersStatusBarHidden : Bool {
        return true
    }
    override var childForStatusBarHidden : UIViewController? {
        return nil
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Set the background color of the view to systemGray5
        view.backgroundColor = .systemGray5

        // Create a UIButton
        let button = UIButton(type: .system)

        // Set the title of the button to "100"
        button.setTitle("100", for: .normal)

        // Set the font of the button title to SFMono with a size of 150
        button.titleLabel?.font = UIFont.monospacedSystemFont(ofSize: 150, weight: .regular)

        // Set the background color of the button to systemYellow
        button.backgroundColor = .systemYellow

        // Set the tint color of the button to black
        button.tintColor = .black

        // Make the button rounded by setting its corner radius to 25
        button.layer.cornerRadius = 25

        // Ensure that the rounded corners are displayed correctly
        button.clipsToBounds = true

        // Define the font descriptor for the custom font
        let fontDescriptor = UIFontDescriptor.preferredFontDescriptor(withTextStyle: .body).withDesign(.monospaced)?.addingAttributes([
            .featureSettings: [
                
                // Set the stylistic alternative for "0"
                [
                UIFontDescriptor.FeatureKey.type: kStylisticAlternativesType,
                UIFontDescriptor.FeatureKey.selector: kStylisticAltThreeOnSelector
                ],
                
                // Set the stylistic alternative for "1"
                [
                UIFontDescriptor.FeatureKey.type: kStylisticAlternativesType,
                UIFontDescriptor.FeatureKey.selector: kStylisticAltFourOnSelector
            ]
                
            ]
        ])

        // Create the custom font
        let font = UIFont(descriptor: fontDescriptor!, size: 150)

        // Create an attributed string with the custom font
        let attributedString = NSAttributedString(string: "100", attributes: [.font: font])

        // Apply the attributed string to the button
        button.setAttributedTitle(attributedString, for: .normal)

        // Add the button to the view
        view.addSubview(button)

        // Center the button in the view
        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
    }
}

Running the code displaying stylistic alternatives for "0" and "1":