Some custom font swift extension ideas.

FontExtension.swift

import UIKit

// Its better to create custom font name as constant to avoid spelling mistakes when using this.
extension String {
    static let appCustomFontRegular = "Montserrat"
    static let appCustomFontBold = "Montserrat-Bold"
}

extension UIFont {
    static func appRegularFont(size: CGFloat) -> UIFont {
        return UIFont(name: String.appCustomFontRegular, size: size) ?? .systemFont(ofSize: size)
    }
    static func appBoldFont(size: CGFloat) -> UIFont {
        return UIFont(name: String.appCustomFontBold, size: size) ?? .boldSystemFont(ofSize: size)
    }
}