UnderlineUnicodeTestEnvironment.swift

func generateUnderlineUnicodeText(from attributedString: NSAttributedString, by key: NSAttributedString.Key) -> String {
    
    var keyRanges = [NSRange]()

    // Find all attributes by difference (key) and take ranges
    attributedString.enumerateAttributes(in: NSRange(location: 0, length: attributedString.length)) { (attributes, range, _) in
        attributes.forEach { (key, value) in
            switch key {
            case key: keyRanges.append(range)
            default: print("Unknown attribute found in the attributed string")}
        }
    }

    var resultString = attributedString.string
    
    // Replace characters on the same underlined
    keyRanges.forEach { range in
        let stringForMutation = attributedString.attributedSubstring(from: range).string
        guard let underlineUnicodeText = stringForMutation.underlineUnicode, stringForMutation.count == underlineUnicodeText.count else {
            fatalError("Error - underlineUnicodeText, unowned symbols or length not same")
        }

        // The reason is that the range points of underline unicode text stored as a UTF-8 surrogate pair.
        // We have to count the additional characters for the right range definition.
        let extraSymbolsCount = (resultString.utf8.count - attributedString.string.utf8.count) / 2
        
        let range = NSRange(location: range.location + extraSymbolsCount, length: range.length)
        guard let swiftRange = Range(range, in: resultString) else {
            fatalError("Error - Range conflict")
        }
        
        resultString.replaceSubrange(swiftRange, with: underlineUnicodeText)
    }

    return resultString
}