NSAttributedString+HTML.swift

extension NSAttributedString {
    convenience init?(htmlString: String) {
        guard let data = htmlString.data(using: .unicode) else { return nil }
        let options: [NSMutableAttributedString.DocumentReadingOptionKey: Any] = [.documentType: NSAttributedString.DocumentType.html]
        do {
            try self.init(data: data, options: options, documentAttributes: nil)
        } catch {
            return nil
        }
    }

    func attributedString(byKeepingAttribute shouldKeepAttribute: ((NSAttributedString.Key, Any, NSRange) -> Bool)) -> NSAttributedString {
        let mas = NSMutableAttributedString(string: string)
        enumerateAttributes(in: NSRange(location: 0, length: length), options: []) { attributes, range, _ in
            for (key, value) in attributes {
                if shouldKeepAttribute(key, value, range) {
                    mas.addAttribute(key, value: value, range: range)
                }
            }
        }
        return NSAttributedString(attributedString: mas)
    }
}
let text = #"Hello, <a href="http://www.example.com">World</a>.<br />こんにちは、<a href="http://www.example.com">世界</a>。"#
let at = NSAttributedString(htmlString: text)?.attributedString { k, _, _ in k == .link }