Swift- Change font on an HTML string that has its own Styles

Swift 4 solution

---

• NSAttributedString extension with convenience initializer
• Enumerates through the attributed string (HTML document) font attributes, and replaces with the provided UIFont
• Preserves original HTML font sizes, or uses font-size from provided UIFont, @see useDocumentFontSize parameter
• This method can simply convert HTML to NSAttributedString, without the overload of manipulating with fonts, just skip the font parameter, @see guard statement

---

extension NSAttributedString {

    convenience init(htmlString html: String, font: UIFont? = nil, useDocumentFontSize: Bool = true) throws {
        let options: [NSAttributedString.DocumentReadingOptionKey : Any] = [
            .documentType: NSAttributedString.DocumentType.html,
            .characterEncoding: String.Encoding.utf8.rawValue
        ]

        let data = html.data(using: .utf8, allowLossyConversion: true)
        guard (data != nil), let fontFamily = font?.familyName, let attr = try? NSMutableAttributedString(data: data!, options: options, documentAttributes: nil) else {
            try self.init(data: data ?? Data(html.utf8), options: options, documentAttributes: nil)
            return
        }

        let fontSize: CGFloat? = useDocumentFontSize ? nil : font!.pointSize
        let range = NSRange(location: 0, length: attr.length)
        attr.enumerateAttribute(.font, in: range, options: .longestEffectiveRangeNotRequired) { attrib, range, _ in
            if let htmlFont = attrib as? UIFont {
                let traits = htmlFont.fontDescriptor.symbolicTraits
                var descrip = htmlFont.fontDescriptor.withFamily(fontFamily)

                if (traits.rawValue & UIFontDescriptorSymbolicTraits.traitBold.rawValue) != 0 {
                    descrip = descrip.withSymbolicTraits(.traitBold)!
                }

                if (traits.rawValue & UIFontDescriptorSymbolicTraits.traitItalic.rawValue) != 0 {
                    descrip = descrip.withSymbolicTraits(.traitItalic)!
                }

                attr.addAttribute(.font, value: UIFont(descriptor: descrip, size: fontSize ?? htmlFont.pointSize), range: range)
            }
        }

        self.init(attributedString: attr)
    }

}

Usage-1 (Replace font)

let attr = try? NSAttributedString(htmlString: "<strong>Hello</strong> World!", font: UIFont.systemFont(ofSize: 34, weight: .thin))

Usage-2 (NSMutableAttributedString example)

let attr = try! NSMutableAttributedString(htmlString: "<strong>Hello</strong> World!", font: UIFont.systemFont(ofSize: 34, weight: .thin))
attr.append(NSAttributedString(string: " MINIMIZE", attributes: [.link: "@m"]))

Usage-3 (Only convert HTML to NSAttributedString)

let attr = try? NSAttributedString(htmlString: "<strong>Hello</strong> World!")