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

I am dinamically getting an HTML string from a Wordpress API and parsing it into an Attributed String to show it in my app. Since the string has its own styles, it shows different fonts and sizes, something that is affecting our design choices.

What I want to do is change the font and its size on the whole attributed string.

I tried doing so in the options of the attributed string, but it does nothing:

let attributedT = try! NSAttributedString(
            data: nContent!.decodeHTML().data(using: String.Encoding.unicode, allowLossyConversion: true)!,
            options: [ NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType, NSFontAttributeName: UIFont(name: "Helvetica", size: 16.0)!],
            documentAttributes: nil)
        contentLbl.attributedText = attributedT

Does anybody have any ideas on how to achieve this?

P.S. I know that I could add a CSS tag to the beginning or end of the string, but would this override other styles in it? Also, if this is a valid solution, could you please provide a sample on how to do it?

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!")

What you want to do, basically, is turn the NSAttributedString into an NSMutableAttributedString.

let attributedT = // ... attributed string
let mutableT = NSMutableAttributedString(attributedString:attributedT)

Now you can call addAttributes to apply attributes, such as a different font, over any desired range, such as the whole thing.

Unfortunately, however, a font without a symbolic trait such as italic is a different font from a font with that symbolic trait. Therefore, you're going to need a utility that copies the existing symbolic traits from a font and applies them to another font:

func applyTraitsFromFont(_ f1: UIFont, to f2: UIFont) -> UIFont? {
    let t = f1.fontDescriptor.symbolicTraits
    if let fd = f2.fontDescriptor.withSymbolicTraits(t) {
        return UIFont.init(descriptor: fd, size: 0)
    }
    return nil
}

Okay, so, armed with that utility, let's try it. I'll start with some simple HTML and convert it to an attributed string, just as you are doing:

let html = "<p>Hello <i>world</i>, hello</p>"
let data = html.data(using: .utf8)!
let att = try! NSAttributedString.init(
    data: data, options: [NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType],
    documentAttributes: nil)
let matt = NSMutableAttributedString(attributedString:att)

As you can see, I've converted to an NSMutableAttributedString, as I advised. Now I'll cycle thru the style runs in terms of font, altering to a different font while using my utility to apply the existing traits:

matt.enumerateAttribute(
    NSFontAttributeName,
    in:NSMakeRange(0,matt.length),
    options:.longestEffectiveRangeNotRequired) { value, range, stop in
        let f1 = value as! UIFont
        let f2 = UIFont(name:"Georgia", size:20)!
        if let f3 = applyTraitsFromFont(f1, to:f2) {
            matt.addAttribute(
                NSFontAttributeName, value:f3, range:range)
        }
    }

Here's the result:

Obviously you could tweak this procedure to be even more sophisticated, depending on your design needs.

The setAttributes will reset all the attributes from HTML. I wrote an extension method to avoid this:

Swift 4

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

    guard let data = html.data(using: .utf8, allowLossyConversion: true) else {
        throw NSError(domain: "Parse Error", code: 0, userInfo: nil)
    }

    if let font = font {
        guard let attr = try? NSMutableAttributedString(data: data, options: options, documentAttributes: nil) else {
            throw NSError(domain: "Parse Error", code: 0, userInfo: nil)
        }
        var attrs = attr.attributes(at: 0, effectiveRange: nil)
        attrs[NSAttributedStringKey.font] = font
        attr.setAttributes(attrs, range: NSRange(location: 0, length: attr.length))
        self.init(attributedString: attr)
    } else {
        try? self.init(data: data, options: options, documentAttributes: nil)
    }
}

Test sample:

let html = "<html><body><h1 style=\"color:red;\">html text here</h1></body></html>"
let font = UIFont.systemFont(ofSize: 16)

var attr = try NSMutableAttributedString(HTMLString: html, font: nil)
var attrs = attr?.attributes(at: 0, effectiveRange: nil)
attrs?[NSAttributedStringKey.font] as? UIFont
// print: <UICTFont: 0x7ff19fd0a530> font-family: "TimesNewRomanPS-BoldMT"; font-weight: bold; font-style: normal; font-size: 24.00pt

attr = try NSMutableAttributedString(HTMLString: html, font: font)
attrs = attr?.attributes(at: 0, effectiveRange: nil)
attrs?[NSAttributedStringKey.font] as? UIFont
// print: <UICTFont: 0x7f8c0cc04620> font-family: ".SFUIText"; font-weight: normal; font-style: normal; font-size: 16.00pt