HighlightedText.swift

extension String {
    func withHighlightedText(highlightedPartsOfString: Array<NSString>, font: UIFont, newFont: UIFont) -> NSAttributedString {
        let nonHighlightedFontAttribute = [NSAttributedString.Key.font:font]
        let highlightedFontAttribute = [NSAttributedString.Key.font:newFont]
        let newText = NSMutableAttributedString(string: self as String, attributes: nonHighlightedFontAttribute)
        for i in 0 ..< highlightedPartsOfString.count {
            newText.addAttributes(highlightedFontAttribute, range: (self as NSString).range(of: highlightedPartsOfString[i] as String))
        }
        return newText
    }
}

//Usage
let boldLabel = UILabel()
let font = UIFont(name: "AvenirNext-Italic", size: 24)!
let boldFont = UIFont(name: "AvenirNext-BoldItalic", size: 24)
boldLabel.attributedText = "testando esse código em negrito".withHighlightedText(
            highlightedPartsOfString: ["código", "negrito"], font: font, newFont: boldFont)