Extension to apply bold attribute to a text. Seen on [stackoverflow|https://stackoverflow.com/a/37992022/1211664]

EasyAttributeString.swift

extension NSMutableAttributedString {
    @discardableResult func bold(_ text: String) -> NSMutableAttributedString {
        let attrs: [NSAttributedStringKey: Any] = [.font: UIFont(name: "AvenirNext-Medium", size: 12)!]
        let boldString = NSMutableAttributedString(string:text, attributes: attrs)
        append(boldString)

        return self
    }

    @discardableResult func normal(_ text: String) -> NSMutableAttributedString {
        let normal = NSAttributedString(string: text)
        append(normal)

        return self
    }
}

let formattedString = NSMutableAttributedString()
formattedString
    .bold("Bold Text")
    .normal(" Normal Text ")
    .bold("Bold Text")

let lbl = UILabel()
lbl.attributedText = formattedString