TransformModel example

TransformModel.swift

@objc class TransformModel: NSObject {

    @objc dynamic var text: String? = ""

    func hasOnlyLetters(_ value: String) -> Bool {
        let letters = String(value
                                .unicodeScalars
                                .filter(CharacterSet
                                            .letters
                                            .contains))
        return letters == value
    }

    func reverse() {

        guard let text = self.text else {
            return
        }

        self.text = String(text.reversed())
    }
}