问如何在Swift 4中更改文本字段中的文本时在每2个字符后添加分隔符

EN

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let text = (textField.text! as NSString).replacingCharacters(in: range, with: string)
    let count = text.count
    if string != "" {
    if count > 17
    {
        return false
    }
    if count % 3 == 0{
        txtField.text?.insert(":", at: String.Index.init(encodedOffset: count - 1))
    }
    return true
  }
  return true
}

class MacAddressField: UITextField {
    override func didMoveToSuperview() {
        keyboardType = .default
        textAlignment = .left
        autocorrectionType = .no
        autocapitalizationType = .none
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
    }
    @objc func editingChanged(_ textField: UITextField) {
        text = text!
            .filter(\.isHexDigit)
            .prefix(12)
            .pairs
            .joined(separator: ":")
            .lowercased()
    }
}

extension Collection {
    func subSequences(limitedTo maxLength: Int) -> [SubSequence] {
        precondition(maxLength > 0, "groups must be greater than zero")
        return .init(sequence(state: startIndex) { start in
            guard start < self.endIndex else { return nil }
            let end = self.index(start, offsetBy: maxLength, limitedBy: self.endIndex) ?? self.endIndex
            defer { start = end }
            return self[start..<end]
        })
    }
    var pairs: [SubSequence] { subSequences(limitedTo: 2) }
}

func textField(_ textField: UITextField,  
               shouldChangeCharactersIn range: NSRange,
               replacementString string: String) -> Bool