quantity_input_formatter.dart

class QuantityInputFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
    if (newValue.text.isEmpty) {
      return const TextEditingValue(
        text: '0',
        selection: TextSelection.collapsed(offset: 1),
      );
    }

    final validValue = _isInteger(newValue.text) ? _convertToValidIntegerFormat(newValue.text) : _convertToValidFloatFormat(newValue.text);
    return TextEditingValue(
      text: validValue,
      selection: TextSelection.collapsed(offset: validValue.length),
    );
  }

  bool _isInteger(String value) {
    return !value.contains('.') && !value.contains(',');
  }

  String _convertToValidIntegerFormat(String value) {
    if (value.length > 1 && value[0] == '0') {
      return value.replaceFirst('0', '');
    }

    return value;
  }

  String _convertToValidFloatFormat(String value) {
    final tmp = value.replaceAll(',', '.').split('.');
    tmp.insert(1, '.');
    return tmp.join();
  }
}