inputFormatter.dart

class DoubleTextInputFormatter extends TextInputFormatter {
  final bool positive;

  final double maxValue;

  DoubleTextInputFormatter({
    this.positive = false,
    this.maxValue = double.maxFinite,
  });

  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    return ((newValue.text.isEmpty) ||
            (newValue.text.startsWith(plus) &&
                (newValue.text.length == 1 ||
                    validateDoubleValue(
                          newValue.text,
                          "...",
                          positive: positive,
                          maxValue: maxValue,
                        ) ==
                        null)) ||
            (!positive &&
                newValue.text.startsWith(minus) &&
                (newValue.text.length == 1 ||
                    validateDoubleValue(
                          newValue.text,
                          "...",
                          positive: positive,
                          maxValue: maxValue,
                        ) ==
                        null)) ||
            (validateDoubleValue(
                  newValue.text,
                  "...",
                  positive: positive,
                  maxValue: maxValue,
                ) ==
                null))
        ? newValue
        : oldValue;
  }
}