text_formatter.dart

class StrictRangeFilteringTextInputFormatter extends FilteringTextInputFormatter {
  final double min;
  final double max;
  final bool allowNegative;
  final bool unlimitedDecimals;
  final int? decimalPlaces;

  StrictRangeFilteringTextInputFormatter({
    required this.min,
    required this.max,
    this.allowNegative = false,
    this.unlimitedDecimals = false,
    this.decimalPlaces,
  }) : super.allow(_buildRegExp(allowNegative, decimalPlaces, unlimitedDecimals));

  static RegExp _buildRegExp(bool allowNegative, int? decimalPlaces, bool unlimitedDecimals) {
    final sign = allowNegative ? '-?' : '';
    final decimal = unlimitedDecimals
        ? '(\\.\\d*)?'
        : (decimalPlaces != null && decimalPlaces > 0
            ? '(\\.\\d{0,$decimalPlaces})?'
            : '');
    final pattern = '^$sign\\d*$decimal\$';
    return RegExp(pattern);
  }

  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
    final filtered = super.formatEditUpdate(oldValue, newValue);
    final text = filtered.text;

    // Allow partial input like "-", ".", "-."
    if (text.isEmpty || text == '-' || text == '.' || text == '-.') {
      return filtered;
    }

    final number = double.tryParse(text);
    if (number == null || number < min || number > max) {
      return oldValue;
    }

    return filtered;
  }
}