DecimalTextInputFormatter class

An advanced TextInputFormatter for numeric fields.

Features:

• Decimal support with precision control via decimalRange (null = unlimited, 0 = integers only, n = up to n fraction digits).
• Negative number support via allowNegative.
• Range validation via min / max with live feedback.
• Live error feedback through errorNotifier.
• State management integration through valueNotifier (parsed num).

Character-level rules (precision, sign, single decimal point) are enforced hard — invalid keystrokes are rejected. Range rules are enforced soft — the value is still accepted while typing, but errorNotifier is updated so the UI can show an inline error.

final errorText = ValueNotifier<String?>(null);
final value = ValueNotifier<num?>(null);

TextField(
  keyboardType: const TextInputType.numberWithOptions(
    decimal: true,
    signed: true,
  ),
  inputFormatters: [
    DecimalTextInputFormatter(
      decimalRange: 2,
      allowNegative: true,
      min: -50,
      max: 100,
      errorNotifier: errorText,
      valueNotifier: value,
    ),
  ],
);

// Show live error:
ValueListenableBuilder<String?>(
  valueListenable: errorText,
  builder: (_, error, __) => Text(error ?? ''),
);