call method
Implementation
String? call(String? text) {
if (text == null) return allowEmpty ? null : message;
String s = text.trim();
if (!allowEmpty && s.isEmpty) return message;
num? v = isReal ? s.toDouble : s.toInt;
if (v == null) return message;
if (minValue != null && v < minValue!) return message;
if (maxValue != null && v > maxValue!) return message;
return null;
}