validate method

Implementation

@override
String? validate(String? input) {
  if (input == null || input.trim().isEmpty) {
    return 'This field is required';
  }

  final num? value = num.tryParse(input);
  if (value == null) return 'Invalid number';

  if (value > max) return 'Value must be at most $max';

  return null;
}