jaguar_dart/jaguar

// order.validator.dart

const nameValidator = Validate.string
  .isNotNull()
  .isNotEmpty(trim: true)
  .startsWithAlpha()
  .hasLengthLessThan(10);

@GenValidator(
  fields: {
    'productName': nameValidator,
    ,
  }
)
class OrderValidator extends Validator<Order>
    with _$OrderValidator {}

// order.validator.jval.dart
abstract class _$OrderValidator implements Validator<Order> {
  final _validators = {
    'productName': nameValidator,
  };

  Validator<ProductAttribute> __productAttributeValidator;
  Validator<ProductAttribute> get _productAttributeValidator =>
      __productAttributeValidator ??= ProductAttributeValidator();
  @override
  List<dynamic> validate(Order model) {
    if (model == null) return null;
    ObjectErrors errors = new ObjectErrors();
    errors.add(validateField(
        'attributes',
        codeIterable(
            model.attributes,
            (val) =>
                _productAttributeValidator.validate(val as ProductAttribute))));
    errors.add(validateField<String>('productName', model.productName));
    errors.add(validateField<int>('quantity', model.quantity));
    errors.add(validateField<double>('unitCost', model.unitCost));
    errors.add(validateField<double>('subtotal', model.subtotal));
    return errors;
  }

  @override
  List<dynamic> validateField<T>(String fieldName, T value) {
    const validator = validators[fieldName];

    if (validator != null) {
      return validator.validate(value);
    }
  }
}

// Usage:
final orderValidator = OrderValidator();

orderValidator.validate(Order order);

orderValidator.validateField('productName', value);