implement validate with default validator

Author: ArturAssisComp

lib/new_api_prototype/core_validators/required_validators.dart

@@ -23,6 +23,13 @@ Validator<T?> isRequired<T extends Object>([
   return finalValidator;
 }
 
+/// This function returns a validator that validates the user input with `v`,
+/// replacing null/empty input with `defaultValue`.
+Validator<T?> validateWithDefault<T extends Object>(
+    T defaultValue, Validator<T> v) {
+  return (T? value) => v(value ?? defaultValue);
+}
+
 String errorIsOptionalTemporary(String vErrorMessage) {
   return 'The field is optional, otherwise, $vErrorMessage';
 }

test/new_api_testing/core_validators/required_validators/validate_with_default_validator_test.dart

@@ -0,0 +1,38 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:form_builder_validators/form_builder_validators.dart';
+
+const String errorMultBy6 = 'error';
+String? isMultipleBy6(int value) {
+  return value % 6 == 0 ? null : errorMultBy6;
+}
+
+void main() {
+  group('Validator: validateWithDefault', () {
+    test('Should validate with valid default', () {
+      final Validator<int?> v = validateWithDefault(0, isMultipleBy6);
+      final Validator<int?> v1 = validateWithDefault(12, isMultipleBy6);
+
+      expect(v(null), isNull);
+      expect(v1(null), isNull);
+
+      expect(v(6), isNull);
+      expect(v1(18), isNull);
+
+      expect(v(2), equals(errorMultBy6));
+      expect(v1(14), equals(errorMultBy6));
+    });
+    test('Should validate with invalid default', () {
+      final Validator<int?> v = validateWithDefault(1, isMultipleBy6);
+      final Validator<int?> v1 = validateWithDefault(15, isMultipleBy6);
+
+      expect(v(null), equals(errorMultBy6));
+      expect(v1(null), equals(errorMultBy6));
+
+      expect(v(6), isNull);
+      expect(v1(18), isNull);
+
+      expect(v(2), equals(errorMultBy6));
+      expect(v1(14), equals(errorMultBy6));
+    });
+  });
+}