Implement contains element validator

Author: ArturAssisComp

lib/new_api_prototype/generic_type_validators.dart

@@ -1,12 +1,21 @@
 import '../localization/l10n.dart';
 import 'constants.dart';
 
-Validator<T> containsElement<T extends Object>(
+/// Returns a [Validator] function that checks whether its input is in the
+/// provided list `values`. If the input is valid, the validator returns `null`,
+/// otherwise, it returns `containsElementMsg`, if provided, or
+/// `FormBuilderLocalizations.current.containsElementErrorText`.
+///
+/// # Errors
+/// - Throws an [AssertionError] if the input `values` is an empty list.
+Validator<T> containsElement<T extends Object?>(
   List<T> values, {
   String? containsElementMsg,
 }) {
+  assert(values.isNotEmpty, 'The list "values" may not be empty.');
+  final Set<T> setOfValues = values.toSet();
   return (T value) {
-    return values.contains(value)
+    return setOfValues.contains(value)
         ? null
         : containsElementMsg ??
             FormBuilderLocalizations.current.containsElementErrorText;

lib/new_api_prototype/new_api_prototype.dart

@@ -1,3 +1,4 @@
+export 'collection_validators.dart';
 export 'composition_validators.dart';
 export 'constants.dart';
 export 'generic_type_validators.dart';

lib/new_api_prototype/string_validators.dart

@@ -1,7 +1,7 @@
 import '../localization/l10n.dart';
+import 'collection_validators.dart';
 import 'composition_validators.dart';
 import 'constants.dart';
-import 'generic_type_validators.dart';
 import 'override_error_msg.dart';
 
 const _minL = minLength;

test/new_api_testing/generic_type_validators/contains_element_validator_test.dart

@@ -0,0 +1,47 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:form_builder_validators/form_builder_validators.dart';
+
+void main() {
+  group('Validator: containsElement', () {
+    group('Validations with default error message', () {
+      test('Should return null when int 0 is provided', () {
+        final Validator<int> validator = containsElement(<int>[0]);
+
+        expect(validator(0), isNull);
+        expect(validator(2),
+            equals(FormBuilderLocalizations.current.containsElementErrorText));
+      });
+      test('Should return null when int 0 or String "2" is provided', () {
+        final Validator<Object> validator = containsElement(<Object>[0, '2']);
+
+        expect(validator(0), isNull);
+        expect(validator('2'), isNull);
+        expect(validator(2),
+            equals(FormBuilderLocalizations.current.containsElementErrorText));
+      });
+      test('Should return null when int 0, int 2, or null is provided', () {
+        final Validator<Object?> validator =
+            containsElement(<Object?>[0, 2, null]);
+
+        expect(validator(0), isNull);
+        expect(validator(2), isNull);
+        expect(validator(null), isNull);
+        expect(validator('2'),
+            equals(FormBuilderLocalizations.current.containsElementErrorText));
+      });
+    });
+
+    test('Should throw assertionError when list input is empty', () {
+      expect(() => containsElement([]), throwsAssertionError);
+    });
+
+    test('Should return custom error message when invalid input is provided',
+        () {
+      const customMessage = 'custom message';
+      final validator =
+          containsElement([1, 2, 3], containsElementMsg: customMessage);
+
+      expect(validator(4), equals(customMessage));
+    });
+  });
+}