implement conditional validator if

Author: ArturAssisComp

lib/new_api_prototype/core_validators/conditional_validators.dart

@@ -0,0 +1,9 @@
+import '../constants.dart';
+
+/// This function returns a validator that conditionally validates the user input.
+/// If `condition` applied to the user input evaluates to true, the user input is
+/// validated by `v`. Otherwise, it is automatically considered valid.
+Validator<T> validateIf<T extends Object?>(
+    bool Function(T value) condition, Validator<T> v) {
+  return (T value) => condition(value) ? v(value) : null;
+}

test/new_api_testing/core_validators/conditional_validators/validate_if_validator_test.dart

@@ -0,0 +1,28 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:form_builder_validators/form_builder_validators.dart';
+import 'package:form_builder_validators/new_api_prototype/core_validators/conditional_validators.dart';
+
+const String errorGt = 'error gt';
+Validator<num> gt(num target) {
+  return (num v) {
+    return v > target ? null : errorGt;
+  };
+}
+
+void main() {
+  group('Validator: validateIf', () {
+    test('Should check if a number is greater than 10 if it is an even int',
+        () {
+      // Only even integers will be validated: ... 0, 2, 4 ...
+      final Validator<num> v =
+          validateIf((num v) => v is int && v % 2 == 0, gt(10));
+
+      expect(v(2.3), isNull);
+
+      expect(v(8), errorGt);
+      expect(v(10), errorGt);
+      expect(v(12), isNull); // This is null because it is valid
+      expect(v(13), isNull); // This is null because it was not validated.
+    });
+  });
+}