implement conditional validator skip if

Author: ArturAssisComp

lib/new_api_prototype/core_validators/conditional_validators.dart

@@ -7,3 +7,12 @@ Validator<T> validateIf<T extends Object?>(
     bool Function(T value) condition, Validator<T> v) {
   return (T value) => condition(value) ? v(value) : null;
 }
+
+/// This function returns a validator that conditionally skips validation for
+/// the user input. If `condition` applied to the user input evaluates to true,
+/// the validation step is skipped and null is returned. Otherwise, it is
+/// validated by 'v'.
+Validator<T> skipIf<T extends Object?>(
+    bool Function(T value) condition, Validator<T> v) {
+  return (T value) => condition(value) ? null : v(value);
+}

test/new_api_testing/core_validators/conditional_validators/skip_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: skipIf', () {
+    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 =
+          skipIf((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.
+    });
+  });
+}