Implement min length validator

Author: ArturAssisComp

lib/new_api_prototype/collection_validators.dart

@@ -1,9 +1,22 @@
 import '../localization/l10n.dart';
 import 'constants.dart';
 
+/// Returns a [Validator] function that checks if its input has a length that is
+/// greater than or equal to `minLength`. If the input satisfies this condition,
+/// the validator returns `null`. Otherwise, it returns the default error message
+/// `FormBuilderLocalizations.current.minLengthErrorText(minLength)`,
+/// if [minLengthMsg] is not provided.
+///
+/// # Caveats
+/// - Objects that are not collections are considered as collections with
+/// length 1.
+///
+/// # Errors
+/// - Throws [AssertionError] if `minLength` is negative.
 Validator<T> minLength<T extends Object>(int minLength,
     {String Function(int)? minLengthMsg}) {
-  return (value) {
+  assert(minLength >= 0, 'The "minLength" parameter may not be negative');
+  return (T value) {
     // I think it makes more sense to say that scalar objects has length 1 instead of 0.
     int valueLength = 1;
 

test/new_api_testing/collection_validators/min_length_validator_test.dart

@@ -0,0 +1,73 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:form_builder_validators/form_builder_validators.dart';
+
+void main() {
+  group('Validator: minLength', () {
+    group('Validators with default error message', () {
+      test('Should validate the input when it is expected to have minLength 0',
+          () {
+        final Validator<List<int>> v = minLength(0);
+        final Validator<Object> v2 = minLength(0);
+
+        expect(v(<int>[]), isNull);
+        expect(v2(<int>[12]), isNull);
+        expect(v2(12), isNull);
+      });
+      test('Should validate the input when it is expected to have minLength 1',
+          () {
+        final Validator<List<int>> v = minLength(1);
+        final Validator<Object> v2 = minLength(1);
+
+        expect(
+            v(<int>[]), FormBuilderLocalizations.current.minLengthErrorText(1));
+        expect(v(<int>[0]), isNull);
+        expect(v(<int>[1, 2]), isNull);
+
+        expect(v2(<String>[]),
+            FormBuilderLocalizations.current.minLengthErrorText(1));
+        expect(v2(<String>['0']), isNull);
+        expect(v2(0), isNull);
+        expect(v2(<Object>[1, '3']), isNull);
+      });
+      test('Should validate the input when it is expected to have minLength 4',
+          () {
+        final Validator<List<String>> v = minLength(4);
+        final Validator<String> v2 = minLength(4);
+
+        expect(v(<String>[]),
+            FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(v(<String>['1']),
+            FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(v(<String>['1', '2']),
+            FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(v(<String>['1', '2', '3']),
+            FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(v(<String>['1', '2', '3', '4']), isNull);
+        expect(v(<String>['1', '2', '3', '4', '5']), isNull);
+        expect(v(<String>['1', '2', '3', '4', '5', '6']), isNull);
+
+        expect(v2(''), FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(v2('1'), FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(
+            v2('12'), FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(
+            v2('123'), FormBuilderLocalizations.current.minLengthErrorText(4));
+        expect(v2('1234'), isNull);
+        expect(v2('12345'), isNull);
+        expect(v2('123456'), isNull);
+      });
+    });
+
+    test('Should validate input returning custom message error when invalid',
+        () {
+      const String customMsg = 'custom msg';
+      final Validator<Object> v = minLength(3, minLengthMsg: (_) => customMsg);
+
+      expect(v(89), equals(customMsg));
+      expect(v(<Object>[1, '2', 3, 4]), isNull);
+    });
+    test('Should throw AssertionError when minLength is negative', () {
+      expect(() => minLength(-2), throwsAssertionError);
+    });
+  });
+}