Implement max length validator

Author: ArturAssisComp

lib/new_api_prototype/collection_validators.dart

@@ -18,9 +18,22 @@ Validator<T> minLength<T extends Object>(int minLength,
   };
 }
 
+/// Returns a [Validator] function that checks if its input has a length that is
+/// less than or equal to `maxLength`. If the input satisfies this condition,
+/// the validator returns `null`. Otherwise, it returns the default error message
+/// `FormBuilderLocalizations.current.maxLengthErrorText(maxLength)`,
+/// if [maxLengthMsg] is not provided.
+///
+/// # Caveats
+/// - Objects that are not collections are considered as collections with
+/// length 1.
+///
+/// # Errors
+/// - Throws [AssertionError] if `maxLength` is negative.
 Validator<T> maxLength<T extends Object>(int maxLength,
     {String Function(int)? maxLengthMsg}) {
-  return (value) {
+  assert(maxLength >= 0, 'The "maxLength" 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/equal_length_validator_test.dart

@@ -54,8 +54,9 @@ void main() {
 
     test('Should validate input returning custom message error when invalid',
         () {
-      final customMsg = 'custom msg';
-      final v = equalLength(3, equalLengthMsg: (_) => customMsg);
+      const String customMsg = 'custom msg';
+      final Validator<Object> v =
+          equalLength(3, equalLengthMsg: (_) => customMsg);
 
       expect(v('hey'), isNull);
       expect(v([1, '2', 3, 4]), equals(customMsg));

test/new_api_testing/collection_validators/max_length_validator_test.dart

@@ -0,0 +1,62 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:form_builder_validators/form_builder_validators.dart';
+
+void main() {
+  group('Validator: maxLength', () {
+    group('Validators with default error message', () {
+      test('Should validate the input when it is expected to have maxLength 0',
+          () {
+        final Validator<List<int>> v = maxLength(0);
+        final Validator<Object> v2 = maxLength(0);
+
+        expect(v(<int>[]), isNull);
+        expect(v2(<int>[12]),
+            FormBuilderLocalizations.current.maxLengthErrorText(0));
+        expect(v2(12), FormBuilderLocalizations.current.maxLengthErrorText(0));
+      });
+      test('Should validate the input when it is expected to have maxLength 1',
+          () {
+        final Validator<List<int>> v = maxLength(1);
+        final Validator<Object> v2 = maxLength(1);
+
+        expect(v(<int>[]), isNull);
+        expect(v(<int>[0]), isNull);
+        expect(v2(<String>['0']), isNull);
+        expect(v2(0), isNull);
+        expect(v(<int>[1, 2]),
+            FormBuilderLocalizations.current.maxLengthErrorText(1));
+        expect(v2(<Object>[1, '3']),
+            FormBuilderLocalizations.current.maxLengthErrorText(1));
+      });
+      test('Should validate the input when it is expected to have maxLength 4',
+          () {
+        final Validator<List<String>> v = maxLength(4);
+        final Validator<String> v2 = maxLength(4);
+
+        expect(v(<String>[]), isNull);
+        expect(v(<String>['1']), isNull);
+        expect(v(<String>['1', '2']), isNull);
+        expect(v(<String>['1', '2', '3']), isNull);
+        expect(v(<String>['1', '2', '3', '4']), isNull);
+
+        expect(v2(''), isNull);
+        expect(v2('1'), isNull);
+        expect(v2('12'), isNull);
+        expect(v2('123'), isNull);
+        expect(v2('1234'), isNull);
+      });
+    });
+
+    test('Should validate input returning custom message error when invalid',
+        () {
+      const String customMsg = 'custom msg';
+      final Validator<Object> v = maxLength(3, maxLengthMsg: (_) => customMsg);
+
+      expect(v('hey'), isNull);
+      expect(v(<Object>[1, '2', 3, 4]), equals(customMsg));
+    });
+    test('Should throw AssertionError when maxLength is negative', () {
+      expect(() => maxLength(-2), throwsAssertionError);
+    });
+  });
+}