feat(single): add Single.asNullable() operator

CHANGELOG.md

@@ -3,6 +3,9 @@
 - **operators**:
     - Add `Stream.asNullable()` operator.
 
+- **single**:
+    - Add `Single.asNullable()` operator.
+
 ## 0.3.0 - Jun 16, 2024
 
 - Accept Dart SDK versions above 3.0 (`sdk: '>=2.12.0 <4.0.0'`).

lib/src/single/as_nullable.dart

@@ -0,0 +1,9 @@
+import 'single.dart';
+
+/// Adapt this `Single<T>` to be a `Single<T?>`.
+extension AsNullableSingleExtension<T extends Object> on Single<T> {
+  /// Adapt this `Single<T>` to be a `Single<T?>`.
+  @pragma('vm:prefer-inline')
+  @pragma('dart2js:tryInline')
+  Single<T?> asNullable() => this;
+}

test/single/as_nullable_test.dart

@@ -0,0 +1,39 @@
+import 'dart:async';
+
+import 'package:rxdart_ext/rxdart_ext.dart';
+import 'package:test/expect.dart';
+import 'package:test/scaffolding.dart';
+
+void main() {
+  group('Single.asNullable', () {
+    test('works', () {
+      final single = Single.value(1);
+      final asNullable = single.asNullable();
+
+      expect(asNullable, isA<Single<int?>>());
+      expect(asNullable, same(single));
+    });
+
+    test('asNullable with publishState', () async {
+      final single = Single.value(1);
+      final stateStream = single.asNullable().publishState(null);
+
+      expect(stateStream, isA<StateConnectableStream<int?>>());
+      expect(stateStream.value, null);
+
+      scheduleMicrotask(() => stateStream.connect());
+      await expectLater(stateStream, emitsInOrder([1]));
+    });
+
+    test('asNullable with publishValueSeeded', () async {
+      final single = Single.value(1);
+      final valueStream = single.asNullable().publishValueSeeded(null);
+
+      expect(valueStream, isA<ValueConnectableStream<int?>>());
+      expect(valueStream.value, null);
+
+      scheduleMicrotask(() => valueStream.connect());
+      await expectLater(valueStream, emitsInOrder([null, 1]));
+    });
+  });
+}