Create Dart replicas for Flutter's notifiers. (#850)

Author: polina-c

.github/workflows/flutter_packages.yaml

@@ -169,7 +169,6 @@ jobs:
           fi
 
       - name: Check for cycles
-        if: matrix.package.path != 'packages/ui_primitives'
         working-directory: ${{ matrix.package.path }}
         run: |
           dart pub global activate layerlens

packages/a2ui_core/CHANGELOG.md

@@ -0,0 +1,5 @@
+# `a2ui_core` Changelog
+
+## 0.0.1
+
+- Initial version.

packages/ui_primitives/LICENSE packages/a2ui_core/LICENSEpackages/ui_primitives/LICENSE renamed to packages/a2ui_core/LICENSE

@@ -0,0 +1,3 @@
+# A2UI Core
+
+Core model for A2UI protocol.

packages/a2ui_core/README.md

@@ -0,0 +1,9 @@
+// Copyright 2025 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export 'src/listenable/error_reporting.dart'
+    show ListenableError, ListenableErrorDetails;
+export 'src/listenable/notifiers.dart'
+    show ChangeNotifier, GenUiListenable, GenUiValueListenable, ValueNotifier;
+export 'src/listenable/primitives.dart' show VoidCallback;

packages/a2ui_core/lib/a2ui_core.dart

@@ -0,0 +1,60 @@
+// Copyright 2025 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+class ListenableErrorReporting {
+  /// Creates an error with the given message.
+  static Error createError(String message) =>
+      ListenableError(ListenableErrorDetails(exception: message));
+
+  /// Reports an error.
+  ///
+  /// Adds the error to [reportedErrors].
+  ///
+  /// The list of errors should be verified and cleaned regularly
+  /// by the framework or application.
+  static void report(ListenableErrorDetails details) =>
+      _reportedErrors.add(details);
+
+  /// Returns the list of reported errors.
+  static Iterable<ListenableErrorDetails> get reportedErrors => _reportedErrors;
+  static final _reportedErrors = <ListenableErrorDetails>[];
+
+  /// Clears the list of reported errors.
+  static void clearReportedErrors() => _reportedErrors.clear();
+}
+
+final class ListenableError extends Error {
+  ListenableError(this.details);
+
+  final ListenableErrorDetails details;
+
+  @override
+  String toString() {
+    return details.toString();
+  }
+}
+
+final class ListenableErrorDetails {
+  ListenableErrorDetails({
+    required this.exception,
+    this.dispatchingObject,
+    this.stack,
+  });
+
+  final Object? dispatchingObject;
+  final Object exception;
+  final StackTrace? stack;
+
+  @override
+  String toString() {
+    final parts = [
+      if (dispatchingObject != null)
+        '$dispatchingObject reported $exception'
+      else
+        '$exception',
+      if (stack != null) '$stack',
+    ];
+    return parts.join('\n');
+  }
+}