[listen] Creates listen package

.github/labeler.yml

@@ -27,8 +27,14 @@
     - any-glob-to-any-file:
       - packages/vector_math/**/*
 
+'p: listen':
+  - changed-files:
+    - any-glob-to-any-file:
+      - packages/listen/**/*
+
 'triage-framework':
   - changed-files:
     - any-glob-to-any-file:
       - packages/vector_math/**/*
       - packages/flutter_hook_config/**/*
+      - packages/listen/**/*

README.md

@@ -39,6 +39,7 @@ These are the packages hosted in this repository:
 |---------|-----|--------|-------|--------|---------------|
 | [flutter\_hook\_config](./packages/flutter_hook_config/) | [![pub package](https://img.shields.io/pub/v/flutter_hook_config.svg)](https://pub.dev/packages/flutter_hook_config) | [![pub points](https://img.shields.io/pub/points/flutter_hook_config)](https://pub.dev/packages/flutter_hook_config/score) | [![downloads](https://img.shields.io/pub/dm/flutter_hook_config)](https://pub.dev/packages/flutter_hook_config/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20flutter_hook_config?label=)](https://github.com/flutter/flutter/labels/p%3A%20flutter_hook_config) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/core-packages/p%3A%20flutter_hook_config?label=)](https://github.com/flutter/core-packages/labels/p%3A%20flutter_hook_config) |
 | [flutter\_template\_images](./packages/flutter_template_images/) | [![pub package](https://img.shields.io/pub/v/flutter_template_images.svg)](https://pub.dev/packages/flutter_template_images) | [![pub points](https://img.shields.io/pub/points/flutter_template_images)](https://pub.dev/packages/flutter_template_images/score) | [![downloads](https://img.shields.io/pub/dm/flutter_template_images)](https://pub.dev/packages/flutter_template_images/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20flutter_template_images?label=)](https://github.com/flutter/flutter/labels/p%3A%20flutter_template_images) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/core-packages/p%3A%20flutter_template_images?label=)](https://github.com/flutter/core-packages/labels/p%3A%20flutter_template_images) |
+| [listen](./packages/listen/) | [![pub package](https://img.shields.io/pub/v/listen.svg)](https://pub.dev/packages/listen) | [![pub points](https://img.shields.io/pub/points/listen)](https://pub.dev/packages/listen/score) | [![downloads](https://img.shields.io/pub/dm/listen)](https://pub.dev/packages/listen/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20listen?label=)](https://github.com/flutter/flutter/labels/p%3A%20listen) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/core-packages/p%3A%20listen?label=)](https://github.com/flutter/core-packages/labels/p%3A%20listen) |
 | [multicast\_dns](./packages/multicast_dns/) | [![pub package](https://img.shields.io/pub/v/multicast_dns.svg)](https://pub.dev/packages/multicast_dns) | [![pub points](https://img.shields.io/pub/points/multicast_dns)](https://pub.dev/packages/multicast_dns/score) | [![downloads](https://img.shields.io/pub/dm/multicast_dns)](https://pub.dev/packages/multicast_dns/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20multicast_dns?label=)](https://github.com/flutter/flutter/labels/p%3A%20multicast_dns) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/core-packages/p%3A%20multicast_dns?label=)](https://github.com/flutter/core-packages/labels/p%3A%20multicast_dns) |
 | [mustache\_template](./third_party/packages/mustache_template/) | [![pub package](https://img.shields.io/pub/v/mustache_template.svg)](https://pub.dev/packages/mustache_template) | [![pub points](https://img.shields.io/pub/points/mustache_template)](https://pub.dev/packages/mustache_template/score) | [![downloads](https://img.shields.io/pub/dm/mustache_template)](https://pub.dev/packages/mustache_template/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20mustache_template?label=)](https://github.com/flutter/flutter/labels/p%3A%20mustache_template) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/core-packages/p%3A%20mustache_template?label=)](https://github.com/flutter/core-packages/labels/p%3A%20mustache_template) |
 | [standard\_message\_codec](./packages/standard_message_codec/) | [![pub package](https://img.shields.io/pub/v/standard_message_codec.svg)](https://pub.dev/packages/standard_message_codec) | [![pub points](https://img.shields.io/pub/points/standard_message_codec)](https://pub.dev/packages/standard_message_codec/score) | [![downloads](https://img.shields.io/pub/dm/standard_message_codec)](https://pub.dev/packages/standard_message_codec/score) | [![GitHub issues by-label](https://img.shields.io/github/issues/flutter/flutter/p%3A%20standard_message_codec?label=)](https://github.com/flutter/flutter/labels/p%3A%20standard_message_codec) | [![GitHub pull requests by-label](https://img.shields.io/github/issues-pr/flutter/core-packages/p%3A%20standard_message_codec?label=)](https://github.com/flutter/core-packages/labels/p%3A%20standard_message_codec) |

packages/listen/.gitignore

@@ -0,0 +1,9 @@
+.children
+.project
+.DS_Store
+packages
+pubspec.lock
+.pub
+.packages
+.dart_tool
+.idea

packages/listen/AUTHORS

@@ -0,0 +1,6 @@
+# Below is a list of people and organizations that have contributed
+# to the project. Names should be added to the list like so:
+#
+#   Name/Organization <email address>
+
+Google Inc.

packages/listen/CHANGELOG.md

@@ -0,0 +1,3 @@
+## 1.0.0-beta.1
+
+- Moves source code from `flutter/flutter` to `flutter/core-packages`.

packages/listen/LICENSE

@@ -0,0 +1,25 @@
+Copyright 2013 The Flutter Authors
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/listen/README.md

@@ -0,0 +1,103 @@
+<?code-excerpt path-base="example/lib"?>
+
+# listen
+
+A package to notify state changes to interested listeners in pure Dart.
+
+## Usage
+
+### Using ValueNotifier
+
+`ValueNotifier` wraps a single value and notifies listeners whenever the value changes:
+
+<?code-excerpt "readme_excerpts.dart (ValueNotifier)"?>
+```dart
+void valueNotifierExample() {
+  final counter = ValueNotifier<int>(0);
+
+  counter.addListener(() {
+    print('Value changed: ${counter.value}');
+  });
+
+  counter.value = 5; // Prints: Value changed: 5
+  counter.value = 10; // Prints: Value changed: 10
+  counter.value = 10; // Does not print because the value is == 10
+
+  counter.dispose();
+}
+
+```
+
+### Using ChangeNotifier
+
+Extend or mix in `ChangeNotifier` to manage state and notify listeners manually:
+
+<?code-excerpt "readme_excerpts.dart (ChangeNotifierClass)"?>
+```dart
+/// A [ChangeNotifier] subclass that encapsulates a list of items and notifies
+/// listeners whenever items are added or removed.
+class ItemListNotifier extends ChangeNotifier {
+  final List<String> _items = <String>[];
+
+  /// The unmodifiable list of current items.
+  List<String> get items => List<String>.unmodifiable(_items);
+
+  /// Adds an [item] to the list and notifies listeners.
+  void addItem(String item) {
+    _items.add(item);
+    notifyListeners();
+  }
+
+  /// Removes an [item] from the list and notifies listeners if it was present.
+  void removeItem(String item) {
+    if (_items.remove(item)) {
+      notifyListeners();
+    }
+  }
+}
+
+```
+
+Then, listen to changes and update state:
+
+<?code-excerpt "readme_excerpts.dart (ChangeNotifierUsage)"?>
+```dart
+void changeNotifierExample() {
+  final listNotifier = ItemListNotifier();
+
+  listNotifier.addListener(() {
+    print('Current items: ${listNotifier.items}');
+  });
+
+  listNotifier.addItem('Apple'); // Prints: Current items: [Apple]
+  listNotifier.addItem('Banana'); // Prints: Current items: [Apple, Banana]
+  listNotifier.removeItem('Apple'); // Prints: Current items: [Banana]
+
+  listNotifier.dispose();
+}
+
+```
+
+### Merging listenables
+
+Use `Listenable.merge` to listen to multiple objects simultaneously:
+
+<?code-excerpt "readme_excerpts.dart (Merge)"?>
+```dart
+void mergeExample() {
+  final first = ValueNotifier<String>('Hello');
+  final second = ValueNotifier<String>('World');
+
+  final merged = Listenable.merge(<Listenable>[first, second]);
+
+  merged.addListener(() {
+    print('Merged listenable triggered: ${first.value} ${second.value}');
+  });
+
+  first.value = 'Hi'; // Prints: Merged listenable triggered: Hi World
+  second.value = 'Dart'; // Prints: Merged listenable triggered: Hi Dart
+
+  first.dispose();
+  second.dispose();
+}
+```

packages/listen/example/lib/counter.dart

@@ -0,0 +1,35 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: avoid_print
+
+import 'package:listen/listen.dart';
+
+/// A simple counter that extends [ChangeNotifier] to notify listeners
+/// whenever its value changes.
+class Counter extends ChangeNotifier {
+  int _count = 0;
+
+  /// The current count value.
+  int get count => _count;
+
+  /// Increments the count by one and notifies listeners.
+  void increment() {
+    _count++;
+    notifyListeners();
+  }
+}
+
+void main() {
+  final counter = Counter();
+
+  counter.addListener(() {
+    print('Counter value changed to: ${counter.count}');
+  });
+
+  counter.increment(); // Prints: Counter value changed to: 1
+  counter.increment(); // Prints: Counter value changed to: 2
+
+  counter.dispose();
+}

packages/listen/example/lib/list_notifier.dart

@@ -0,0 +1,43 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: avoid_print
+
+import 'package:listen/listen.dart';
+
+/// A [ChangeNotifier] subclass that encapsulates a list of items and notifies
+/// listeners whenever items are added or removed.
+class ItemListNotifier extends ChangeNotifier {
+  final List<String> _items = <String>[];
+
+  /// The unmodifiable list of current items.
+  List<String> get items => List<String>.unmodifiable(_items);
+
+  /// Adds an [item] to the list and notifies listeners.
+  void addItem(String item) {
+    _items.add(item);
+    notifyListeners();
+  }
+
+  /// Removes an [item] from the list and notifies listeners if it was present.
+  void removeItem(String item) {
+    if (_items.remove(item)) {
+      notifyListeners();
+    }
+  }
+}
+
+void main() {
+  final listNotifier = ItemListNotifier();
+
+  listNotifier.addListener(() {
+    print('Current items: ${listNotifier.items}');
+  });
+
+  listNotifier.addItem('Apple'); // Prints: Current items: [Apple]
+  listNotifier.addItem('Banana'); // Prints: Current items: [Apple, Banana]
+  listNotifier.removeItem('Apple'); // Prints: Current items: [Banana]
+
+  listNotifier.dispose();
+}

packages/listen/example/lib/listenable_merge.dart

@@ -0,0 +1,24 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: avoid_print
+
+import 'package:listen/listen.dart';
+
+void main() {
+  final first = ValueNotifier<String>('Hello');
+  final second = ValueNotifier<String>('World');
+
+  final merged = Listenable.merge(<Listenable>[first, second]);
+
+  merged.addListener(() {
+    print('Merged listenable triggered: ${first.value} ${second.value}');
+  });
+
+  first.value = 'Hi'; // Prints: Merged listenable triggered: Hi World
+  second.value = 'Dart'; // Prints: Merged listenable triggered: Hi Dart
+
+  first.dispose();
+  second.dispose();
+}

packages/listen/example/lib/main.dart

@@ -0,0 +1,20 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ignore_for_file: avoid_print
+
+import 'package:listen/listen.dart';
+
+void main() {
+  final counter = ValueNotifier<int>(0);
+
+  counter.addListener(() {
+    print('Counter changed to: ${counter.value}');
+  });
+
+  counter.value = 1; // Prints: Counter changed to: 1
+  counter.value = 2; // Prints: Counter changed to: 2
+
+  counter.dispose();
+}

packages/listen/example/lib/readme_excerpts.dart

@@ -0,0 +1,91 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This file exists solely to host compiled excerpts for README.md, and is not
+// intended for use as an actual example application.
+
+// ignore_for_file: avoid_print
+
+import 'package:listen/listen.dart';
+
+/// Demonstrates [ValueNotifier] usage for README.
+// #docregion ValueNotifier
+void valueNotifierExample() {
+  final counter = ValueNotifier<int>(0);
+
+  counter.addListener(() {
+    print('Value changed: ${counter.value}');
+  });
+
+  counter.value = 5; // Prints: Value changed: 5
+  counter.value = 10; // Prints: Value changed: 10
+  counter.value = 10; // Does not print because the value is == 10
+
+  counter.dispose();
+}
+
+// #enddocregion ValueNotifier
+
+// #docregion ChangeNotifierClass
+/// A [ChangeNotifier] subclass that encapsulates a list of items and notifies
+/// listeners whenever items are added or removed.
+class ItemListNotifier extends ChangeNotifier {
+  final List<String> _items = <String>[];
+
+  /// The unmodifiable list of current items.
+  List<String> get items => List<String>.unmodifiable(_items);
+
+  /// Adds an [item] to the list and notifies listeners.
+  void addItem(String item) {
+    _items.add(item);
+    notifyListeners();
+  }
+
+  /// Removes an [item] from the list and notifies listeners if it was present.
+  void removeItem(String item) {
+    if (_items.remove(item)) {
+      notifyListeners();
+    }
+  }
+}
+
+// #enddocregion ChangeNotifierClass
+
+/// Demonstrates [ChangeNotifier] usage for README.
+// #docregion ChangeNotifierUsage
+void changeNotifierExample() {
+  final listNotifier = ItemListNotifier();
+
+  listNotifier.addListener(() {
+    print('Current items: ${listNotifier.items}');
+  });
+
+  listNotifier.addItem('Apple'); // Prints: Current items: [Apple]
+  listNotifier.addItem('Banana'); // Prints: Current items: [Apple, Banana]
+  listNotifier.removeItem('Apple'); // Prints: Current items: [Banana]
+
+  listNotifier.dispose();
+}
+
+// #enddocregion ChangeNotifierUsage
+
+/// Demonstrates [Listenable.merge] usage for README.
+// #docregion Merge
+void mergeExample() {
+  final first = ValueNotifier<String>('Hello');
+  final second = ValueNotifier<String>('World');
+
+  final merged = Listenable.merge(<Listenable>[first, second]);
+
+  merged.addListener(() {
+    print('Merged listenable triggered: ${first.value} ${second.value}');
+  });
+
+  first.value = 'Hi'; // Prints: Merged listenable triggered: Hi World
+  second.value = 'Dart'; // Prints: Merged listenable triggered: Hi Dart
+
+  first.dispose();
+  second.dispose();
+}
+// #enddocregion Merge