feat: Adding ComponentNotifier API (#1889)

This adds a proposal for a new API for flame, the ComponentNotifier.

This API offers the user change notifiers classes that are tied to FlameGame and its components so the user can be notified when a component is added, removed or updated.

This will enable users to:

    Take the benefit of reactive programming inside the game
    Have a simple way of watching certain states from the game, on Flutter Widgets

One important note here is that this proposal does not mean to replace integrations like flame_bloc, but rather provider an simple and out of the box solution, without any need of additional packages, since change notifiers are provided by flutter itself.

Opening this as draft for now to get feedback on the implementation, will write tests and docs once we have the final implementation.

doc/flame/components.md

@@ -1085,6 +1085,94 @@ Check the example app
 for details on how to use it.
 
 
+## ComponentsNotifier
+
+Most of the time just accessing children and their attributes is enough to build the logic of
+your game.
+
+But sometimes, reactivity can help the developer to simplify and write better code, to help with
+that Flame provides the `ComponentsNotifier`, which is an implementation of a
+`ChangeNotifier` that notifies listeners every time a component is added, removed or manually
+changed.
+
+For example, lets say that we want to show a game over text when the player's lives reach zero.
+
+To make the component automatically report when new instances are added or removed, the `Notifier`
+mixin can be applied to the component class:
+
+```dart
+class Player extends SpriteComponent with Notifier {}
+```
+
+Then to listen to changes on that component the `componentsNotifier` method from `FlameGame` can
+be used:
+
+```dart
+class MyGame extends FlameGame {
+  int lives = 2;
+
+  Future<void> onLoad() {
+  final playerNotifier = componentsNotifier<Player>()
+    ..addListener(() {
+      final player = playerNotifier.single;
+      if (player == null) {
+        lives--;
+        if (lives == 0) {
+          add(GameOverComponent());
+        } else {
+          add(Player());
+        }
+      }
+    });
+  }
+}
+```
+
+A `Notifier` component can also manually notify its listeners that something changed. Lets expand
+the example above to make a hud component to blink when the player has half of their health. In
+order to do so, we need that the `Player` component notify a change manually, example:
+
+```dart
+class Player extends SpriteComponent with Notifier {
+  double health = 1;
+
+  void takeHit() {
+    health -= .1;
+    if (health == 0) {
+      removeFromParent();
+    } else if (health <= .5) {
+      notifyListeners();
+    }
+  }
+}
+```
+
+Then our hud component could look like:
+
+```dart
+class Hud extends PositionComponent with HasGameRef {
+
+  Future<void> onLoad() {
+  final playerNotifier = gameRef.componentsNotifier<Player>()
+    ..addListener(() {
+      final player = playerNotifier.single;
+      if (player != null) {
+        if (player.health <= .5) {
+          add(BlinkEffect());
+        }
+      }
+    });
+  }
+}
+```
+
+`ComponentsNotifier`s can also come in handy to rebuild widgets when state changes inside a
+`FlameGame`, to help with that Flame provides a `ComponentsNotifierBuilder` widget.
+
+To see an example of its use check the running example
+[here](https://github.com/flame-engine/flame/tree/main/examples/lib/stories/components/components_notifier_example.dart);
+
+
 ## ClipComponent
 
 A `ClipComponent` is a component that will clip the canvas to its size and shape. This means that

examples/lib/stories/components/components.dart

@@ -1,6 +1,8 @@
 import 'package:dashbook/dashbook.dart';
 import 'package:examples/commons/commons.dart';
 import 'package:examples/stories/components/clip_component_example.dart';
+import 'package:examples/stories/components/components_notifier_example.dart';
+import 'package:examples/stories/components/components_notifier_provider_example.dart';
 import 'package:examples/stories/components/composability_example.dart';
 import 'package:examples/stories/components/debug_example.dart';
 import 'package:examples/stories/components/game_in_game_example.dart';
@@ -52,5 +54,17 @@ void addComponentsStories(Dashbook dashbook) {
       (_) => GameWidget(game: LookAtSmoothExample()),
       codeLink: baseLink('components/look_at_smooth_example.dart'),
       info: LookAtExample.description,
+    )
+    ..add(
+      'Component Notifier',
+      (_) => const ComponentsNotifierExampleWidget(),
+      codeLink: baseLink('components/component_notifier_example.dart'),
+      info: ComponentsNotifierExampleWidget.description,
+    )
+    ..add(
+      'Component Notifier (with provider)',
+      (_) => const ComponentsNotifierProviderExampleWidget(),
+      codeLink: baseLink('components/component_notifier_provider_example.dart'),
+      info: ComponentsNotifierProviderExampleWidget.description,
     );
 }

examples/lib/stories/components/components_notifier_example.dart

@@ -0,0 +1,111 @@
+import 'package:flame/components.dart';
+import 'package:flame/game.dart';
+import 'package:flame/widgets.dart';
+import 'package:flutter/material.dart';
+
+class ComponentsNotifierExampleWidget extends StatefulWidget {
+  const ComponentsNotifierExampleWidget({super.key});
+
+  static String description = '''
+      Showcases how the components notifier can be used between
+      a flame game instance and widgets.
+
+      Tap the red dots to defeat the enemies and see the hud being updated
+      to reflect the current state of the game.
+''';
+
+  @override
+  State<ComponentsNotifierExampleWidget> createState() =>
+      _ComponentsNotifierExampleWidgetState();
+}
+
+class _ComponentsNotifierExampleWidgetState
+    extends State<ComponentsNotifierExampleWidget> {
+  @override
+  void initState() {
+    super.initState();
+
+    game = ComponentNotifierExample();
+  }
+
+  late final ComponentNotifierExample game;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Stack(
+        children: [
+          Positioned.fill(
+            child: GameWidget(game: game),
+          ),
+          Positioned(
+            left: 16,
+            top: 16,
+            child: ComponentsNotifierBuilder<Enemy>(
+              notifier: game.componentsNotifier<Enemy>(),
+              builder: (context, notifier) {
+                return GameHud(
+                  remainingEnemies: notifier.components.length,
+                  onReplay: game.replay,
+                );
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class GameHud extends StatelessWidget {
+  const GameHud({
+    super.key,
+    required this.remainingEnemies,
+    required this.onReplay,
+  });
+
+  final int remainingEnemies;
+  final VoidCallback onReplay;
+
+  @override
+  Widget build(BuildContext context) {
+    return Card(
+      child: Padding(
+        padding: const EdgeInsets.all(16),
+        child: remainingEnemies == 0
+            ? ElevatedButton(
+                onPressed: onReplay,
+                child: const Text('Play again'),
+              )
+            : Text('Remaining enemies: $remainingEnemies'),
+      ),
+    );
+  }
+}
+
+class Enemy extends CircleComponent with Tappable, Notifier {
+  Enemy({super.position})
+      : super(
+          radius: 20,
+          paint: Paint()..color = const Color(0xFFFF0000),
+        );
+
+  @override
+  bool onTapUp(_) {
+    removeFromParent();
+    return true;
+  }
+}
+
+class ComponentNotifierExample extends FlameGame with HasTappables {
+  @override
+  Future<void> onLoad() async {
+    replay();
+  }
+
+  void replay() {
+    add(Enemy(position: Vector2(100, 100)));
+    add(Enemy(position: Vector2(200, 100)));
+    add(Enemy(position: Vector2(300, 100)));
+  }
+}

examples/lib/stories/components/components_notifier_provider_example.dart

@@ -0,0 +1,105 @@
+import 'package:flame/components.dart';
+import 'package:flame/game.dart';
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+class ComponentsNotifierProviderExampleWidget extends StatefulWidget {
+  const ComponentsNotifierProviderExampleWidget({super.key});
+
+  static String description = '''
+      Similar to the Components Notifier example, but uses provider
+      instead of the built in ComponentsNotifierBuilder widget.
+''';
+
+  @override
+  State<ComponentsNotifierProviderExampleWidget> createState() =>
+      _ComponentsNotifierProviderExampleWidgetState();
+}
+
+class _ComponentsNotifierProviderExampleWidgetState
+    extends State<ComponentsNotifierProviderExampleWidget> {
+  @override
+  void initState() {
+    super.initState();
+
+    game = ComponentNotifierExample();
+  }
+
+  late final ComponentNotifierExample game;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: MultiProvider(
+        providers: [
+          Provider<ComponentNotifierExample>.value(value: game),
+          ChangeNotifierProvider<ComponentsNotifier<Enemy>>(
+            create: (_) => game.componentsNotifier<Enemy>(),
+          ),
+        ],
+        child: Stack(
+          children: [
+            Positioned.fill(
+              child: GameWidget(game: game),
+            ),
+            const Positioned(
+              left: 16,
+              top: 16,
+              child: GameHud(),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class GameHud extends StatelessWidget {
+  const GameHud({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final enemies = context.watch<ComponentsNotifier<Enemy>>().components;
+
+    return Card(
+      child: Padding(
+        padding: const EdgeInsets.all(16),
+        child: enemies.isEmpty
+            ? ElevatedButton(
+                child: const Text('Play again'),
+                onPressed: () {
+                  context.read<ComponentNotifierExample>().replay();
+                },
+              )
+            : Text('Remaining enemies: ${enemies.length}'),
+      ),
+    );
+  }
+}
+
+class Enemy extends CircleComponent with Tappable, Notifier {
+  Enemy({super.position})
+      : super(
+          radius: 20,
+          paint: Paint()..color = const Color(0xFFFF0000),
+        );
+
+  @override
+  bool onTapUp(_) {
+    removeFromParent();
+    return true;
+  }
+}
+
+class ComponentNotifierExample extends FlameGame with HasTappables {
+  @override
+  Future<void> onLoad() async {
+    replay();
+  }
+
+  void replay() {
+    add(Enemy(position: Vector2(100, 100)));
+    add(Enemy(position: Vector2(200, 100)));
+    add(Enemy(position: Vector2(300, 100)));
+  }
+}

examples/pubspec.yaml

@@ -23,6 +23,7 @@ dependencies:
   google_fonts: ^2.3.2
   meta: ^1.8.0
   padracing: ^1.0.0
+  provider: ^6.0.3
   rogue_shooter: ^0.1.0
   trex_game: ^0.1.0
 

packages/flame/lib/components.dart

@@ -3,6 +3,7 @@ export 'src/anchor.dart';
 export 'src/collisions/has_collision_detection.dart';
 export 'src/collisions/hitboxes/screen_hitbox.dart';
 export 'src/components/clip_component.dart';
+export 'src/components/components_notifier.dart';
 export 'src/components/core/component.dart';
 export 'src/components/core/component_set.dart';
 export 'src/components/core/position_type.dart';
@@ -21,6 +22,7 @@ export 'src/components/mixins/has_game_ref.dart' show HasGameRef;
 export 'src/components/mixins/has_paint.dart';
 export 'src/components/mixins/hoverable.dart';
 export 'src/components/mixins/keyboard_handler.dart';
+export 'src/components/mixins/notifier.dart';
 export 'src/components/mixins/parent_is_a.dart';
 export 'src/components/mixins/single_child_particle.dart';
 export 'src/components/mixins/tappable.dart';