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.

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)));
+  }
+}