feat: Add autoResize for SpriteGroupComponent (#2442)

This PR adds an autoResize parameter to SpriteGroupComponent which will allow Flame users to control if the size of the component should be re-calculated to fit the size of current active sprite in the group. This new parameter auto-populates when not specified, based value of size while construction. If size is not given, autoResize will be true and vice-versa.

Caveat:
If someone goes and replaces the actual Sprite at a particular key within SpriteGroupComponent.sprites map and if that key is the current state, it will not trigger an auto-resize. This seems like a very unlikely use-case, so I am not worried about it too much.

packages/flame/lib/src/components/sprite_group_component.dart

@@ -11,33 +11,64 @@ export '../sprite_animation.dart';
 class SpriteGroupComponent<T> extends PositionComponent
     with HasPaint
     implements SizeProvider {
-  /// Key with the current playing animation
-  T? current;
+  T? _current;
 
   /// Map with the available states for this sprite group
   Map<T, Sprite>? sprites;
 
+  bool _autoResize;
+
   /// Creates a component with an empty animation which can be set later
   SpriteGroupComponent({
     this.sprites,
-    this.current,
+    T? current,
+    bool? autoResize,
     Paint? paint,
     super.position,
-    super.size,
+    Vector2? size,
     super.scale,
     super.angle,
     super.nativeAngle,
     super.anchor,
     super.children,
     super.priority,
-  }) {
+  })  : assert(
+          (size == null) == (autoResize ?? size == null),
+          '''If size is set, autoResize should be false or size should be null when autoResize is true.''',
+        ),
+        _current = current,
+        _autoResize = autoResize ?? size == null,
+        super(size: size ?? sprites?[current]?.srcSize) {
     if (paint != null) {
       this.paint = paint;
     }
   }
 
   Sprite? get sprite => sprites?[current];
 
+  /// Returns the current group state.
+  T? get current => _current;
+
+  /// The the group state to given state.
+  ///
+  /// Will update [size] if [autoResize] is true.
+  set current(T? value) {
+    _current = value;
+    _resizeToSprite();
+  }
+
+  /// Returns current value of auto resize flag.
+  bool get autoResize => _autoResize;
+
+  /// Sets the given value of autoResize flag.
+  ///
+  /// Will update the [size] to fit srcSize of
+  /// current [sprite] if set to  true.
+  set autoResize(bool value) {
+    _autoResize = value;
+    _resizeToSprite();
+  }
+
   @override
   @mustCallSuper
   void onMount() {
@@ -60,4 +91,15 @@ class SpriteGroupComponent<T> extends PositionComponent
       overridePaint: paint,
     );
   }
+
+  /// Updates the size to current [sprite]'s srcSize if [autoResize] is true.
+  void _resizeToSprite() {
+    if (_autoResize) {
+      if (sprite != null) {
+        size.setFrom(sprite!.srcSize);
+      } else {
+        size.setZero();
+      }
+    }
+  }
 }

packages/flame/test/components/sprite_group_component_test.dart

@@ -1,6 +1,6 @@
 import 'package:flame/components.dart';
 import 'package:flame_test/flame_test.dart';
-import 'package:test/test.dart';
+import 'package:flutter_test/flutter_test.dart';
 
 enum _SpriteState {
   idle,
@@ -34,4 +34,60 @@ Future<void> main() async {
       expect(component.sprite, sprite2);
     });
   });
+
+  group('SpriteComponent.autoResize', () {
+    test('mutual exclusive with size while construction', () {
+      expect(
+        () => SpriteGroupComponent<_SpriteState>(
+          autoResize: true,
+          size: Vector2.all(2),
+        ),
+        throwsAssertionError,
+      );
+
+      expect(
+        () => SpriteGroupComponent<_SpriteState>(autoResize: false),
+        throwsAssertionError,
+      );
+    });
+
+    test('default value set correctly when not provided explicitly', () {
+      final component1 = SpriteGroupComponent<_SpriteState>();
+      final component2 = SpriteGroupComponent<_SpriteState>(
+        size: Vector2.all(2),
+      );
+
+      expect(component1.autoResize, true);
+      expect(component2.autoResize, false);
+    });
+
+    test('resizes on current state change', () {
+      final sprite1 = Sprite(image);
+      final sprite2 = Sprite(image, srcSize: Vector2.all(15));
+
+      final component = SpriteGroupComponent<_SpriteState>(
+        sprites: {_SpriteState.idle: sprite1, _SpriteState.running: sprite2},
+        current: _SpriteState.idle,
+      );
+      expect(component.size, sprite1.srcSize);
+
+      component.current = _SpriteState.running;
+      expect(component.size, sprite2.srcSize);
+    });
+
+    test('resizes only when true', () {
+      final sprite1 = Sprite(image);
+      final sprite2 = Sprite(image, srcSize: Vector2.all(15));
+      final component = SpriteGroupComponent<_SpriteState>(
+        sprites: {_SpriteState.idle: sprite1, _SpriteState.running: sprite2},
+        current: _SpriteState.idle,
+      )..autoResize = false;
+
+      component.current = _SpriteState.running;
+      expect(component.size, sprite1.srcSize);
+
+      component.autoResize = true;
+      expect(component.size, sprite2.srcSize);
+    });
+  });
 }