feat: implementing composable blueprints

lib/flame/blueprint.dart

@@ -14,6 +14,8 @@ const _attachedErrorMessage = "Can't add to attached Blueprints";
 /// the [FlameGame] level.
 abstract class Blueprint<T extends FlameGame> {
   final List<Component> _components = [];
+  final List<Blueprint> _blueprints = [];
+
   bool _isAttached = false;
 
   /// Called before the the [Component]s managed
@@ -25,7 +27,10 @@ abstract class Blueprint<T extends FlameGame> {
   @mustCallSuper
   Future<void> attach(T game) async {
     build(game);
-    await game.addAll(_components);
+    await Future.wait([
+      game.addAll(_components),
+      ..._blueprints.map(game.addFromBlueprint).toList(),
+    ]);
     _isAttached = true;
   }
 
@@ -41,8 +46,23 @@ abstract class Blueprint<T extends FlameGame> {
     _components.add(component);
   }
 
+  /// Adds a list of [Blueprint]s to this blueprint.
+  void addAllBlueprints(List<Blueprint> blueprints) {
+    assert(!_isAttached, _attachedErrorMessage);
+    _blueprints.addAll(blueprints);
+  }
+
+  /// Adds a single [Blueprint] to this blueprint.
+  void addBlueprint(Blueprint blueprint) {
+    assert(!_isAttached, _attachedErrorMessage);
+    _blueprints.add(blueprint);
+  }
+
   /// Returns a copy of the components built by this blueprint
   List<Component> get components => List.unmodifiable(_components);
+
+  /// Returns a copy of the children blueprints
+  List<Blueprint> get blueprints => List.unmodifiable(_blueprints);
 }
 
 /// A [Blueprint] that provides additional

test/flame/blueprint_test.dart

@@ -14,6 +14,13 @@ class MyBlueprint extends Blueprint {
   }
 }
 
+class MyComposedBlueprint extends Blueprint {
+  @override
+  void build(_) {
+    addBlueprint(MyBlueprint());
+  }
+}
+
 class MyForge2dBlueprint extends Forge2DBlueprint {
   @override
   void build(_) {
@@ -24,6 +31,11 @@ class MyForge2dBlueprint extends Forge2DBlueprint {
 
 void main() {
   group('Blueprint', () {
+    setUpAll(() {
+      registerFallbackValue(MyBlueprint());
+      registerFallbackValue(Component());
+    });
+
     test('components can be added to it', () {
       final blueprint = MyBlueprint()..build(MockPinballGame());
 
@@ -38,6 +50,14 @@ void main() {
       verify(() => mockGame.addAll(any())).called(1);
     });
 
+    test('adds components from a child Blueprint the to a game on attach', () {
+      final mockGame = MockPinballGame();
+      when(() => mockGame.addAll(any())).thenAnswer((_) async {});
+      MyComposedBlueprint().attach(mockGame);
+
+      verify(() => mockGame.addAll(any())).called(2);
+    });
+
     test(
       'throws assertion error when adding to an already attached blueprint',
       () async {