chore: modified doc for Layered and added one test for nested

packages/pinball_components/lib/src/components/layer.dart

@@ -8,6 +8,9 @@ import 'package:flutter/material.dart';
 /// [BodyComponent]s with compatible [Layer]s can collide with each other,
 /// ignoring others. This compatibility depends on bit masking operation
 /// between layers. For more information read: https://en.wikipedia.org/wiki/Mask_(computing).
+///
+/// A parent [Layered] have priority against its children's layer. Them won't be
+/// changed but will be ignored.
 /// {@endtemplate}
 mixin Layered<T extends Forge2DGame> on BodyComponent<T> {
   Layer _layer = Layer.all;

packages/pinball_components/test/src/components/layer_test.dart

@@ -14,6 +14,24 @@ class TestBodyComponent extends BodyComponent with Layered {
   }
 }
 
+class TestNestedBodyComponent extends BodyComponent with Layered {
+  TestNestedBodyComponent({required this.childLayer});
+
+  final Layer childLayer;
+
+  @override
+  Body createBody() {
+    final fixtureDef = FixtureDef(CircleShape());
+    return world.createBody(BodyDef())..createFixture(fixtureDef);
+  }
+
+  @override
+  Future<void> onLoad() async {
+    await super.onLoad();
+    await add(TestBodyComponent()..layer = childLayer);
+  }
+}
+
 void main() {
   final flameTester = FlameTester(Forge2DGame.new);
 
@@ -121,6 +139,24 @@ void main() {
         expect(component.layer, equals(Layer.all));
       },
     );
+
+    flameTester.test(
+      'nested children will keep their layer',
+      (game) async {
+        const parentLayer = Layer.jetpack;
+        const childLayer = Layer.board;
+
+        final component = TestNestedBodyComponent(childLayer: childLayer)
+          ..layer = parentLayer;
+
+        await game.ensureAdd(component);
+        expect(childLayer, isNot(equals(parentLayer)));
+
+        for (final child in component.children) {
+          expect((child as TestBodyComponent).layer, equals(childLayer));
+        }
+      },
+    );
   });
 
   group('LayerMaskBits', () {