feat: adding has mounted to component

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

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:collection';
 
 import 'package:flutter/painting.dart';
@@ -53,6 +54,8 @@ class Component {
   bool get hasChildren => _children?.isNotEmpty ?? false;
   ComponentSet? _children;
 
+  Completer<void>? _mountCompleter;
+
   @protected
   _LifecycleManager get lifecycle => _lifecycleManager ??= _LifecycleManager();
   _LifecycleManager? _lifecycleManager;
@@ -203,6 +206,17 @@ class Component {
   /// ```
   void onMount() {}
 
+  /// A future that will complete once the component is mounted on its parent
+  Future<void> get hasMounted {
+    if (isMounted) {
+      return Future.value();
+    }
+
+    _mountCompleter ??= Completer<void>();
+
+    return _mountCompleter!.future;
+  }
+
   /// This method is called periodically by the game engine to request that your
   /// component updates itself.
   ///
@@ -258,6 +272,7 @@ class Component {
   /// Changes the current parent for another parent and prepares the tree under
   /// the new root.
   void changeParent(Component component) {
+    _mountCompleter = null;
     parent?.remove(this);
     nextParent = component;
   }
@@ -403,6 +418,7 @@ class Component {
     if (existingChild || _state == LifecycleState.removed) {
       onGameResize(findGame()!.canvasSize);
     }
+    _mountCompleter?.complete();
     onMount();
     _state = LifecycleState.mounted;
     if (!existingChild) {

packages/flame/test/components/component_lifecycle_test.dart

@@ -47,6 +47,52 @@ void main() {
       );
     });
 
+    flameGame.test('component hasMounted completes', (game) async {
+      final component = _MyComponent();
+      await game.add(component);
+      final hasMounted = component.hasMounted;
+
+      await game.ready();
+
+      return expectLater(hasMounted, completes);
+    });
+
+    flameGame.test(
+      'component hasMounted completes even after the '
+      'component is already mounted',
+      (game) async {
+        final component = _MyComponent();
+        await game.add(component);
+        await game.ready();
+
+        final hasMounted = component.hasMounted;
+
+        return expectLater(hasMounted, completes);
+      },
+    );
+
+    flameGame.test(
+      'component hasMounted completes when changing parent',
+      (game) async {
+        final parent = _MyComponent('parent');
+        final child = _MyComponent('child');
+        parent.add(child);
+        game.add(parent);
+
+        var hasMounted = child.hasMounted;
+        await game.ready();
+
+        await expectLater(hasMounted, completes);
+
+        child.changeParent(game);
+        hasMounted = child.hasMounted;
+        game.update(0);
+        await game.ready();
+
+        await expectLater(hasMounted, completes);
+      },
+    );
+
     // Obsolete scenario, when we used to have a separate "prepare" stage
     flameGame.test('parent prepares the component', (game) async {
       final parent = _MyComponent('parent');