Add game_template (#1180)

Adds a template / sample for games built in Flutter, with all the bells and whistles, like ads, in-app purchases, audio, main menu, settings, and so on.

Co-authored-by: Parker Lougheed
Co-authored-by: Shams Zakhour

game_template/lib/src/app_lifecycle/app_lifecycle.dart

@@ -0,0 +1,63 @@
+// Copyright 2022, the Flutter project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:flutter/widgets.dart';
+import 'package:logging/logging.dart';
+import 'package:provider/provider.dart';
+
+class AppLifecycleObserver extends StatefulWidget {
+  final Widget child;
+
+  const AppLifecycleObserver({required this.child, Key? key}) : super(key: key);
+
+  @override
+  _AppLifecycleObserverState createState() => _AppLifecycleObserverState();
+}
+
+class _AppLifecycleObserverState extends State<AppLifecycleObserver>
+    with WidgetsBindingObserver {
+  static final _log = Logger('AppLifecycleObserver');
+
+  final ValueNotifier<AppLifecycleState> lifecycleListenable =
+      ValueNotifier(AppLifecycleState.inactive);
+
+  @override
+  Widget build(BuildContext context) {
+    // Using InheritedProvider because we don't want to use Consumer
+    // or context.watch or anything like that to listen to this. We want
+    // to manually add listeners. We're interested in the _events_ of lifecycle
+    // state changes, and not so much in the state itself. (For example,
+    // we want to stop sound when the app goes into the background, and
+    // restart sound again when the app goes back into focus. We're not
+    // rebuilding any widgets.)
+    //
+    // Provider, by default, throws when one
+    // is trying to provide a Listenable (such as ValueNotifier) without using
+    // something like ValueListenableProvider. InheritedProvider is more
+    // low-level and doesn't have this problem.
+    return InheritedProvider<ValueNotifier<AppLifecycleState>>.value(
+      value: lifecycleListenable,
+      child: widget.child,
+    );
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    _log.info(() => 'didChangeAppLifecycleState: $state');
+    lifecycleListenable.value = state;
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance!.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance!.addObserver(this);
+    _log.info('Subscribed to app lifecycle updates');
+  }
+}