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/player_progress/persistence/local_storage_player_progress_persistence.dart

@@ -0,0 +1,26 @@
+// 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:shared_preferences/shared_preferences.dart';
+
+import 'player_progress_persistence.dart';
+
+/// An implementation of [PlayerProgressPersistence] that uses
+/// `package:shared_preferences`.
+class LocalStoragePlayerProgressPersistence extends PlayerProgressPersistence {
+  final Future<SharedPreferences> instanceFuture =
+      SharedPreferences.getInstance();
+
+  @override
+  Future<int> getHighestLevelReached() async {
+    final prefs = await instanceFuture;
+    return prefs.getInt('highestLevelReached') ?? 0;
+  }
+
+  @override
+  Future<void> saveHighestLevelReached(int level) async {
+    final prefs = await instanceFuture;
+    await prefs.setInt('highestLevelReached', level);
+  }
+}

game_template/lib/src/player_progress/persistence/memory_player_progress_persistence.dart

@@ -0,0 +1,23 @@
+// 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 'player_progress_persistence.dart';
+
+/// An in-memory implementation of [PlayerProgressPersistence].
+/// Useful for testing.
+class MemoryOnlyPlayerProgressPersistence implements PlayerProgressPersistence {
+  int level = 0;
+
+  @override
+  Future<int> getHighestLevelReached() async {
+    await Future<void>.delayed(const Duration(milliseconds: 500));
+    return level;
+  }
+
+  @override
+  Future<void> saveHighestLevelReached(int level) async {
+    await Future<void>.delayed(const Duration(milliseconds: 500));
+    this.level = level;
+  }
+}

game_template/lib/src/player_progress/persistence/player_progress_persistence.dart

@@ -0,0 +1,13 @@
+// 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.
+
+/// An interface of persistence stores for the player's progress.
+///
+/// Implementations can range from simple in-memory storage through
+/// local preferences to cloud saves.
+abstract class PlayerProgressPersistence {
+  Future<int> getHighestLevelReached();
+
+  Future<void> saveHighestLevelReached(int level);
+}