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/player_progress.dart

@@ -0,0 +1,57 @@
+// 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 'dart:async';
+
+import 'package:flutter/foundation.dart';
+
+import 'persistence/player_progress_persistence.dart';
+
+/// Encapsulates the player's progress.
+class PlayerProgress extends ChangeNotifier {
+  static const maxHighestScoresPerPlayer = 10;
+
+  final PlayerProgressPersistence _store;
+
+  int _highestLevelReached = 0;
+
+  /// Creates an instance of [PlayerProgress] backed by an injected
+  /// persistence [store].
+  PlayerProgress(PlayerProgressPersistence store) : _store = store;
+
+  /// The highest level that the player has reached so far.
+  int get highestLevelReached => _highestLevelReached;
+
+  /// Fetches the latest data from the backing persistence store.
+  Future<void> getLatestFromStore() async {
+    final level = await _store.getHighestLevelReached();
+    if (level > _highestLevelReached) {
+      _highestLevelReached = level;
+      notifyListeners();
+    } else if (level < _highestLevelReached) {
+      await _store.saveHighestLevelReached(_highestLevelReached);
+    }
+  }
+
+  /// Resets the player's progress so it's like if they just started
+  /// playing the game for the first time.
+  void reset() {
+    _highestLevelReached = 0;
+    notifyListeners();
+    _store.saveHighestLevelReached(_highestLevelReached);
+  }
+
+  /// Registers [level] as reached.
+  ///
+  /// If this is higher than [highestLevelReached], it will update that
+  /// value and save it to the injected persistence store.
+  void setLevelReached(int level) {
+    if (level > _highestLevelReached) {
+      _highestLevelReached = level;
+      notifyListeners();
+
+      unawaited(_store.saveHighestLevelReached(level));
+    }
+  }
+}