feat(flame_audio): Adding an easy way of updating the prefix from FlameAudio (#2751)

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# Description
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It was already possible to update the prefixes of FlameAudio assets, but
it required a certain understanding of `FlameAudio` and `Audioplayers`.

This PR adds a simple helper method that makes it easier to change
without requiring much context of the internals of the package.

## Checklist
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- [x] I have followed the [Contributor Guide] when preparing my PR.
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- [ ] I have updated/added relevant documentation in `docs` and added
dartdoc comments with `///`.
- [ ] I have updated/added relevant examples in `examples` or `docs`.


## Breaking Change?
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- [x] No, this PR is not a breaking change.

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packages/flame_audio/lib/flame_audio.dart

@@ -4,13 +4,46 @@ import 'package:flame_audio/bgm.dart';
 
 export 'package:audioplayers/audioplayers.dart';
 
+/// A typedef for a function that creates an [AudioCache] instance.
+typedef AudioCacheFactory = AudioCache Function({required String prefix});
+
+/// A typedef for a function that creates a [Bgm] instance.
+typedef BgmFactory = Bgm Function({required AudioCache audioCache});
+
 /// This utility class holds static references to some global audio objects.
 ///
 /// You can use as a helper to very simply play a sound or a background music.
 /// Alternatively you can create your own instances and control them yourself.
 class FlameAudio {
+  /// The factory used to create the global [AudioCache] instance.
+  ///
+  /// Useful to override the default [AudioCache] constructor in testing
+  /// or edge cases where the developer needs control on how [AudioCache]
+  /// are created.
+  static AudioCacheFactory audioCacheFactory = AudioCache.new;
+
+  /// The factory used to create the global [Bgm] instance.
+  ///
+  /// Useful to override the default [Bgm] constructor in testing
+  /// or edge cases where the developer needs control on how [Bgm]
+  /// are created.
+  static BgmFactory bgmFactory = Bgm.new;
+
   /// Access a shared instance of the [AudioCache] class.
-  static AudioCache audioCache = AudioCache(prefix: 'assets/audio/');
+  static AudioCache audioCache = audioCacheFactory(
+    prefix: 'assets/audio/',
+  );
+
+  /// Access a shared instance of the [Bgm] class.
+  ///
+  /// This will use the same global audio cache from [FlameAudio].
+  static final Bgm bgm = bgmFactory(audioCache: audioCache);
+
+  /// Updates the prefix in the global [AudioCache] and [bgm] instances.
+  static void updatePrefix(String prefix) {
+    audioCache.prefix = prefix;
+    bgm.audioPlayer.audioCache.prefix = prefix;
+  }
 
   static Future<AudioPlayer> _preparePlayer(
     String file,
@@ -74,11 +107,6 @@ class FlameAudio {
     );
   }
 
-  /// Access a shared instance of the [Bgm] class.
-  ///
-  /// This will use the same global audio cache from [FlameAudio].
-  static final Bgm bgm = Bgm(audioCache: audioCache);
-
   /// Creates a new Audio Pool using Flame's global Audio Cache.
   static Future<AudioPool> createPool(
     String sound, {

packages/flame_audio/pubspec.yaml

@@ -22,3 +22,6 @@ dependencies:
 dev_dependencies:
   dartdoc: ^6.2.2
   flame_lint: ^1.1.0
+  flutter_test:
+    sdk: flutter
+  mocktail: ^1.0.0

packages/flame_audio/test/flame_audio_test.dart

@@ -0,0 +1,43 @@
+import 'package:flame_audio/bgm.dart';
+import 'package:flame_audio/flame_audio.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mocktail/mocktail.dart';
+
+class _MockAudioCache extends Mock implements AudioCache {}
+
+class _MockAudioPlayer extends Mock implements AudioPlayer {}
+
+class _MockBgmCache extends Mock implements Bgm {}
+
+void main() {
+  group('FlameAudio', () {
+    test('starts the audioCache with the default prefix', () {
+      expect(
+        FlameAudio.audioCache.prefix,
+        equals('assets/audio/'),
+      );
+    });
+
+    group('updatePrefix', () {
+      test('updates the prefix on both bgm and audioCache', () {
+        final audioCache = _MockAudioCache();
+
+        FlameAudio.audioCache = audioCache;
+
+        final bgm = _MockBgmCache();
+        final bgmAudioPlayer = _MockAudioPlayer();
+        when(() => bgm.audioPlayer).thenReturn(bgmAudioPlayer);
+        final bgmAudioCache = _MockAudioCache();
+        when(() => bgmAudioPlayer.audioCache).thenReturn(bgmAudioCache);
+
+        FlameAudio.bgmFactory = ({required AudioCache audioCache}) => bgm;
+
+        const newPrefix = 'newPrefix/';
+        FlameAudio.updatePrefix(newPrefix);
+
+        verify(() => audioCache.prefix = newPrefix).called(1);
+        verify(() => bgmAudioCache.prefix = newPrefix).called(1);
+      });
+    });
+  });
+}