Make some AppWidgetControl variables private

lib/logic/app_widget.dart

@@ -10,41 +10,45 @@ class AppWidgetControl extends Control {
   static AppWidgetControl instance = AppWidgetControl();
   @visibleForTesting
   static const appWidgetName = 'MusicPlayerAppWidget';
+  static const _songUriKey = 'song';
+  static const _playingKey = 'playing';
+
+  StreamSubscription<Song>? _currentSongListener;
+  StreamSubscription<bool>? _playingStateListener;
 
-  StreamSubscription<Song>? currentSongListener;
-  StreamSubscription<bool>? playingStateListener;
   /// The last song content uri sent to the widget.
-  String? lastSongContentUri;
+  String? _lastSongContentUri;
+
   /// The last playing state sent to the widget.
-  bool? lastPlayingState;
+  bool? _lastPlayingState;
 
   @override
   void init() {
     super.init();
-    lastSongContentUri = null;
-    lastPlayingState = null;
-    currentSongListener = PlaybackControl.instance.onSongChange
-        .listen((song) => update(song, MusicPlayer.instance.playing));
-    playingStateListener = MusicPlayer.instance.playingStream.listen(
-        (playing) => update(PlaybackControl.instance.currentSong, playing));
+    _lastSongContentUri = null;
+    _lastPlayingState = null;
+    _currentSongListener =
+        PlaybackControl.instance.onSongChange.listen((song) => update(song, MusicPlayer.instance.playing));
+    _playingStateListener =
+        MusicPlayer.instance.playingStream.listen((playing) => update(PlaybackControl.instance.currentSong, playing));
   }
-  
+
   @override
   void dispose() {
-    currentSongListener?.cancel();
-    playingStateListener?.cancel();
+    _currentSongListener?.cancel();
+    _playingStateListener?.cancel();
     super.dispose();
   }
 
   /// Update the widgets with the current [song] and [playing] state.
   Future<void> update(Song song, bool playing) async {
-    if (playing == lastPlayingState && song.contentUri == lastSongContentUri) {
+    if (playing == _lastPlayingState && song.contentUri == _lastSongContentUri) {
       return;
     }
-    lastSongContentUri = song.contentUri;
-    lastPlayingState = playing;
-    await HomeWidget.saveWidgetData('song', song.contentUri);
-    await HomeWidget.saveWidgetData('playing', playing);
+    _lastSongContentUri = song.contentUri;
+    _lastPlayingState = playing;
+    await HomeWidget.saveWidgetData(_songUriKey, song.contentUri);
+    await HomeWidget.saveWidgetData(_playingKey, playing);
     await HomeWidget.updateWidget(name: appWidgetName);
   }
 }