added Bgm class

lib/bgm.dart

@@ -0,0 +1,101 @@
+import 'dart:io';
+
+import 'package:audioplayers/audioplayers.dart';
+import 'package:flame/flame.dart';
+import 'package:flutter/widgets.dart';
+
+/// The looping **b**ack**g**round **m**usic class.
+///
+/// This class helps with looping background music management that reacts to
+/// application lifecycle state changes. On construction, the instance is added
+/// as an observer to the [WidgetsBinding] instance. A [dispose] function is
+/// provided in case this functionality needs to be unloaded but the app needs
+/// to keep running.
+class Bgm extends WidgetsBindingObserver {
+  AudioPlayer audioPlayer;
+  bool isPlaying = false;
+
+  Bgm() {
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  /// Dispose the [WidgetsBinding] observer.
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+  }
+
+  /// Plays and loops a background music file specified by [filename].
+  ///
+  /// The volume can be specified in the optional named parameter [volume]
+  /// where `0` means off and `1` means max.
+  ///
+  /// It is safe to call this function even when a current BGM track is
+  /// playing.
+  void play(String filename, {double volume}) async {
+    volume ??= 1;
+
+    if (audioPlayer.state != AudioPlayerState.STOPPED) {
+      audioPlayer.stop();
+    }
+
+    isPlaying = true;
+    audioPlayer = await Flame.audio.loopLongAudio(
+      filename,
+      volume: volume,
+    );
+  }
+
+  /// Stops the currently playing background music track (if any).
+  void stop() {
+
+  }
+
+  /// Resumes the currently played (but resumed) background music.
+  void resume() {
+    isPlaying = true;
+    audioPlayer.resume();
+  }
+
+  /// Pauses the background music without unloading or resetting the audio
+  /// player.
+  void pause() {
+    isPlaying = false;
+    audioPlayer.pause();
+  }
+
+  /// Prefetch an audio and store it in the cache.
+  ///
+  /// Alias of `FlameAudio.load();`.
+  Future<File> load(String file) => Flame.audio.load(file);
+
+  /// Prefetch a list of audios and store them in the cache.
+  ///
+  /// Alias of `FlameAudio.loadAll();`.
+  Future<List<File>> loadAll(List<String> files) => Flame.audio.loadAll(files);
+
+  /// Clears the file in the cache.
+  ///
+  /// Alias of `FlameAudio.clear();`.
+  void clear(String file) => Flame.audio.clear(file);
+
+  /// Clears all the audios in the cache.
+  /// Alias of `FlameAudio.clearAll();`.
+  ///
+  void clearAll() => Flame.audio.clearAll();
+
+  /// Handler for AppLifecycleState changes.
+  ///
+  /// This function handles the automatic pause and resume when the app
+  /// lifecycle state changes. There is NO NEED to call this function directly
+  /// directly.
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.resumed) {
+      if (isPlaying && audioPlayer.state == AudioPlayerState.PAUSED) {
+        audioPlayer.resume();
+      }
+    } else {
+      audioPlayer.pause();
+    }
+  }
+}