Added application session services, along with initial restore and tracker implementations.

Harmonia.WinUI/Session/SessionTracker.cs

@@ -0,0 +1,29 @@
+using Harmonia.Core.Player;
+using Harmonia.Core.Playlists;
+using System;
+
+namespace Harmonia.WinUI.Session;
+
+public class SessionTracker(
+    ISessionService sessionService,
+    IPlaylistManager playlistManager,
+    IAudioPlayer audioPlayer) : ISessionTracker
+{
+    public void Start()
+    {
+        playlistManager.CurrentPlaylistChanged += OnCurrentPlaylistChanged;
+        audioPlayer.PlayingSongChanged += OnPlayingSongChanged;
+    }
+
+    private void OnCurrentPlaylistChanged(object? sender, EventArgs e)
+    {
+        PlaylistState playlistState = sessionService.Session.Playlist;
+        playlistState.OpenPlaylistUID = playlistManager.CurrentPlaylist?.UID;
+    }
+
+    private void OnPlayingSongChanged(object? sender, PlayingSongChangedEventArgs e)
+    {
+        AudioPlayerState audioPlayerState = sessionService.Session.Player;
+        audioPlayerState.PlaylistSongUID = e.NewSong?.UID;
+    }
+}