Added playing song info view and view model.

Harmonia.UI/Views/MainWindow.axaml.cs

@@ -1,11 +1,61 @@
 using Avalonia.Controls;
+using Harmonia.Core.Engine;
+using Harmonia.Core.Models;
+using Harmonia.Core.Player;
+using System.ComponentModel;
+using System.Linq;
 
 namespace Harmonia.UI.Views;
 
 public partial class MainWindow : Window
 {
-    public MainWindow()
+    private readonly IAudioPlayer _audioPlayer;
+
+    private const string ApplicationTitle = "Harmonia";
+
+    public MainWindow(IAudioPlayer audioPlayer)
     {
+        _audioPlayer = audioPlayer;
+        _audioPlayer.PropertyChanged += OnAudioPlayerPropertyChanged;
+
+        Title = ApplicationTitle;
+
         InitializeComponent();
     }
+
+    private void OnAudioPlayerPropertyChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        switch (e.PropertyName)
+        {
+            case nameof(_audioPlayer.State):
+                Title = GetUpdatedAppTitle();
+                //PropertyChanged(nameof(Title));
+                break;
+        }
+    }
+
+    private string GetUpdatedAppTitle()
+    {
+        return _audioPlayer.State switch
+        {
+            AudioPlaybackState.Stopped => ApplicationTitle,
+            _ => $"{GetSongArtistsAndTitle()} - {ApplicationTitle}"
+        };
+    }
+
+    private string GetSongArtistsAndTitle()
+    {
+        if (_audioPlayer.PlayingSong == null)
+            return string.Empty;
+
+        Song song = _audioPlayer.PlayingSong.Song;
+
+        string[] values =
+        [
+            string.Join(" / ", song.Artists ?? song.AlbumArtists),
+            song.Title ?? song.ShortFileName
+        ];
+
+        return string.Join(" - ", values.Where(value => string.IsNullOrWhiteSpace(value) == false));
+    }
 }