Show total songs and total duration for each playlist.

Harmonia.WinUI/ViewModels/PlaylistItemViewModel.cs

@@ -0,0 +1,47 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+using Harmonia.Core.Playlists;
+using System;
+using System.Linq;
+
+namespace Harmonia.WinUI.ViewModels;
+
+public partial class PlaylistItemViewModel : ObservableObject
+{
+    public Playlist Playlist { get; private set; }
+
+    private string _summary = string.Empty;
+    public string Summary
+    {
+        get
+        {
+            return _summary;
+        }
+        set
+        {
+            SetProperty(ref _summary, value);
+        }
+    }
+
+    public PlaylistItemViewModel(Playlist playlist)
+    {
+        Playlist = playlist;
+        Playlist.PlaylistUpdated += OnPlaylistUpdated;
+        UpdateMetaData();
+    }
+
+    public void Unsubscribe()
+    {
+        Playlist.PlaylistUpdated -= OnPlaylistUpdated;
+    }
+
+    private void OnPlaylistUpdated(object? sender, EventArgs e)
+    {
+        UpdateMetaData();
+    }
+
+    private void UpdateMetaData()
+    {
+        TimeSpan total = TimeSpan.FromSeconds(Playlist.Songs.Sum(s => s.Song.Length.TotalSeconds));
+        Summary = $"{Playlist.Songs.Count} songs / {total:h\\:mm\\:ss}";
+    }
+}