Added AvaloniaUI project.

Harmonia.UI/ViewModels/MainViewModel.cs

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+namespace Harmonia.UI.ViewModels;
+
+public partial class MainViewModel : ViewModelBase
+{
+    public string Greeting => "Welcome to Avalonia!";
+}

Harmonia.UI/ViewModels/PlaybackBarViewModel.cs

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+using Avalonia.Media.Imaging;
+using Avalonia.Threading;
+using Harmonia.Core.Caching;
+using Harmonia.Core.Imaging;
+using Harmonia.Core.Models;
+using Harmonia.Core.Player;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Harmonia.UI.ViewModels;
+
+public partial class PlaybackBarViewModel : ViewModelBase
+{
+    private readonly IAudioPlayer _audioPlayer;
+    private readonly IAudioImageCache _audioImageCache;
+
+    private bool _isPositionChangeInProgress;
+    private CancellationTokenSource? _audioImageCancellationTokenSource;
+
+    private Song? _song;
+    public Song? Song
+    {
+        get
+        {
+            return _song;
+        }
+        private set
+        {
+            _song = value;
+            OnPropertyChanged();
+
+            CurrentPosition = 0; // What if player is being loaded and returning to save state?
+            Position = 0;
+            MaxPosition = value?.Length.TotalSeconds ?? 0;
+        }
+    }
+
+    private Bitmap? _songImageSource;
+    public Bitmap? SongImageSource
+    {
+        get
+        {
+            return _songImageSource;
+        }
+        private set
+        {
+            _songImageSource = value;
+            OnPropertyChanged();
+        }
+    }
+
+    private double _currentPosition;
+    public double CurrentPosition
+    {
+        get
+        {
+            return _currentPosition;
+        }
+        set
+        {
+            _currentPosition = value;
+            OnPropertyChanged();
+
+            if (_isPositionChangeInProgress)
+                _audioPlayer.Position = value;
+        }
+    }
+
+    private double _position;
+    public double Position
+    {
+        get
+        {
+            return _position;
+        }
+        private set
+        {
+            _position = value;
+            OnPropertyChanged();
+        }
+    }
+
+    private double _maxPosition;
+    public double MaxPosition
+    {
+        get
+        {
+            return _maxPosition;
+        }
+        set
+        {
+            _maxPosition = value;
+            OnPropertyChanged();
+        }
+    }
+
+    public bool IsRandom
+    {
+        get
+        {
+            return _audioPlayer.IsRandom;
+        }
+        private set
+        {
+            _audioPlayer.IsRandom = value;
+            OnPropertyChanged();
+        }
+    }
+
+    public string Greeting => "Welcome to Harmonia!";
+
+    public PlaybackBarViewModel(IAudioPlayer audioPlayer, IAudioImageCache audioImageCache)
+    {
+        _audioPlayer = audioPlayer;
+        _audioPlayer.PlayingSongChanged += OnAudioPlayerPlayingSongChanged;
+
+        _audioImageCache = audioImageCache;
+    }
+
+    private void OnAudioPlayerPlayingSongChanged(object? sender, EventArgs e)
+    {
+        Song = _audioPlayer.PlayingSong?.Song;
+        Task.Run(UpdateImage);
+    }
+
+    private async Task UpdateImage()
+    {
+        // TODO: Show default picture
+        if (Song == null)
+            return;
+
+        if (_audioImageCancellationTokenSource != null)
+            await _audioImageCancellationTokenSource.CancelAsync();
+
+        _audioImageCancellationTokenSource = new();
+        CancellationToken cancellationToken = _audioImageCancellationTokenSource.Token;
+
+        SongPictureInfo? songPictureInfo = await _audioImageCache.GetAsync(Song, cancellationToken);
+
+        if (songPictureInfo == null)
+            return;
+
+        await Dispatcher.UIThread.InvokeAsync(() => SetSongImageSource(songPictureInfo));
+    }
+
+    private void SetSongImageSource(SongPictureInfo songPictureInfo)
+    {
+        SongImageSource = new(songPictureInfo.Stream);
+    }
+
+    public void Play()
+    {
+        _audioPlayer.Play();
+    }
+
+    public void Pause()
+    {
+        _audioPlayer.Pause();
+    }
+
+    public void Stop()
+    {
+        _audioPlayer.Stop();
+        CurrentPosition = 0;
+        Position = 0;
+    }
+
+    public void Previous()
+    {
+        _audioPlayer.PreviousAsync();
+    }
+
+    public void Next()
+    {
+        _audioPlayer.NextAsync();
+    }
+}

Harmonia.UI/ViewModels/ViewModelBase.cs

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+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Harmonia.UI.ViewModels;
+
+public class ViewModelBase : ObservableObject
+{
+}

Harmonia.UI/ViewModels/ViewModelLocator.cs

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+using Microsoft.Extensions.DependencyInjection;
+
+namespace Harmonia.UI.ViewModels;
+
+public class ViewModelLocator
+{
+    public static MainViewModel MainViewModel
+        => App.ServiceProvider.GetRequiredService<MainViewModel>();
+
+    public static PlaybackBarViewModel PlaybackBarViewModel
+        => App.ServiceProvider.GetRequiredService<PlaybackBarViewModel>();
+}