Added AvaloniaUI project.

Harmonia.UI/App.axaml

@@ -0,0 +1,18 @@
+<Application xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+			 xmlns:vm="clr-namespace:Harmonia.UI.ViewModels"
+             x:Class="Harmonia.UI.App"
+             RequestedThemeVariant="Default">
+             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->
+
+    <Application.Styles>
+        <FluentTheme />
+    </Application.Styles>
+
+	<Application.Resources>
+		<ResourceDictionary>
+			<vm:ViewModelLocator x:Key="Locator" />
+	    </ResourceDictionary>
+	</Application.Resources>
+
+</Application>

Harmonia.UI/App.axaml.cs

@@ -0,0 +1,62 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Data.Core.Plugins;
+using Avalonia.Markup.Xaml;
+using Harmonia.Core.Extensions;
+using Harmonia.UI.ViewModels;
+using Harmonia.UI.Views;
+using Microsoft.Extensions.DependencyInjection;
+using System;
+
+namespace Harmonia.UI;
+
+public partial class App : Application
+{
+    public static IServiceProvider ServiceProvider { get; private set; }
+
+    static App()
+    {
+        ServiceCollection services = new();
+
+        services.AddSingleton<MainViewModel>();
+        services.AddSingleton<MainWindow>();
+        services.AddSingleton<PlaybackBarViewModel>();
+
+        services.AddHarmonia();
+
+        ServiceProvider = services.BuildServiceProvider();
+    }
+
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        // Line below is needed to remove Avalonia data validation.
+        // Without this line you will get duplicate validations from both Avalonia and CT
+        BindingPlugins.DataValidators.RemoveAt(0);
+
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            //desktop.MainWindow = new MainWindow
+            //{
+            //    DataContext = new MainViewModel()
+            //};
+
+            desktop.MainWindow = ServiceProvider.GetRequiredService<MainWindow>();
+        }
+        else if (ApplicationLifetime is ISingleViewApplicationLifetime singleViewPlatform)
+        {
+            //singleViewPlatform.MainView = new MainView
+            //{
+            //    DataContext = new MainViewModel()
+            //};
+
+            singleViewPlatform.MainView = ServiceProvider.GetRequiredService<MainWindow>();
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}

Harmonia.UI/Harmonia.UI.csproj

@@ -0,0 +1,33 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <AvaloniaResource Include="Assets\**" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia" Version="$(AvaloniaVersion)" />
+    <PackageReference Include="Avalonia.Themes.Fluent" Version="$(AvaloniaVersion)" />
+    <PackageReference Include="Avalonia.Fonts.Inter" Version="$(AvaloniaVersion)" />
+    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.2.0" />
+
+    <!--Condition below is needed to remove Avalonia.Diagnostics package from build output in Release configuration.-->
+    <PackageReference Condition="'$(Configuration)' == 'Debug'" Include="Avalonia.Diagnostics" Version="$(AvaloniaVersion)" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="9.0.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Harmonia.Core\Harmonia.Core.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Update="Views\PlaybackBar.axaml.cs">
+      <DependentUpon>PlaybackBar.axaml</DependentUpon>
+    </Compile>
+  </ItemGroup>
+</Project>

Harmonia.UI/ViewModels/MainViewModel.cs

@@ -0,0 +1,6 @@
+namespace Harmonia.UI.ViewModels;
+
+public partial class MainViewModel : ViewModelBase
+{
+    public string Greeting => "Welcome to Avalonia!";
+}

Harmonia.UI/ViewModels/PlaybackBarViewModel.cs

@@ -0,0 +1,178 @@
+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

@@ -0,0 +1,7 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Harmonia.UI.ViewModels;
+
+public class ViewModelBase : ObservableObject
+{
+}

Harmonia.UI/ViewModels/ViewModelLocator.cs

@@ -0,0 +1,12 @@
+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>();
+}

Harmonia.UI/Views/MainView.axaml

@@ -0,0 +1,22 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:vm="clr-namespace:Harmonia.UI.ViewModels"
+			 xmlns:views="clr-namespace:Harmonia.UI.Views"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+			 DataContext="{x:Static vm:ViewModelLocator.MainViewModel}"
+             x:Class="Harmonia.UI.Views.MainView"
+             x:DataType="vm:MainViewModel">
+  <Design.DataContext>
+    <!-- This only sets the DataContext for the previewer in an IDE,
+         to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
+    <vm:MainViewModel />
+  </Design.DataContext>
+
+	<StackPanel>
+		<TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
+		<views:PlaybackBar></views:PlaybackBar>
+	</StackPanel>
+
+</UserControl>

Harmonia.UI/Views/MainView.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace Harmonia.UI.Views;
+
+public partial class MainView : UserControl
+{
+    public MainView()
+    {
+        InitializeComponent();
+    }
+}

Harmonia.UI/Views/MainWindow.axaml

@@ -0,0 +1,12 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:Harmonia.UI.ViewModels"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:views="clr-namespace:Harmonia.UI.Views"
+        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+        x:Class="Harmonia.UI.Views.MainWindow"
+        Icon="/Assets/avalonia-logo.ico"
+        Title="Harmonia.UI">
+        <views:MainView />
+</Window>

Harmonia.UI/Views/MainWindow.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace Harmonia.UI.Views;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+}

Harmonia.UI/Views/PlaybackBar.axaml

@@ -0,0 +1,17 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:vm="clr-namespace:Harmonia.UI.ViewModels"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+			 DataContext="{x:Static vm:ViewModelLocator.PlaybackBarViewModel}"
+             x:Class="Harmonia.UI.Views.PlaybackBar"
+             x:DataType="vm:PlaybackBarViewModel">
+  <Design.DataContext>
+    <!-- This only sets the DataContext for the previewer in an IDE,
+         to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
+    <vm:MainViewModel />
+  </Design.DataContext>
+
+  <TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
+</UserControl>

Harmonia.UI/Views/PlaybackBar.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace Harmonia.UI.Views;
+
+public partial class PlaybackBar : UserControl
+{
+    public PlaybackBar()
+    {
+        InitializeComponent();
+    }
+}