avalonia

name	avalonia
description	Avalonia UI reference guide for PhotoManager: UI conventions, MVVM patterns, and cross-platform UI patterns. Use this skill when working on UI code, adding image-processing methods, or needing to understand the Avalonia UI layer of the application.

You are an expert on the Avalonia UI usage in PhotoManager. Answer or implement the following: ${input:question:What Avalonia UI topic or task can I help you with?}

Use the facts below as your authoritative reference.

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UI Project Structure

PhotoManager.UI/
├── App.axaml / App.axaml.cs       — DI container, Serilog setup, single-instance enforcement
├── Program.cs                      — Application entry point (BuildAvaloniaApp)
├── Windows/                        — {Feature}Window  (ShowDialog = modal, Show = modeless)
│   ├── MainWindow.axaml/.cs        — Main shell, async catalog background work
│   ├── FindDuplicatedAssetsWindow  — Duplicate detection workflow
│   ├── SyncAssetsWindow            — Sync workflow
│   ├── FolderNavigationWindow      — Folder picker
│   └── AboutWindow                 — About dialog
├── Controls/                       — {Feature}UserControl  (reusable composed controls)
│   ├── ViewerUserControl           — Full-screen image viewer
│   ├── ThumbnailsUserControl       — Virtualized thumbnail list
│   └── FolderNavigationControl     — TreeView folder browser
├── ViewModels/                     — {Feature}ViewModel, base classes, Enums/
├── Converters/                     — {Feature}Converter
├── Models/                         — {Action}EventHandler delegates + {Action}EventArgs classes
├── Services/                       — UI-layer services (single-instance, theme)
└── Configuration/                  — UI settings classes

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MVVM Patterns

ViewModel Base Classes

• BaseViewModel — implements INotifyPropertyChanged; call NotifyPropertyChanged(nameof(Prop))
• BaseProcessViewModel<TC, TR> — for multi-step process workflows (Describe → Configure → Run → Results)
• All ViewModels inherit one of the above; no external MVVM framework is used

Property Pattern (C# 11 field keyword)

public SortCriteria SortCriteria
{
    get;
    private set
    {
        field = value;
        NotifyPropertyChanged(nameof(SortCriteria));
    }
}

Visibility — Prefer Computed bool Properties

public bool IsThumbnailsVisible => AppMode == AppMode.Thumbnails;

Collections

• ObservableCollection<T> — dynamic UI lists
• SortableObservableCollection<T> (project type in ViewModels/) — raises Reset on Sort()

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Dependency Injection

• DI chain in App.axaml.cs: AddInfrastructure() → AddDomain() → AddApplication() → AddUi()
• App.ServiceProvider (static) is the escape hatch for AXAML-instantiated UserControls:

  _logger = App.ServiceProvider?.GetService<ILogger<ThumbnailsUserControl>>()
            ?? NullLogger<ThumbnailsUserControl>.Instance;
  

• Singletons registered in UiServiceCollectionExtensions: MainWindow, ApplicationViewModel
• Dialog windows and their ViewModels are created on-demand in code-behind (not via DI):

  FindDuplicatedAssetsViewModel vm = new(_application);
  FindDuplicatedAssetsWindow window = new(vm, _loggerFactory.CreateLogger<FindDuplicatedAssetsWindow>());
  await window.ShowDialog(this);
  

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Async & Thread Safety

• Background work launched in window Opened event handler
• Always use Dispatcher.UIThread.InvokeAsync() to marshal domain callbacks back to the UI thread
• Always use .ConfigureAwait(true) in UI async methods (preserves synchronization context)

_catalogTask = ViewModel.CatalogAssets(
    e => Dispatcher.UIThread.InvokeAsync(() => ViewModel.NotifyCatalogChange(e)),
    _cancellationTokenSource.Token);
await _catalogTask.ConfigureAwait(true);

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Value Converters

All converters live in Converters/ and follow this pattern:

Converter	Input → Output
FileNameConverter	string → escapes _ for display
FileSizeConverter	long (bytes) → formatted KB/MB/GB string
PixelSizeConverter	Asset → "W×H pixels" string
TernaryConverter	(bool, object) multi-binding → conditional value
VisibilityConverter	type name → true or false (IsVisible)

• ConvertBack always throw new NotImplementedException() for read-only converters
• Register as resources in the AXAML that uses them

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Custom Events (in Models/)

public delegate void FolderAddedEventHandler(object sender, FolderAddedEventArgs e);

public class FolderAddedEventArgs : EventArgs
{
    public required Folder Folder { get; init; }
}

Pattern: {Action}EventHandler delegate + {Action}EventArgs class.

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AXAML Patterns

• Theme: Avalonia Fluent theme with configurable System/Light/Dark mode
• Global styles in App.axaml (Application.Styles)
• Performance: VirtualizingStackPanel in ItemsControl for large photo lists
• DataGrid: Avalonia.Controls.DataGrid for tabular data
• No resource dictionaries beyond App.axaml for global resources
• Image rendering: Use Stretch="Uniform" or Stretch="UniformToFill" on Image controls

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Cross-Platform Image Handling

The image pipeline uses SkiaSharp for cross-platform image operations (replacing WPF's BitmapImage):

• BitmapHelper.cs uses SkiaSharp (SKBitmap, SKImage, SKCodec) for image loading and rotation
• ExifHelper.cs uses SkiaSharp for EXIF orientation reading
• Image data is passed as byte[] between layers, decoded to Avalonia.Media.Imaging.Bitmap only in the UI layer
• The Rotation enum is a custom domain enum (not WPF's System.Windows.Media.Imaging.Rotation)

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Naming Conventions

Element	Pattern	Example
Window	{Feature}Window	FindDuplicatedAssetsWindow
UserControl	{Feature}UserControl	ThumbnailsUserControl
ViewModel	{Feature}ViewModel	ApplicationViewModel
Process ViewModel	{Feature}ProcessViewModel	SyncAssetsViewModel
Converter	{Feature}Converter	FileSizeConverter
Event delegate	{Action}EventHandler	FolderAddedEventHandler
Event args	{Action}EventArgs	FolderAddedEventArgs