feat: initialize image viewer, add demo.

src/Ursa/Controls/ImageViewer/ImageViewer.cs

@@ -0,0 +1,153 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+using Avalonia.Media;
+
+namespace Ursa.Controls;
+
+[TemplatePart(PART_Image, typeof(Image))]
+[TemplatePart(PART_Layer, typeof(VisualLayerManager))]
+public class ImageViewer: TemplatedControl
+{
+    public const string PART_Image = "PART_Image";
+    public const string PART_Layer = "PART_Layer";
+    
+    private Image _image = null!;
+    private VisualLayerManager? _layer;
+    private Point? _lastClickPoint;
+    private Point? _lastReleasePoint;
+
+    public static readonly StyledProperty<object?> OverlayerProperty = AvaloniaProperty.Register<ImageViewer, object?>(
+        nameof(Overlayer));
+
+    public object? Overlayer
+    {
+        get => GetValue(OverlayerProperty);
+        set => SetValue(OverlayerProperty, value);
+    }
+
+    public static readonly StyledProperty<IImage?> SourceProperty = Image.SourceProperty.AddOwner<ImageViewer>();
+    public IImage? Source
+    {
+        get => GetValue(SourceProperty);
+        set => SetValue(SourceProperty, value);
+    }
+
+    private double _scale  = 1;
+
+    public static readonly DirectProperty<ImageViewer, double> ScaleProperty = AvaloniaProperty.RegisterDirect<ImageViewer, double>(
+        nameof(Scale), o => o.Scale, unsetValue: 1);
+
+    public double Scale
+    {
+        get => _scale;
+        private set => SetAndRaise(ScaleProperty, ref _scale, value);
+    }
+
+    private double _translateX;
+
+    public static readonly DirectProperty<ImageViewer, double> TranslateXProperty = AvaloniaProperty.RegisterDirect<ImageViewer, double>(
+        nameof(TranslateX), o => o.TranslateX, unsetValue: 0);
+
+    public double TranslateX
+    {
+        get => _translateX;
+        private set => SetAndRaise(TranslateXProperty, ref _translateX, value);
+    }
+
+    private double _translateY;
+
+    public static readonly DirectProperty<ImageViewer, double> TranslateYProperty = AvaloniaProperty.RegisterDirect<ImageViewer, double>(
+        nameof(TranslateY), o => o.TranslateY);
+
+    public double TranslateY
+    {
+        get => _translateY;
+        private set => SetAndRaise(TranslateYProperty, ref _translateY, value);
+    }
+    
+
+    static ImageViewer()
+    {
+        OverlayerProperty.Changed.AddClassHandler<ImageViewer>((o, e) => o.OnOverlayerChanged(e));
+        SourceProperty.Changed.AddClassHandler<ImageViewer>((o, e) => o.OnSourceChanged(e));
+    }
+    
+    private void OnOverlayerChanged(AvaloniaPropertyChangedEventArgs args)
+    {
+        var control = args.GetNewValue<object?>();
+        if (control is Control c)
+        {
+            AdornerLayer.SetAdorner(this, c);
+        }
+    }
+
+    private void OnSourceChanged(AvaloniaPropertyChangedEventArgs args)
+    {
+        IImage image = args.GetNewValue<IImage>();
+        Size size = image.Size;
+        double width = this.Width;
+        double height = this.Height;
+        Scale = Math.Min(width/size.Width, height/size.Height);
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        _image = e.NameScope.Get<Image>(PART_Image);
+        _layer = e.NameScope.Get<VisualLayerManager>(PART_Layer);
+        Scale = 1;
+        if (Overlayer is Control c)
+        {
+            AdornerLayer.SetAdorner(this, c);
+        }
+    }
+
+
+
+    protected override void OnPointerWheelChanged(PointerWheelEventArgs e)
+    {
+        base.OnPointerWheelChanged(e);
+        if(e.Delta.Y > 0)
+        {
+            Scale *= 1.1;
+        }
+        else
+        {
+            var scale = Scale;
+            scale /= 1.1;
+            if (scale < 0.1) scale = 0.1;
+            Scale = scale;
+        }
+    }
+
+    protected override void OnPointerMoved(PointerEventArgs e)
+    {
+        base.OnPointerMoved(e);
+        if (e.Pointer.Captured == this && _lastClickPoint != null)
+        {
+            Point p = e.GetPosition(this);
+            double deltaX = p.X - _lastClickPoint.Value.X;
+            double deltaY = p.Y - _lastClickPoint.Value.Y;
+            TranslateX = deltaX + (_lastReleasePoint?.X ?? 0);
+            TranslateY = deltaY + (_lastReleasePoint?.Y ?? 0);
+        }
+        
+    }
+
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        base.OnPointerPressed(e);
+        e.Pointer.Capture(this);
+        _lastClickPoint = e.GetPosition(this);
+    }
+
+    protected override void OnPointerReleased(PointerReleasedEventArgs e)
+    {
+        base.OnPointerReleased(e);
+        e.Pointer.Capture(null);
+        _lastReleasePoint = new Point(TranslateX, TranslateY);
+    }
+}