Added a fallback image property to ImageBrushLoader

AsyncImageLoader.Avalonia/ImageBrushLoader.cs

@@ -25,9 +25,11 @@ private static async void OnSourceChanged(ImageBrush imageBrush, AvaloniaPropert
 
         Bitmap? bitmap = null;
         try {
-            if (!string.IsNullOrWhiteSpace(newValue)) {
+            if (!string.IsNullOrWhiteSpace(newValue))
                 bitmap = await AsyncImageLoader.ProvideImageAsync(newValue!);
-            }
+
+            if (bitmap == null && GetFallbackImage(imageBrush) is Bitmap fallback)
+                bitmap = fallback;
         }
         catch (Exception e) {
             Logger?.Log("ImageBrushLoader", "ImageBrushLoader image resolution failed: {0}", e);
@@ -42,6 +44,32 @@ private static async void OnSourceChanged(ImageBrush imageBrush, AvaloniaPropert
     public static readonly AttachedProperty<string?> SourceProperty =
         AvaloniaProperty.RegisterAttached<ImageBrush, string?>("Source", typeof(ImageLoader));
 
+    /// <summary>
+    /// Attached property that provides a fallback <see cref="Bitmap"/> to use when <see cref="SourceProperty"/> is null or empty.
+    /// </summary>
+    public static readonly AttachedProperty<Bitmap?> FallbackImageProperty =
+        AvaloniaProperty.RegisterAttached<ImageBrush, Bitmap?>("FallbackImage", typeof(Bitmap));
+
+    /// <summary>
+    /// Gets the fallback <see cref="Bitmap"/> attached to the specified <see cref="ImageBrush"/>.
+    /// Returns <c>null</c> if no fallback image has been set.
+    /// </summary>
+    /// <param name="element">The <see cref="ImageBrush"/> to read the fallback image from.</param>
+    /// <returns>The fallback <see cref="Bitmap"/>, or <c>null</c> if none is set.</returns>
+    public static Bitmap? GetFallbackImage(ImageBrush element) {
+        return element.GetValue(FallbackImageProperty);
+    }
+
+    /// <summary>
+    /// Sets the fallback <see cref="Bitmap"/> on the specified <see cref="ImageBrush"/>.
+    /// The fallback image is used when the <see cref="SourceProperty"/> value is null or empty.
+    /// </summary>
+    /// <param name="element">The <see cref="ImageBrush"/> to set the fallback image on.</param>
+    /// <param name="value">The <see cref="Bitmap"/> to use as the fallback</param>
+    public static void SetFallbackImage(ImageBrush element, Bitmap? value) {
+        element.SetValue(FallbackImageProperty, value);
+    }
+
     public static string? GetSource(ImageBrush element) {
         return element.GetValue(SourceProperty);
     }