Implement WriteableBitmap caching/reuse and disposal in ColorSlider

src/Avalonia.Controls.ColorPicker/ColorSlider/ColorSlider.cs

@@ -2,6 +2,7 @@
 using Avalonia.Controls.Metadata;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Media.Imaging;
 using Avalonia.Utilities;
 
 namespace Avalonia.Controls.Primitives
@@ -31,6 +32,8 @@ namespace Avalonia.Controls.Primitives
 
         protected bool ignorePropertyChanged = false;
 
+        private WriteableBitmap? _backgroundBitmap;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="ColorSlider"/> class.
         /// </summary>
@@ -38,6 +41,26 @@ namespace Avalonia.Controls.Primitives
         {
         }
 
+        /// <inheritdoc/>
+        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToVisualTree(e);
+
+            // Bitmaps were released when detached from the visual tree so they must be re-built
+            UpdateBackground();
+        }
+
+        /// <inheritdoc/>
+        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnDetachedFromVisualTree(e);
+
+            // Clean-up all bitmaps
+            // https://github.com/AvaloniaUI/Avalonia/issues/9051
+            _backgroundBitmap?.Dispose();
+            _backgroundBitmap = null;
+        }
+
         /// <summary>
         /// Updates the visual state of the control by applying latest PseudoClasses.
         /// </summary>
@@ -110,7 +133,19 @@ namespace Avalonia.Controls.Primitives
 
                 if (bitmap != null)
                 {
-                    Background = new ImageBrush(ColorPickerHelpers.CreateBitmapFromPixelData(bitmap, pixelWidth, pixelHeight));
+                    if (_backgroundBitmap != null)
+                    {
+                        // Re-use the existing WriteableBitmap
+                        // This assumes the height, width and byte counts are the same and must be set to null
+                        // elsewhere if that assumption is ever not true.
+                        ColorPickerHelpers.UpdateBitmapFromPixelData(_backgroundBitmap, bitmap);
+                    }
+                    else
+                    {
+                        _backgroundBitmap = ColorPickerHelpers.CreateBitmapFromPixelData(bitmap, pixelWidth, pixelHeight);
+                    }
+
+                    Background = new ImageBrush(_backgroundBitmap);
                 }
             }
         }
@@ -399,6 +434,11 @@ namespace Avalonia.Controls.Primitives
             }
             else if (change.Property == BoundsProperty)
             {
+                // If the control's overall dimensions have changed the background bitmap size also needs to change.
+                // This means the existing bitmap must be released to be recreated correctly in UpdateBackground().
+                _backgroundBitmap?.Dispose();
+                _backgroundBitmap = null;
+
                 UpdateBackground();
             }
             else if (change.Property == ValueProperty ||

src/Avalonia.Controls.ColorPicker/Helpers/ColorPickerHelpers.cs

@@ -617,7 +617,6 @@ namespace Avalonia.Controls.Primitives
                 PixelFormat.Bgra8888,
                 AlphaFormat.Premul);
 
-            // Warning: This is highly questionable
             using (var frameBuffer = bitmap.Lock())
             {
                 Marshal.Copy(bgraPixelData.ToArray(), 0, frameBuffer.Address, bgraPixelData.Count);
@@ -625,5 +624,23 @@ namespace Avalonia.Controls.Primitives
 
             return bitmap;
         }
+
+        /// <summary>
+        /// Updates the given <see cref="WriteableBitmap"/> with new, raw BGRA pre-multiplied alpha pixel data.
+        /// WARNING: The bitmap's width, height and byte count MUST not have changed and MUST be enforced externally.
+        /// </summary>
+        /// <param name="bitmap">The existing <see cref="WriteableBitmap"/> to update.</param>
+        /// <param name="bgraPixelData">The bitmap (in raw BGRA pre-multiplied alpha pixels).</param>
+        public static void UpdateBitmapFromPixelData(
+            WriteableBitmap bitmap,
+            IList<byte> bgraPixelData)
+        {
+            using (var frameBuffer = bitmap.Lock())
+            {
+                Marshal.Copy(bgraPixelData.ToArray(), 0, frameBuffer.Address, bgraPixelData.Count);
+            }
+
+            return;
+        }
     }
 }