perf: Add new ArrayList to avoid extra copy creating bitmaps

src/Uno.UI/Microsoft/UI/Xaml/Controls/ColorPicker/ArrayList.cs

@@ -0,0 +1,87 @@
+using System;
+
+namespace Microsoft.UI.Xaml.Controls
+{
+	/// <summary>
+	/// Simplified implementation of List<typeparamref name="T"/> that supports direct access to the internal array.
+	/// </summary>
+	/// <remarks>
+	/// This is needed in the ColorPicker to avoid an extra copy when converting a list of bytes into a WritableBitmap.
+	/// The performance gains justify creation of this class and a slight deviation from WinUI.
+	/// This class is not considered safe to use outside of the ColorPicker so it is not in a generic location.
+	/// </remarks>
+	/// <typeparam name="T">The type of elements in the list.</typeparam>
+	internal class ArrayList<T>
+	{
+		private T[] _list;
+
+		public ArrayList()
+		{
+			_list = System.Array.Empty<T>();
+		}
+
+		public ArrayList(int capacity)
+		{
+			if (capacity < 0)
+			{
+				throw new ArgumentOutOfRangeException("Capacity cannot be less than zero.");
+			}
+            else if (capacity == 0)
+			{
+				_list = System.Array.Empty<T>();
+			}
+			else
+			{
+				_list = new T[capacity];
+			}
+        }
+
+		internal T[] Array
+		{
+			get => _list;
+		}
+
+		public int Capacity
+		{
+			set
+			{
+				if (value < Count)
+				{
+					throw new IndexOutOfRangeException("Cannot resize the list smaller than it was.");
+				}
+
+				if (value != Count)
+				{
+					if (value > 0)
+					{
+						T[] newList = new T[value];
+
+						if (Count > 0)
+						{
+							System.Array.Copy(_list, 0, newList, 0, Count);
+						}
+
+						_list = newList;
+					}
+					else
+					{
+						_list = System.Array.Empty<T>();
+					}
+				}
+			}
+		}
+
+		public int Count { get; private set; } = 0;
+
+		public void Add(T item)
+		{
+			if (Count == _list.Length)
+			{
+				System.Array.Resize(ref _list, _list.Length * 2);;
+			}
+
+			_list[Count] = item;
+			Count++;
+		}
+	}
+}

src/Uno.UI/Microsoft/UI/Xaml/Controls/ColorPicker/ColorHelpers.cs

@@ -382,7 +382,7 @@ public static async void CreateCheckeredBackgroundAsync(
 			int width,
 			int height,
 			Color checkerColor,
-			List<byte> bgraCheckeredPixelData,
+			ArrayList<byte> bgraCheckeredPixelData,
 			IAsyncAction asyncActionToAssign,
 			CoreDispatcher dispatcherHelper,
 			Action<WriteableBitmap> completedFunction)
@@ -460,7 +460,7 @@ await dispatcherHelper.RunAsync(CoreDispatcherPriority.Normal, () =>
 		public static WriteableBitmap CreateBitmapFromPixelData(
 			int pixelWidth,
 			int pixelHeight,
-			List<byte> bgraPixelData)
+			ArrayList<byte> bgraPixelData)
 		{
 			// IBufferByteAccess isn't included in any WinMD file, because its sole method - Buffer() -
 			// allows direct pointer access, which isn't applicable to C#.  In C#, there's a separate ToStream()
@@ -470,10 +470,10 @@ public static WriteableBitmap CreateBitmapFromPixelData(
 
 			// Uno Doc:
 			// Since Uno uses C#, the method of converting to a bitmap was changed to what is below.
-			// WARNING: The .ToArray() copy is a performance hit. It would be better to just pass arrays to this method.
+			// A new 'ArrayList' class is used to avoid an extra copy using List<T>.ToArray() here.
 			using (Stream stream = bitmap.PixelBuffer.AsStream())
 			{
-				_ = stream.WriteAsync(bgraPixelData.ToArray(), 0, bgraPixelData.Count);
+				_ = stream.WriteAsync(bgraPixelData.Array, 0, bgraPixelData.Count);
 			}
 
 			// Uno Doc: The following code is not supported in C# and is removed.

src/Uno.UI/Microsoft/UI/Xaml/Controls/ColorPicker/ColorPicker.cs

@@ -1352,7 +1352,7 @@ private void CreateColorPreviewCheckeredBackground()
 				{
 					int width = (int)Math.Round(colorPreviewRectangleGrid.ActualWidth);
 					int height = (int)Math.Round(colorPreviewRectangleGrid.ActualHeight);
-					List<byte> bgraCheckeredPixelData = new List<byte>();
+					ArrayList<byte> bgraCheckeredPixelData = new ArrayList<byte>();
 					var strongThis = this;
 
 					ColorHelpers.CreateCheckeredBackgroundAsync(
@@ -1380,7 +1380,7 @@ private void CreateAlphaSliderCheckeredBackground()
 				{
 					int width = (int)Math.Round(alphaSliderBackgroundRectangle.ActualWidth);
 					int height = (int)Math.Round(alphaSliderBackgroundRectangle.ActualHeight);
-					List<byte> bgraCheckeredPixelData = new List<byte>();
+					ArrayList<byte> bgraCheckeredPixelData = new ArrayList<byte>();
 					var strongThis = this;
 
 					ColorHelpers.CreateCheckeredBackgroundAsync(

src/Uno.UI/Microsoft/UI/Xaml/Controls/ColorPicker/ColorSpectrum.cs

@@ -1016,12 +1016,15 @@ private async void CreateBitmapsAndColorMap()
 
 			// The middle 4 are only needed and used in the case of hue as the third dimension.
 			// Saturation and luminosity need only a min and max.
-			List<byte> bgraMinPixelData = new List<byte>();
-			List<byte> bgraMiddle1PixelData = new List<byte>();
-			List<byte> bgraMiddle2PixelData = new List<byte>();
-			List<byte> bgraMiddle3PixelData = new List<byte>();
-			List<byte> bgraMiddle4PixelData = new List<byte>();
-			List<byte> bgraMaxPixelData = new List<byte>();
+
+			byte[] test = Array.Empty<byte>();
+
+			ArrayList<byte> bgraMinPixelData = new ArrayList<byte>();
+			ArrayList<byte> bgraMiddle1PixelData = new ArrayList<byte>();
+			ArrayList<byte> bgraMiddle2PixelData = new ArrayList<byte>();
+			ArrayList<byte> bgraMiddle3PixelData = new ArrayList<byte>();
+			ArrayList<byte> bgraMiddle4PixelData = new ArrayList<byte>();
+			ArrayList<byte> bgraMaxPixelData = new ArrayList<byte>();
 			List<Hsv> newHsvValues = new List<Hsv>();
 
 			// Uno Docs: size_t not available in C# so types were changed
@@ -1212,12 +1215,12 @@ private void FillPixelForBox(
 			double maxSaturation,
 			double minValue,
 			double maxValue,
-			List<byte> bgraMinPixelData,
-			List<byte> bgraMiddle1PixelData,
-			List<byte> bgraMiddle2PixelData,
-			List<byte> bgraMiddle3PixelData,
-			List<byte> bgraMiddle4PixelData,
-			List<byte> bgraMaxPixelData,
+			ArrayList<byte> bgraMinPixelData,
+			ArrayList<byte> bgraMiddle1PixelData,
+			ArrayList<byte> bgraMiddle2PixelData,
+			ArrayList<byte> bgraMiddle3PixelData,
+			ArrayList<byte> bgraMiddle4PixelData,
+			ArrayList<byte> bgraMaxPixelData,
 			List<Hsv> newHsvValues)
 		{
 			double hMin = minHue;
@@ -1372,12 +1375,12 @@ private void FillPixelForRing(
 			double maxSaturation,
 			double minValue,
 			double maxValue,
-			List<byte> bgraMinPixelData,
-			List<byte> bgraMiddle1PixelData,
-			List<byte> bgraMiddle2PixelData,
-			List<byte> bgraMiddle3PixelData,
-			List<byte> bgraMiddle4PixelData,
-			List<byte> bgraMaxPixelData,
+			ArrayList<byte> bgraMinPixelData,
+			ArrayList<byte> bgraMiddle1PixelData,
+			ArrayList<byte> bgraMiddle2PixelData,
+			ArrayList<byte> bgraMiddle3PixelData,
+			ArrayList<byte> bgraMiddle4PixelData,
+			ArrayList<byte> bgraMaxPixelData,
 			List<Hsv> newHsvValues)
 		{
 			double hMin = minHue;