Reduced memory usage in UniformGrid.MeasureOverride

Author: Sergio0694

Microsoft.Toolkit.Uwp.UI.Controls/UniformGrid/TakenSpotsReferenceHolder.cs

@@ -13,7 +13,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Controls
     /// <see cref="UniformGrid.GetFreeSpot"/> iterator.
     /// This is used so we can better isolate our logic and make it easier to test.
     /// </summary>
-    internal class TakenSpotsReferenceHolder
+    internal sealed class TakenSpotsReferenceHolder
     {
         /// <summary>
         /// The <see cref="BitArray"/> instance used to efficiently track empty spots.
@@ -53,8 +53,16 @@ public TakenSpotsReferenceHolder(int rows, int columns)
         /// <param name="j">The horizontal offset.</param>
         public bool this[int i, int j]
         {
-            get => spotsTaken[(i * Width) + j];
-            set => spotsTaken[(i * Width) + j] = value;
+            get => this.spotsTaken[(i * Width) + j];
+            set => this.spotsTaken[(i * Width) + j] = value;
+        }
+
+        /// <summary>
+        /// Resets the current reference holder.
+        /// </summary>
+        public void Reset()
+        {
+            this.spotsTaken.SetAll(false);
         }
     }
 }

Microsoft.Toolkit.Uwp.UI.Controls/UniformGrid/UniformGrid.cs

@@ -23,6 +23,11 @@ public partial class UniformGrid : Grid
         // Internal list we use to keep track of items that we don't have space to layout.
         private List<UIElement> _overflow = new List<UIElement>();
 
+        /// <summary>
+        /// The <see cref="TakenSpotsReferenceHolder"/> instance in use, if any.
+        /// </summary>
+        private TakenSpotsReferenceHolder _spotref;
+
         /// <inheritdoc/>
         protected override Size MeasureOverride(Size availableSize)
         {
@@ -39,7 +44,20 @@ protected override Size MeasureOverride(Size availableSize)
             SetupRowDefinitions(rows);
             SetupColumnDefinitions(columns);
 
-            var spotref = new TakenSpotsReferenceHolder(rows, columns);
+            TakenSpotsReferenceHolder spotref;
+
+            // If the last spot holder matches the size currently in use, just reset
+            // that instance and reuse it to avoid allocating a new bit array.
+            if (_spotref != null && _spotref.Height == rows && _spotref.Width == columns)
+            {
+                spotref = _spotref;
+
+                spotref.Reset();
+            }
+            else
+            {
+                spotref = _spotref = new TakenSpotsReferenceHolder(rows, columns);
+            }
 
             // Figure out which children we should automatically layout and where available openings are.
             foreach (var child in visible)