perf(android): Adjust clipping computation performance

src/Uno.UI/UI/Xaml/UIElement.Android.cs

@@ -22,29 +22,55 @@ namespace Windows.UI.Xaml
 	public partial class UIElement : BindableView
 	{
 		/// <summary>
-		/// Returns true if this element has children and they are all native (non-UIElements), false if it has no children or if at
-		/// least one is a UIElement.
+		/// Keeps the count of native children (non-UIElements), for clipping purpoess.
 		/// </summary>
-		private bool AreChildrenNativeViewsOnly
+		private int _nativeChildrenCount;
+		private bool _nativeClipChildren;
+
+		private Rect _previousClip = Rect.Empty;
+
+		private void ComputeAreChildrenNativeViewsOnly()
 		{
-			get
+			var nativeClipChildren = (this as IShadowChildrenProvider).ChildrenShadow.Count == _nativeChildrenCount;
+
+			if (_nativeClipChildren != nativeClipChildren)
 			{
-				var shadow = (this as IShadowChildrenProvider).ChildrenShadow;
-				if (shadow.Count == 0)
-				{
-					return false;
-				}
+				_nativeClipChildren = nativeClipChildren;
 
-				foreach (var child in shadow)
-				{
-					if (child is UIElement)
-					{
-						return false;
-					}
-				}
+				// Non-UIElements typically expect to be clipped, and display incorrectly otherwise
+				// This won't work when UIElements and non-UIElements are mixed in the same Panel,
+				// but it should cover most cases in practice, and anyway should be superceded when
+				// IFrameworkElement will be removed.
+				SetClipChildren(FeatureConfiguration.UIElement.AlwaysClipNativeChildren ? _nativeClipChildren : false);
+			}
+		}
 
-				return true;
+		protected override void OnLocalViewRemoved(View view)
+		{
+			base.OnLocalViewRemoved(view);
+
+			if (!(view is UIElement))
+			{
+				_nativeChildrenCount--;
 			}
+
+			ComputeAreChildrenNativeViewsOnly();
+		}
+
+		/// <summary>
+		/// Invoked when a child view has been added.
+		/// </summary>
+		/// <param name="view">The view being removed</param>
+		protected override void OnChildViewAdded(View view)
+		{
+			base.OnChildViewAdded(view);
+
+			if(!(view is UIElement))
+			{
+				_nativeChildrenCount++;
+			}
+
+			ComputeAreChildrenNativeViewsOnly();
 		}
 
 		public UIElement()
@@ -54,7 +80,6 @@ public UIElement()
 			InitializePointers();
 		}
 
-
 		/// <summary>
 		/// Determines if InvalidateMeasure has been called
 		/// </summary>
@@ -67,18 +92,20 @@ public UIElement()
 
 		partial void ApplyNativeClip(Rect rect)
 		{
-			// Non-UIElements typically expect to be clipped, and display incorrectly otherwise
-			// This won't work when UIElements and non-UIElements are mixed in the same Panel,
-			// but it should cover most cases in practice, and anyway should be superceded when
-			// IFrameworkElement will be removed.
-			SetClipChildren(FeatureConfiguration.UIElement.AlwaysClipNativeChildren ? AreChildrenNativeViewsOnly : false);
-
 			if (rect.IsEmpty)
 			{
-				ViewCompat.SetClipBounds(this, null);
+				if (_previousClip != rect)
+				{
+					_previousClip = rect;
+
+					ViewCompat.SetClipBounds(this, null);
+				}
+
 				return;
 			}
 
+			_previousClip = rect;
+
 			var physicalRect = rect.LogicalToPhysicalPixels();
 			if (FrameRoundingAdjustment is { } fra)
 			{