Resolve Layout changes during native measure/arrange pass

Xamarin.Forms.Core/Device.cs

@@ -90,6 +90,8 @@ public static IPlatformServices PlatformServices
 			set { s_platformServices = value; }
 		}
 
+		public static IPlatformInvalidate PlatformInvalidator { get; set; }
+
 		[EditorBrowsable(EditorBrowsableState.Never)]
 		public static IReadOnlyList<string> Flags { get; private set; }
 
@@ -283,5 +285,10 @@ public static class Styles
 
 			public static readonly Style CaptionStyle = new Style(typeof(Label)) { BaseResourceKey = CaptionStyleKey };
 		}
+
+		public static void Invalidate(VisualElement visualElement) 
+		{
+			PlatformInvalidator?.Invalidate(visualElement);
+		}
 	}
 }

Xamarin.Forms.Core/IPlatformInvalidate.cs

@@ -0,0 +1,8 @@
+namespace Xamarin.Forms.Internals
+{
+	public interface IPlatformInvalidate
+
+	{
+		void Invalidate(VisualElement visualElement);
+	}
+}

Xamarin.Forms.Core/Layout.cs

@@ -326,32 +326,44 @@ internal virtual void OnChildMeasureInvalidated(VisualElement child, Invalidatio
 			}
 
 			s_resolutionList.Add(new KeyValuePair<Layout, int>(this, GetElementDepth(this)));
-			if (!s_relayoutInProgress)
-			{
-				s_relayoutInProgress = true;
 
-				// Rather than recomputing the layout for each change as it happens, we accumulate them in 
+			if (Device.PlatformInvalidator == null && !s_relayoutInProgress)
+			{
+				// Rather than recomputing the layout for each change as it happens, we accumulate them in
 				// s_resolutionList and schedule a single layout update operation to handle them all at once.
 				// This avoids a lot of unnecessary layout operations if something is triggering many property
 				// changes at once (e.g., a BindingContext change)
 
+				s_relayoutInProgress = true;
+
 				if (Dispatcher != null)
 				{
 					Dispatcher.BeginInvokeOnMainThread(ResolveLayoutChanges);
 				}
 				else
 				{
 					Device.BeginInvokeOnMainThread(ResolveLayoutChanges);
-				}			
+				}
+			}
+			else
+			{
+				// If the platform supports PlatformServices2, queueing is unnecessary; the layout changes
+				// will be handled during the Layout's next Measure/Arrange pass
+				Device.Invalidate(this);
 			}
 		}
 
-		internal void ResolveLayoutChanges()
+		public void ResolveLayoutChanges()
 		{
-			// if thread safety mattered we would need to lock this and compareexchange above
+			s_relayoutInProgress = false;
+
+			if (s_resolutionList.Count == 0)
+			{
+				return;
+			}	
+
 			IList<KeyValuePair<Layout, int>> copy = s_resolutionList;
 			s_resolutionList = new List<KeyValuePair<Layout, int>>();
-			s_relayoutInProgress = false;
 
 			foreach (KeyValuePair<Layout, int> kvp in copy)
 			{

Xamarin.Forms.Core/VisualElement.cs

@@ -663,7 +663,10 @@ public void BatchCommit()
 		{
 			_batched = Math.Max(0, _batched - 1);
 			if (!Batched)
+			{
 				BatchCommitted?.Invoke(this, new EventArg<VisualElement>(this));
+				Device.Invalidate(this);
+			}
 		}
 
 		ResourceDictionary _resources;

Xamarin.Forms.Platform.Android/Forms.cs

@@ -296,7 +296,11 @@ static void SetupInit(
 			// We want this to be updated when we have a new activity (e.g. on a configuration change)
 			// because AndroidPlatformServices needs a current activity to launch URIs from
 			Profile.FramePartition("Device.PlatformServices");
-			Device.PlatformServices = new AndroidPlatformServices(activity);
+
+			var androidServices = new AndroidPlatformServices(activity);
+
+			Device.PlatformServices = androidServices;
+			Device.PlatformInvalidator = androidServices;
 
 			// use field and not property to avoid exception in getter
 			if (Device.info != null)
@@ -610,7 +614,7 @@ protected override Expression VisitMember(MemberExpression node)
 			}
 		}
 
-		class AndroidPlatformServices : IPlatformServices
+		class AndroidPlatformServices : IPlatformServices, IPlatformInvalidate
 		{
 			double _buttonDefaultSize;
 			double _editTextDefaultSize;
@@ -917,6 +921,18 @@ public SizeRequest GetNativeSize(VisualElement view, double widthConstraint, dou
 				return Platform.Android.Platform.GetNativeSize(view, widthConstraint, heightConstraint);
 			}
 
+			public void Invalidate(VisualElement visualElement)
+			{
+				var renderer = visualElement.GetRenderer();
+				if (renderer == null || renderer.View.IsDisposed())
+				{
+					return;
+				}
+
+				renderer.View.Invalidate();
+				renderer.View.RequestLayout();
+			}
+
 			public OSAppTheme RequestedTheme
             {
                 get

Xamarin.Forms.Platform.Android/Platform.cs

@@ -1312,6 +1312,16 @@ protected override void Dispose(bool disposing)
 
 			bool ILayoutChanges.HasLayoutOccurred => _hasLayoutOccurred;
 
+			protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
+			{
+				if (Element is Layout layout)
+				{
+					layout.ResolveLayoutChanges();
+				}
+
+				base.OnMeasure(widthMeasureSpec, heightMeasureSpec);
+			}
+
 			protected override void OnLayout(bool changed, int left, int top, int right, int bottom)
 			{
 				base.OnLayout(changed, left, top, right, bottom);

Xamarin.Forms.Platform.Android/VisualElementTracker.cs

@@ -336,6 +336,9 @@ void UpdateIsVisible()
 				aview.Visibility = ViewStates.Visible;
 			if (!view.IsVisible && aview.Visibility != ViewStates.Gone)
 				aview.Visibility = ViewStates.Gone;
+
+			aview.Invalidate();
+			aview.RequestLayout();
 		}
 
 		void UpdateNativeView(object sender, EventArgs e)

Xamarin.Forms.Platform.UAP/Forms.cs

@@ -49,7 +49,12 @@ public static void Init(IActivatedEventArgs launchActivatedEventArgs, IEnumerabl
 
 			Device.SetIdiom(TargetIdiom.Tablet);
 			Device.SetFlowDirection(GetFlowDirection());
-			Device.PlatformServices = new WindowsPlatformServices(Window.Current.Dispatcher);
+
+			var platformServices = new WindowsPlatformServices(Window.Current.Dispatcher);
+
+			Device.PlatformServices = platformServices;
+			Device.PlatformInvalidator = platformServices;
+
 			Device.SetFlags(s_flags);
 			Device.Info = new WindowsDeviceInfo();
 

Xamarin.Forms.Platform.UAP/LayoutRenderer.cs

@@ -80,5 +80,11 @@ void UpdateClipToBounds()
 				Clip = new RectangleGeometry { Rect = new WRect(0, 0, ActualWidth, ActualHeight) };
 			}
 		}
+
+		protected override Windows.Foundation.Size MeasureOverride(Windows.Foundation.Size availableSize)
+		{
+			Element?.ResolveLayoutChanges();
+			return base.MeasureOverride(availableSize);
+		}
 	}
 }

Xamarin.Forms.Platform.UAP/WindowsBasePlatformServices.cs

@@ -25,7 +25,7 @@
 
 namespace Xamarin.Forms.Platform.UWP
 {
-	internal abstract class WindowsBasePlatformServices : IPlatformServices
+	internal abstract class WindowsBasePlatformServices : IPlatformServices, IPlatformInvalidate
 	{
 		const string WrongThreadError = "RPC_E_WRONG_THREAD";
 		readonly CoreDispatcher _dispatcher;
@@ -256,6 +256,17 @@ await dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => {
 			return await taskCompletionSource.Task;
 		}
 
+		public void Invalidate(VisualElement visualElement)
+		{
+			var renderer = Platform.GetRenderer(visualElement);
+			if (renderer == null)
+			{
+				return;
+			}
+
+			renderer.ContainerElement.InvalidateMeasure();
+		}
+
 		public OSAppTheme RequestedTheme => Windows.UI.Xaml.Application.Current.RequestedTheme == ApplicationTheme.Dark ? OSAppTheme.Dark : OSAppTheme.Light;
 	}
 }

Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewController.cs

@@ -66,7 +66,7 @@ protected override void Dispose(bool disposing)
 
 				_emptyUIView?.Dispose();
 				_emptyUIView = null;
-	
+
 				_emptyViewFormsElement = null;
 
 				ItemsViewLayout?.Dispose();

Xamarin.Forms.Platform.iOS/Forms.cs

@@ -180,8 +180,12 @@ public static void Init()
 			}
 #endif
 			Device.SetFlags(s_flags);
-			Device.PlatformServices = new IOSPlatformServices();
+			var platformServices = new IOSPlatformServices();
+
+			Device.PlatformServices = platformServices;
+
 #if __MOBILE__
+			Device.PlatformInvalidator = platformServices;
 			Device.Info = new IOSDeviceInfo();
 #else
 			Device.Info = new Platform.macOS.MacDeviceInfo();
@@ -227,6 +231,9 @@ protected override Expression VisitMember(MemberExpression node)
 		}
 
 		class IOSPlatformServices : IPlatformServices
+#if __MOBILE__
+			, IPlatformInvalidate
+#endif
 		{
 			readonly double _fontScalingFactor = 1;
 			public IOSPlatformServices()
@@ -782,6 +789,18 @@ static UIViewController GetCurrentViewController(bool throwIfNull = true)
 
 				return viewController;
 			}
+
+			public void Invalidate(VisualElement visualElement)
+			{
+				var renderer = Platform.iOS.Platform.GetRenderer(visualElement);
+
+				if (renderer == null)
+				{
+					return;
+				}
+
+				renderer.NativeView.SetNeedsLayout();
+			}
 #endif
 		}
 	}

Xamarin.Forms.Platform.iOS/Platform.cs

@@ -582,6 +582,26 @@ public override UIView HitTest(CGPoint point, UIEvent uievent)
 
 				return result;
 			}
+
+			void ResolveLayoutChanges() 
+			{
+				if (Element is Layout layout)
+				{
+					layout.ResolveLayoutChanges();
+				}
+			}
+
+			public override void LayoutSubviews()
+			{
+				ResolveLayoutChanges();
+				base.LayoutSubviews();
+			}
+
+			public override CGSize SizeThatFits(CGSize size)
+			{
+				ResolveLayoutChanges();
+				return base.SizeThatFits(size);
+			}
 		}
 
 		internal static string ResolveMsAppDataUri(Uri uri)