Fix up the Layout Manager

src/Avalonia.Layout/LayoutManager.cs

@@ -14,8 +14,8 @@ namespace Avalonia.Layout
     /// </summary>
     public class LayoutManager : ILayoutManager
     {
-        private readonly HashSet<ILayoutable> _toMeasure = new HashSet<ILayoutable>();
-        private readonly HashSet<ILayoutable> _toArrange = new HashSet<ILayoutable>();
+        private readonly Queue<ILayoutable> _toMeasure = new Queue<ILayoutable>();
+        private readonly Queue<ILayoutable> _toArrange = new Queue<ILayoutable>();
         private bool _queued;
         private bool _running;
 
@@ -30,8 +30,18 @@ public void InvalidateMeasure(ILayoutable control)
             Contract.Requires<ArgumentNullException>(control != null);
             Dispatcher.UIThread.VerifyAccess();
 
-            _toMeasure.Add(control);
-            _toArrange.Add(control);
+            if (!control.IsAttachedToVisualTree)
+            {
+#if DEBUG
+                throw new AvaloniaInternalException(
+                    "LayoutManager.InvalidateMeasure called on a control that is detached from the visual tree.");
+#else
+                return;
+#endif
+            }
+
+            _toMeasure.Enqueue(control);
+            _toArrange.Enqueue(control);
             QueueLayoutPass();
         }
 
@@ -41,7 +51,17 @@ public void InvalidateArrange(ILayoutable control)
             Contract.Requires<ArgumentNullException>(control != null);
             Dispatcher.UIThread.VerifyAccess();
 
-            _toArrange.Add(control);
+            if (!control.IsAttachedToVisualTree)
+            {
+#if DEBUG
+                throw new AvaloniaInternalException(
+                    "LayoutManager.InvalidateArrange called on a control that is detached from the visual tree.");
+#else
+                return;
+#endif
+            }
+
+            _toArrange.Enqueue(control);
             QueueLayoutPass();
         }
 
@@ -108,62 +128,73 @@ private void ExecuteMeasurePass()
         {
             while (_toMeasure.Count > 0)
             {
-                var next = _toMeasure.First();
-                Measure(next);
+                var control = _toMeasure.Dequeue();
+
+                if (!control.IsMeasureValid && control.IsAttachedToVisualTree)
+                {
+                    Measure(control);
+                }
             }
         }
 
         private void ExecuteArrangePass()
         {
             while (_toArrange.Count > 0 && _toMeasure.Count == 0)
             {
-                var next = _toArrange.First();
-                Arrange(next);
+                var control = _toArrange.Dequeue();
+
+                if (!control.IsArrangeValid && control.IsAttachedToVisualTree)
+                {
+                    Arrange(control);
+                }
             }
         }
 
         private void Measure(ILayoutable control)
         {
-            var root = control as ILayoutRoot;
-            var parent = control.VisualParent as ILayoutable;
-
-            if (root != null)
-            {
-                root.Measure(root.MaxClientSize);
-            }
-            else if (parent != null)
+            // Controls closest to the visual root need to be arranged first. We don't try to store
+            // ordered invalidation lists, instead we traverse the tree upwards, measuring the
+            // controls closest to the root first. This has been shown by benchmarks to be the
+            // fastest and most memory-efficent algorithm.
+            if (control.VisualParent is ILayoutable parent)
             {
                 Measure(parent);
             }
 
-            if (!control.IsMeasureValid)
+            // If the control being measured has IsMeasureValid == true here then its measure was
+            // handed by an ancestor and can be ignored. The measure may have also caused the
+            // control to be removed.
+            if (!control.IsMeasureValid && control.IsAttachedToVisualTree)
             {
-                control.Measure(control.PreviousMeasure.Value);
+                if (control is ILayoutRoot root)
+                {
+                    root.Measure(Size.Infinity);
+                }
+                else
+                {
+                    control.Measure(control.PreviousMeasure.Value);
+                }
             }
-
-            _toMeasure.Remove(control);
         }
 
         private void Arrange(ILayoutable control)
         {
-            var root = control as ILayoutRoot;
-            var parent = control.VisualParent as ILayoutable;
-
-            if (root != null)
-            {
-                root.Arrange(new Rect(root.DesiredSize));
-            }
-            else if (parent != null)
+            if (control.VisualParent is ILayoutable parent)
             {
                 Arrange(parent);
             }
 
-            if (control.PreviousArrange.HasValue)
+            if (!control.IsArrangeValid && control.IsAttachedToVisualTree)
             {
-                control.Arrange(control.PreviousArrange.Value);
+                if (control is ILayoutRoot root)
+                {
+                    root.Arrange(new Rect(control.DesiredSize));
+                }
+                else
+                {
+                    control.Arrange(control.PreviousArrange.Value);
+                }
             }
-
-            _toArrange.Remove(control);
         }
 
         private void QueueLayoutPass()

src/Avalonia.Layout/Layoutable.cs

@@ -378,8 +378,12 @@ public void InvalidateMeasure()
 
                 IsMeasureValid = false;
                 IsArrangeValid = false;
-                LayoutManager.Instance?.InvalidateMeasure(this);
-                InvalidateVisual();
+
+                if (((ILayoutable)this).IsAttachedToVisualTree)
+                {
+                    LayoutManager.Instance?.InvalidateMeasure(this);
+                    InvalidateVisual();
+                }
             }
         }
 
@@ -393,8 +397,12 @@ public void InvalidateArrange()
                 Logger.Verbose(LogArea.Layout, this, "Invalidated arrange");
 
                 IsArrangeValid = false;
-                LayoutManager.Instance?.InvalidateArrange(this);
-                InvalidateVisual();
+
+                if (((ILayoutable)this).IsAttachedToVisualTree)
+                {
+                    LayoutManager.Instance?.InvalidateArrange(this);
+                    InvalidateVisual();
+                }
             }
         }
 

tests/Avalonia.Benchmarks/Avalonia.Benchmarks.csproj

@@ -49,6 +49,7 @@
     <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="Layout\Measure.cs" />
     <Compile Include="Styling\ApplyStyling.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
@@ -100,7 +101,7 @@
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.9.2" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.10.8" />
   </ItemGroup>
   <Import Project="$(MSBuildThisFileDirectory)..\..\src\Shared\nuget.workaround.targets" />
 </Project>

tests/Avalonia.Benchmarks/Layout/Measure.cs

@@ -0,0 +1,65 @@
+using System;
+using System.Collections.Generic;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.UnitTests;
+using BenchmarkDotNet.Attributes;
+
+namespace Avalonia.Benchmarks.Layout
+{
+    [MemoryDiagnoser]
+    public class Measure : IDisposable
+    {
+        private IDisposable _app;
+        private TestRoot root;
+        private List<Control> controls = new List<Control>();
+
+        public Measure()
+        {
+            _app = UnitTestApplication.Start(TestServices.RealLayoutManager);
+
+            var panel = new StackPanel();
+            root = new TestRoot { Child = panel };
+            controls.Add(panel);
+            CreateChildren(panel, 3, 5);
+            LayoutManager.Instance.ExecuteInitialLayoutPass(root);
+        }
+
+        public void Dispose()
+        {
+            _app.Dispose();
+        }
+
+        [Benchmark]
+        public void Remeasure_Half()
+        {
+            var random = new Random(1);
+
+            foreach (var control in controls)
+            {
+                if (random.Next(2) == 0)
+                {
+                    control.InvalidateMeasure();
+                }
+            }
+
+            LayoutManager.Instance.ExecuteLayoutPass();
+        }
+
+        private void CreateChildren(IPanel parent, int childCount, int iterations)
+        {
+            for (var i = 0; i < childCount; ++i)
+            {
+                var control = new StackPanel();
+                parent.Children.Add(control);
+
+                if (iterations > 0)
+                {
+                    CreateChildren(control, childCount, iterations - 1);
+                }
+
+                controls.Add(control);
+            }
+        }
+    }
+}

tests/Avalonia.Benchmarks/Styling/ApplyStyling.cs

@@ -11,6 +11,7 @@
 
 namespace Avalonia.Benchmarks.Styling
 {
+    [MemoryDiagnoser]
     public class ApplyStyling : IDisposable
     {
         private IDisposable _app;