Assert control invalidation behavior.

Controls not attached to the visual tree should not notify the `LayoutManager` that they have had their layout invalidated. Similarly when added to the visual tree their parents and themselves should have their layout invalidated.

src/Avalonia.Layout/LayoutManager.cs

@@ -30,12 +30,19 @@ namespace Avalonia.Layout
             Contract.Requires<ArgumentNullException>(control != null);
             Dispatcher.UIThread.VerifyAccess();
 
-            if (control.IsAttachedToVisualTree)
+            if (!control.IsAttachedToVisualTree)
             {
-                _toMeasure.Enqueue(control);
-                _toArrange.Enqueue(control);
-                QueueLayoutPass();
+#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();
         }
 
         /// <inheritdoc/>
@@ -44,11 +51,18 @@ namespace Avalonia.Layout
             Contract.Requires<ArgumentNullException>(control != null);
             Dispatcher.UIThread.VerifyAccess();
 
-            if (control.IsAttachedToVisualTree)
+            if (!control.IsAttachedToVisualTree)
             {
-                _toArrange.Enqueue(control);
-                QueueLayoutPass();
+#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();
         }
 
         /// <inheritdoc/>

src/Avalonia.Layout/Layoutable.cs

@@ -378,8 +378,12 @@ namespace Avalonia.Layout
 
                 IsMeasureValid = false;
                 IsArrangeValid = false;
-                LayoutManager.Instance?.InvalidateMeasure(this);
-                InvalidateVisual();
+
+                if (((ILayoutable)this).IsAttachedToVisualTree)
+                {
+                    LayoutManager.Instance?.InvalidateMeasure(this);
+                    InvalidateVisual();
+                }
             }
         }
 
@@ -393,8 +397,12 @@ namespace Avalonia.Layout
                 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.Layout.UnitTests/LayoutableTests.cs

@@ -0,0 +1,90 @@
+using System;
+using Avalonia.Controls;
+using Moq;
+using Xunit;
+
+namespace Avalonia.Layout.UnitTests
+{
+    public class LayoutableTests
+    {
+        [Fact]
+        public void Only_Calls_LayoutManager_InvalidateMeasure_Once()
+        {
+            var target = new Mock<ILayoutManager>();
+
+            using (Start(target.Object))
+            {
+                var control = new Decorator();
+                var root = new LayoutTestRoot { Child = control };
+
+                root.Measure(Size.Infinity);
+                root.Arrange(new Rect(root.DesiredSize));
+                target.ResetCalls();
+
+                control.InvalidateMeasure();
+                control.InvalidateMeasure();
+
+                target.Verify(x => x.InvalidateMeasure(control), Times.Once());
+            }
+        }
+
+        [Fact]
+        public void Only_Calls_LayoutManager_InvalidateArrange_Once()
+        {
+            var target = new Mock<ILayoutManager>();
+
+            using (Start(target.Object))
+            {
+                var control = new Decorator();
+                var root = new LayoutTestRoot { Child = control };
+
+                root.Measure(Size.Infinity);
+                root.Arrange(new Rect(root.DesiredSize));
+                target.ResetCalls();
+
+                control.InvalidateArrange();
+                control.InvalidateArrange();
+
+                target.Verify(x => x.InvalidateArrange(control), Times.Once());
+            }
+        }
+
+        [Fact]
+        public void Attaching_Control_To_Tree_Invalidates_Parent_Measure()
+        {
+            var target = new Mock<ILayoutManager>();
+
+            using (Start(target.Object))
+            {
+                var control = new Decorator();
+                var root = new LayoutTestRoot { Child = control };
+
+                root.Measure(Size.Infinity);
+                root.Arrange(new Rect(root.DesiredSize));
+                Assert.True(control.IsMeasureValid);
+
+                root.Child = null;
+                root.Measure(Size.Infinity);
+                root.Arrange(new Rect(root.DesiredSize));
+
+                Assert.False(control.IsMeasureValid);
+                Assert.True(root.IsMeasureValid);
+
+                target.ResetCalls();
+
+                root.Child = control;
+
+                Assert.False(root.IsMeasureValid);
+                Assert.False(control.IsMeasureValid);
+                target.Verify(x => x.InvalidateMeasure(root), Times.Once());
+            }
+        }
+
+        private IDisposable Start(ILayoutManager layoutManager)
+        {
+            var result = AvaloniaLocator.EnterScope();
+            AvaloniaLocator.CurrentMutable.Bind<ILayoutManager>().ToConstant(layoutManager);
+            return result;
+        }
+    }
+}