Preserve bound actions during detach triggers

docfx/articles/interactions-custom/core/detached-from-logical-tree-trigger.md

@@ -1,3 +1,5 @@
 # DetachedFromLogicalTreeTrigger
 
 Executes when the associated object is detached from the logical tree.
+
+The trigger executes before its behavior/action logical scope is released, so bound actions retain their data context for the detach callback.

docfx/articles/interactions-custom/core/detached-from-visual-tree-trigger.md

@@ -1,3 +1,5 @@
 # DetachedFromVisualTreeTrigger
 
 Executes when the associated object is detached from the visual tree.
+
+The trigger executes before its behavior/action logical scope is released. Bound actions therefore retain their data context for the detach callback, including when content leaves the visual tree because a `TabControl` selection changes.

src/Xaml.Behaviors.Interactivity/StyledElement/StyledElementBehavior.cs

@@ -118,12 +118,17 @@ void IBehaviorEventsHandler.AttachedToVisualTreeEventHandler()
 
     void IBehaviorEventsHandler.DetachedFromVisualTreeEventHandler()
     {
-        if (AssociatedObject is not TopLevel)
+        try
         {
-            DetachBehaviorFromLogicalTree();
+            OnDetachedFromVisualTree();
+        }
+        finally
+        {
+            if (AssociatedObject is not TopLevel)
+            {
+                DetachBehaviorFromLogicalTree();
+            }
         }
-
-        OnDetachedFromVisualTree();
     }
 
     void IBehaviorEventsHandler.AttachedToLogicalTreeEventHandler()
@@ -135,12 +140,17 @@ void IBehaviorEventsHandler.AttachedToLogicalTreeEventHandler()
 
     void IBehaviorEventsHandler.DetachedFromLogicalTreeEventHandler()
     {
-        if (AssociatedObject is not TopLevel)
+        try
         {
-            DetachBehaviorFromLogicalTree();
+            OnDetachedFromLogicalTree();
+        }
+        finally
+        {
+            if (AssociatedObject is not TopLevel)
+            {
+                DetachBehaviorFromLogicalTree();
+            }
         }
-
-        OnDetachedFromLogicalTree();
     }
 
     void IBehaviorEventsHandler.LoadedEventHandler() => OnLoaded();

tests/Xaml.Behaviors.Interactions.UnitTests/Custom/DetachedFromVisualTreeTrigger001.axaml

@@ -0,0 +1,24 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:local="using:Avalonia.Xaml.Interactions.UnitTests.Custom"
+        x:Class="Avalonia.Xaml.Interactions.UnitTests.Custom.DetachedFromVisualTreeTrigger001"
+        x:DataType="local:DetachedTriggerBindingSource">
+  <Window.DataContext>
+    <local:DetachedTriggerBindingSource />
+  </Window.DataContext>
+
+  <TabControl x:Name="TargetTabs">
+    <TabItem Header="Tab 1">
+      <TextBlock Text="Tab 1">
+        <Interaction.Behaviors>
+          <DetachedFromVisualTreeTrigger>
+            <InvokeCommandAction Command="{Binding DetachedCommand}" />
+          </DetachedFromVisualTreeTrigger>
+        </Interaction.Behaviors>
+      </TextBlock>
+    </TabItem>
+    <TabItem Header="Tab 2">
+      <TextBlock Text="Tab 2" />
+    </TabItem>
+  </TabControl>
+</Window>

tests/Xaml.Behaviors.Interactions.UnitTests/Custom/DetachedFromVisualTreeTrigger001.axaml.cs

@@ -0,0 +1,25 @@
+using System.Windows.Input;
+using Avalonia.Controls;
+using Avalonia.Xaml.Interactions.UnitTests.Core;
+
+namespace Avalonia.Xaml.Interactions.UnitTests.Custom;
+
+public partial class DetachedFromVisualTreeTrigger001 : Window
+{
+    public DetachedFromVisualTreeTrigger001()
+    {
+        InitializeComponent();
+    }
+}
+
+public sealed class DetachedTriggerBindingSource
+{
+    public DetachedTriggerBindingSource()
+    {
+        DetachedCommand = new Command(_ => DetachedCount++);
+    }
+
+    public ICommand DetachedCommand { get; }
+
+    public int DetachedCount { get; private set; }
+}

tests/Xaml.Behaviors.Interactions.UnitTests/Custom/DetachedFromVisualTreeTriggerTests.cs

@@ -0,0 +1,25 @@
+using Avalonia.Headless;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using Xunit;
+
+namespace Avalonia.Xaml.Interactions.UnitTests.Custom;
+
+public class DetachedFromVisualTreeTriggerTests
+{
+    [AvaloniaFact]
+    public void SwitchingTabs_ExecutesBoundDetachedCommand()
+    {
+        var window = new DetachedFromVisualTreeTrigger001();
+        var source = Assert.IsType<DetachedTriggerBindingSource>(window.DataContext);
+
+        window.Show();
+        window.CaptureRenderedFrame();
+
+        window.TargetTabs.SelectedIndex = 1;
+        Dispatcher.UIThread.RunJobs();
+        window.CaptureRenderedFrame();
+
+        Assert.Equal(1, source.DetachedCount);
+    }
+}

tests/Xaml.Behaviors.Interactivity.UnitTests/StyledElementBehaviorTests.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia.Controls;
 using Avalonia.Headless.XUnit;
 using Avalonia.Threading;
@@ -33,6 +34,29 @@ public void Detach_ClearsLogicalParentAndTemplatedParent()
         window.Close();
     }
 
+    [AvaloniaFact]
+    public void DetachVisualTree_ClearsLogicalScopeWhenCallbackThrows()
+    {
+        var behavior = new ThrowingDetachBehavior();
+        var button = new Button();
+        var templatedParent = new ContentControl();
+        var window = new Window { Content = button };
+
+        TemplatedParentHelper.SetTemplatedParent(button, templatedParent);
+        Interaction.GetBehaviors(button).Add(behavior);
+        window.Show();
+
+        Assert.Throws<InvalidOperationException>(() =>
+            ((IBehaviorEventsHandler)behavior).DetachedFromVisualTreeEventHandler());
+
+        Assert.Null(behavior.Parent);
+        Assert.Null(behavior.TemplatedParent);
+
+        behavior.Detach();
+        Interaction.SetBehaviors(button, null);
+        window.Close();
+    }
+
     [AvaloniaFact]
     public void TopLevelClose_PreservesLogicalParentUntilDeferredDetach()
     {
@@ -57,4 +81,12 @@ public void TopLevelClose_PreservesLogicalParentUntilDeferredDetach()
     }
 
     private sealed class TestStyledElementBehavior : StyledElementBehavior;
+
+    private sealed class ThrowingDetachBehavior : StyledElementBehavior
+    {
+        protected override void OnDetachedFromVisualTree()
+        {
+            throw new InvalidOperationException("Expected test exception.");
+        }
+    }
 }