Improve FocusBehavior: support list view lazy render. support control x:load

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/FocusBehavior/FocusBehaviorPage.xaml

@@ -10,6 +10,7 @@
     <interactivity:Interaction.Behaviors>
         <behaviors:FocusBehavior>
             <behaviors:FocusTarget Control="{x:Bind disabledItem}" />
+            <behaviors:FocusTarget Control="{x:Bind xLoadItem}" />
             <behaviors:FocusTarget Control="{x:Bind emptyList}" />
             <behaviors:FocusTarget Control="{x:Bind enabledItem}" />
         </behaviors:FocusBehavior>
@@ -27,5 +28,13 @@
         </ListView>
         <Button x:Name="enabledItem"
                 Content="I can get the focus" />
+        <StackPanel Orientation="Horizontal">
+            <ToggleSwitch x:Name="LoadControl"
+                          OffContent="Load the item"
+                          OnContent="Unload the item" />
+            <Button x:Name="xLoadItem"
+                    x:Load="{x:Bind LoadControl.IsOn, Mode=OneWay}"
+                    Content="I can get the focus when loaded" />
+        </StackPanel>
     </StackPanel>
 </Page>

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/FocusBehavior/FocusBehaviorXaml.bind

@@ -9,6 +9,7 @@
   <interactivity:Interaction.Behaviors>
     <behaviors:FocusBehavior>
       <behaviors:FocusTarget Control="{x:Bind disabledItem}" />
+      <behaviors:FocusTarget Control="{x:Bind xLoadItem}" />
       <behaviors:FocusTarget Control="{x:Bind emptyList}" />
       <behaviors:FocusTarget Control="{x:Bind enabledItem}" />
     </behaviors:FocusBehavior>
@@ -27,5 +28,13 @@
     </ListView>
     <Button x:Name="enabledItem"
             Content="I can get the focus" />
+    <StackPanel Orientation="Horizontal">
+      <ToggleSwitch x:Name="LoadControl"
+                    OffContent="Load the item"
+                    OnContent="Unload the item" />
+      <Button x:Name="xLoadItem"
+              x:Load="{x:Bind LoadControl.IsOn, Mode=OneWay}"
+              Content="I can get the focus when loaded" />
+    </StackPanel>
   </StackPanel>
 </Page>

Microsoft.Toolkit.Uwp.UI.Behaviors/Focus/FocusBehavior.cs

@@ -18,6 +18,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Behaviors
     /// The focus will be set following the <see cref="Targets"/> order. The first control being ready
     /// and accepting the focus will receive it.
     /// The focus can be set to another control with a higher priority if it loads before <see cref="FocusEngagementTimeout"/>.
+    /// The focus can be set to another control if some controls will be loaded/unloaded later.
     /// </summary>
     [ContentProperty(Name = nameof(Targets))]
     public sealed class FocusBehavior : BehaviorBase<UIElement>
@@ -67,10 +68,27 @@ public TimeSpan FocusEngagementTimeout
         }
 
         /// <inheritdoc/>
-        protected override void OnAssociatedObjectLoaded() => ApplyFocus();
+        protected override void OnAssociatedObjectLoaded()
+        {
+            foreach (var target in Targets)
+            {
+                target.ControlChanged += OnTargetControlChanged;
+            }
+
+            ApplyFocus();
+        }
 
         /// <inheritdoc/>
-        protected override void OnDetaching() => Stop();
+        protected override bool Uninitialize()
+        {
+            foreach (var target in Targets)
+            {
+                target.ControlChanged -= OnTargetControlChanged;
+            }
+
+            Stop();
+            return true;
+        }
 
         private static void OnTargetsPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
         {
@@ -92,24 +110,39 @@ private void ApplyFocus()
             }
 
             var focusedControlIndex = -1;
+            var listViewBaseControls = 0;
             for (var i = 0; i < Targets.Count; i++)
             {
                 var control = Targets[i].Control;
+                if (control is null)
+                {
+                    continue;
+                }
+
                 if (control.IsLoaded)
                 {
                     if (control.Focus(FocusState.Programmatic))
                     {
                         focusedControlIndex = i;
                         break;
                     }
+
+                    if (control is ListViewBase listViewBase)
+                    {
+                        // The list may not have any item yet, we wait until the first item is rendered.
+                        listViewBase.ContainerContentChanging -= OnContainerContentChanging;
+                        listViewBase.ContainerContentChanging += OnContainerContentChanging;
+                        listViewBaseControls++;
+                    }
                 }
                 else
                 {
+                    control.Loaded -= OnControlLoaded;
                     control.Loaded += OnControlLoaded;
                 }
             }
 
-            if (focusedControlIndex == 0 || Targets.All(t => t.Control?.IsLoaded == true))
+            if (focusedControlIndex == 0 || (listViewBaseControls == 0 && Targets.All(t => t.Control?.IsLoaded == true)))
             {
                 // The first control has received the focus or all the control are loaded and none can take the focus: we stop.
                 Stop();
@@ -137,6 +170,14 @@ private void OnEngagementTimerTick(object sender, object e)
 
         private void OnControlLoaded(object sender, RoutedEventArgs e) => ApplyFocus();
 
+        private void OnTargetControlChanged(object sender, EventArgs e) => ApplyFocus();
+
+        private void OnContainerContentChanging(ListViewBase sender, ContainerContentChangingEventArgs args)
+        {
+            sender.ContainerContentChanging -= OnContainerContentChanging;
+            ApplyFocus();
+        }
+
         private void Stop(FocusTargetList targets = null)
         {
             if (_timer != null)
@@ -153,6 +194,11 @@ private void Stop(FocusTargetList targets = null)
                 }
 
                 target.Control.Loaded -= OnControlLoaded;
+
+                if (target.Control is ListViewBase listViewBase)
+                {
+                    listViewBase.ContainerContentChanging -= OnContainerContentChanging;
+                }
             }
         }
     }
@@ -167,7 +213,7 @@ public sealed class FocusTargetList : List<FocusTarget>
     /// <summary>
     /// A target for the <see cref="FocusBehavior"/>.
     /// </summary>
-    public sealed partial class FocusTarget : DependencyObject
+    public sealed class FocusTarget : DependencyObject
     {
         /// <summary>
         /// The DP to store the <see cref="Control"/> property value.
@@ -176,7 +222,13 @@ public sealed partial class FocusTarget : DependencyObject
             nameof(Control),
             typeof(Control),
             typeof(FocusTarget),
-            new PropertyMetadata(null));
+            new PropertyMetadata(null, OnControlChanged));
+
+        /// <summary>
+        /// Raised when <see cref="Control"/> property changed.
+        /// It can change if we use x:Load on the control.
+        /// </summary>
+        public event EventHandler ControlChanged;
 
         /// <summary>
         /// Gets or sets the control that will receive the focus.
@@ -186,6 +238,12 @@ public Control Control
             get => (Control)GetValue(ControlProperty);
             set => SetValue(ControlProperty, value);
         }
+
+        private static void OnControlChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            var target = (FocusTarget)d;
+            target.ControlChanged?.Invoke(target, EventArgs.Empty);
+        }
     }
 #pragma warning restore SA1402 // File may only contain a single type
 }