Update FocusTracker to no longer use a DispatcherQueueTimer to constantly look for focused element

Fixes #3884
Now uses FocusManager.GotFocus event from 1809 (new min SDK version)

Author: michael-hawker

Microsoft.Toolkit.Uwp.DeveloperTools/FocusTracker/FocusTracker.cs

@@ -34,19 +34,19 @@ public partial class FocusTracker : Control
 
         private static void OnIsActiveChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
         {
-            var focusTracker = d as FocusTracker;
-
-            if (e.NewValue != null && (bool)e.NewValue)
+            if (d is FocusTracker focusTracker)
             {
-                focusTracker?.Start();
-            }
-            else
-            {
-                focusTracker?.Stop();
+                if (e.NewValue != null && (bool)e.NewValue)
+                {
+                    focusTracker.Start();
+                }
+                else
+                {
+                    focusTracker.Stop();
+                }
             }
         }
 
-        private DispatcherQueueTimer updateTimer;
         private TextBlock controlName;
         private TextBlock controlType;
         private TextBlock controlAutomationName;
@@ -69,32 +69,42 @@ public FocusTracker()
             DefaultStyleKey = typeof(FocusTracker);
         }
 
+        /// <summary>
+        /// Update the visual state of the control when its template is changed.
+        /// </summary>
+        protected override void OnApplyTemplate()
+        {
+            controlName = GetTemplateChild("ControlName") as TextBlock;
+            controlType = GetTemplateChild("ControlType") as TextBlock;
+            controlAutomationName = GetTemplateChild("ControlAutomationName") as TextBlock;
+            controlFirstParentWithName = GetTemplateChild("ControlFirstParentWithName") as TextBlock;
+        }
+
         private void Start()
         {
-            if (updateTimer == null)
+            // Get currently focused control once when we start
+            if (Windows.Foundation.Metadata.ApiInformation.IsPropertyPresent("Windows.UI.Xaml.UIElement", "XamlRoot") && XamlRoot != null)
             {
-                updateTimer = DispatcherQueue.GetForCurrentThread().CreateTimer();
-                updateTimer.Tick += UpdateTimer_Tick;
+                FocusOnControl(FocusManager.GetFocusedElement(XamlRoot) as FrameworkElement);
+            }
+            else
+            {
+                FocusOnControl(FocusManager.GetFocusedElement() as FrameworkElement);
             }
 
-            updateTimer.Start();
+            // Then use FocusManager event from 1809 to listen to updates
+            FocusManager.GotFocus += FocusManager_GotFocus;
         }
 
         private void Stop()
         {
-            updateTimer?.Stop();
+            FocusManager.GotFocus -= FocusManager_GotFocus;
             ClearContent();
         }
 
-        /// <summary>
-        /// Update the visual state of the control when its template is changed.
-        /// </summary>
-        protected override void OnApplyTemplate()
+        private void FocusManager_GotFocus(object sender, FocusManagerGotFocusEventArgs e)
         {
-            controlName = GetTemplateChild("ControlName") as TextBlock;
-            controlType = GetTemplateChild("ControlType") as TextBlock;
-            controlAutomationName = GetTemplateChild("ControlAutomationName") as TextBlock;
-            controlFirstParentWithName = GetTemplateChild("ControlFirstParentWithName") as TextBlock;
+            FocusOnControl(e.NewFocusedElement as FrameworkElement);
         }
 
         private void ClearContent()
@@ -105,19 +115,8 @@ private void ClearContent()
             controlFirstParentWithName.Text = string.Empty;
         }
 
-        private void UpdateTimer_Tick(object sender, object e)
+        private void FocusOnControl(FrameworkElement focusedControl)
         {
-            FrameworkElement focusedControl;
-
-            if (Windows.Foundation.Metadata.ApiInformation.IsPropertyPresent("Windows.UI.Xaml.UIElement", "XamlRoot") && XamlRoot != null)
-            {
-                focusedControl = FocusManager.GetFocusedElement(XamlRoot) as FrameworkElement;
-            }
-            else
-            {
-                focusedControl = FocusManager.GetFocusedElement() as FrameworkElement;
-            }
-
             if (focusedControl == null)
             {
                 ClearContent();