Merged PR 1497281: Reload nav view category when invoked, even if the selection didn't actually change

Switched over to using the ItemInvoked event on NavigationView so that we have the chance to reload the category, even if the selection didn't actually change.

SampleGallery/SampleGalleryNavViewHost.xaml

@@ -15,7 +15,7 @@
         <contract5Present:NavigationView 
             x:Name="NavView"  
             IsSettingsVisible="True"
-            SelectionChanged="NavViewSelectionChanged">
+            ItemInvoked="NavViewItemInvoked">
 
             <Frame x:Name="ContentFrame" Margin="24"/>
                         

SampleGallery/SampleGalleryNavViewHost.xaml.cs

@@ -6,6 +6,7 @@ using System.Linq;
 using System.Runtime.InteropServices.WindowsRuntime;
 using Windows.Foundation;
 using Windows.Foundation.Collections;
+using Windows.System;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Controls.Primitives;
@@ -34,7 +35,7 @@ namespace CompositionSampleGallery
                 return _itemsSource;
             }
             set
-            {                
+            {
                 NavView.MenuItems.Clear();
                 _itemsSource = value;
 
@@ -99,36 +100,48 @@ namespace CompositionSampleGallery
             BackStackStateChanged?.Invoke(this, EventArgs.Empty);
         }
 
-        private void NavViewSelectionChanged(NavigationView sender, NavigationViewSelectionChangedEventArgs args)
+        private void OnLoaded(object sender, RoutedEventArgs e)
         {
-            NavigationItem navItem = (NavigationItem)((NavigationViewItem)args.SelectedItem).DataContext;
+            NavView.SelectedItem = NavView.MenuItems[0];
+            NavigateToSelectedItem();
+        }
 
-            // Verify nav item exists for navigation info. Settings page will return no navigation item.
-            if (navItem != null)
+        private void NavViewItemInvoked(NavigationView sender, NavigationViewItemInvokedEventArgs args)
+        {
+            if (args.IsSettingsInvoked)
             {
-                Dictionary<string, string> properties = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
-                properties.Add("TargetView", navItem.Category.ToString());
-                Shared.AppTelemetryClient.TrackEvent("Navigate", properties, null);
-            }
-
-            if (args.IsSettingsSelected)
-            {
-                ContentFrame.Navigate(typeof(Settings));
+                Navigate(typeof(Settings), null);
+                ClearBackstack();
             }
             else
             {
-                ContentFrame.Navigate(navItem.PageType, navItem);
-            }
+                // The NavigationViewItemInvokedEventArgs only give us the string label of the item that was invoked.  Rather than writing
+                // awkward code to string compare the arg against each category name, we instead can just fetch the selected item
+                // off of the navigation view.  However, if the selection is actually changing as a result of this invocation, the NavView doesn't
+                // reflect that change until *after* this event has fired.  So, use the thread's DispatcherQueue to defer the navigation until 
+                // after the nav view has processed the potential selection change.
 
+                DispatcherQueue.GetForCurrentThread().TryEnqueue(DispatcherQueuePriority.High, () => { NavigateToSelectedItem(); } );                
+            }
+        }
+
+        private void NavigateToSelectedItem()
+        {
+            NavigationItem navItem = (NavigationItem)((NavigationViewItem)NavView.SelectedItem).DataContext;
+
+            Dictionary<string, string> properties = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+            properties.Add("TargetView", navItem.Category.ToString());
+            Shared.AppTelemetryClient.TrackEvent("Navigate", properties, null);
+            Navigate(navItem.PageType, navItem);
+            ClearBackstack();
+        }
+
+        private void ClearBackstack()
+        {
             // Reset the backstack when a new category is selected to avoid having to coordinate the cateogory 
             // selection as we navigate back through the backstack
             ContentFrame.BackStack.Clear();
             FireBackStackChangedEvent();
         }
-
-        private void OnLoaded(object sender, RoutedEventArgs e)
-        {
-            NavView.SelectedItem = NavView.MenuItems[0];
-        }
     }
 }