BugFix: Fixes BasemapGallery View Style in MAUI Windows (#596)

* MAUI Windows :
Enabled Grid View for BasemapGallery in MAUI Windows.
Changed BasemaGallery's CollectionView ItemLayout to fixed GridItemsLayout and manipulated span and spacing values with BasemapGallery View Style.

* Refactor ItemsLayout update logic in BasemapGallery.
Includes a specific workaround for iOS to avoid a `NullReferenceException` when changing the span of `GridItemsLayout`.

src/Toolkit/Toolkit.Maui/BasemapGallery/BasemapGallery.Appearance.cs

@@ -45,7 +45,7 @@ static BasemapGallery()
             Border border = new Border { Padding = 4, StrokeThickness = 1, StrokeShape = new Rectangle() };
             Grid outerScrimContainer = new Grid();
             outerScrimContainer.RowDefinitions.Add(new RowDefinition { Height = new GridLength(86) });
-            outerScrimContainer.RowDefinitions.Add(new RowDefinition { Height = new GridLength(40)});
+            outerScrimContainer.RowDefinitions.Add(new RowDefinition { Height = new GridLength(40) });
             outerScrimContainer.RowSpacing = 4;
 
             Image thumbnail = new Image { WidthRequest = 64, HeightRequest = 64, Aspect = Aspect.AspectFill, BackgroundColor = Colors.Transparent, HorizontalOptions = LayoutOptions.Center };
@@ -61,9 +61,9 @@ static BasemapGallery()
             Grid scrimGrid = new Grid();
             scrimGrid.SetAppThemeColor(BackgroundColorProperty, Colors.White, Colors.Black);
             outerScrimContainer.Children.Add(scrimGrid);
-            Grid.SetRowSpan(scrimGrid, 2);
-            
-
+            Grid.SetRowSpan(scrimGrid, 2);
+
+
             thumbnail.SetBinding(Image.SourceProperty, nameof(BasemapGalleryItem.ThumbnailData), converter: ImageSourceConverter);
             nameLabel.SetBinding(Label.TextProperty, nameof(BasemapGalleryItem.Name));
             scrimGrid.SetBinding(OpacityProperty, nameof(BasemapGalleryItem.IsValid), mode: BindingMode.OneWay, converter: OpacityConverter);
@@ -194,7 +194,8 @@ protected override void OnApplyTemplate()
         _loadingScrim = GetTemplateChild("PART_LoadingScrim") as View;
         if (ListView != null)
         {
-            ListView.BindingContext = _controller;
+            ListView.BindingContext = _controller;
+            ListView.ItemsLayout = new GridItemsLayout(ItemsLayoutOrientation.Vertical);
         }
     }
 
@@ -280,14 +281,8 @@ private void HandleTemplateChange(double currentWidth)
             case BasemapGalleryViewStyle.Automatic:
                 styleAfterUpdate = currentWidth >= ViewStyleWidthThreshold ? BasemapGalleryViewStyle.Grid : BasemapGalleryViewStyle.List;
                 break;
-        }
-
-        // This check may be removable once UWP collectionview supports dynamic item sizing: https://gist.github.com/hartez/7d0edd4182dbc7de65cebc6c67f72e14
-        if (DeviceInfo.Platform == DevicePlatform.WinUI)
-        {
-            styleAfterUpdate = BasemapGalleryViewStyle.List;
-        }
-
+        }
+
         if (styleAfterUpdate == BasemapGalleryViewStyle.Grid)
         {
             gridSpanAfterUpdate = System.Math.Max((int)(currentWidth / 128), 1);
@@ -298,13 +293,13 @@ private void HandleTemplateChange(double currentWidth)
             if (styleAfterUpdate == BasemapGalleryViewStyle.List)
             {
                 ListView.ItemTemplate = ListItemTemplate;
-                ListView.ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Vertical) { ItemSpacing = 0 };
+                UpdateItemsLayout(1, 0, 0);
                 ListView.Margin = new Thickness(0);
             }
             else
             {
-                ListView.ItemTemplate = GridItemTemplate;
-                ListView.ItemsLayout = new GridItemsLayout(gridSpanAfterUpdate, ItemsLayoutOrientation.Vertical) { HorizontalItemSpacing = 4, VerticalItemSpacing = 4 };
+                ListView.ItemTemplate = DefaultGridDataTemplate;
+                UpdateItemsLayout(gridSpanAfterUpdate, 4, 4);
                 ListView.Margin = new Thickness(4, 4, 0, 0);
             }
 
@@ -313,6 +308,31 @@ private void HandleTemplateChange(double currentWidth)
         }
     }
 
+    private void UpdateItemsLayout(int span, double verticalSpacing, double horizontalSpacing)
+    {
+        if (ListView is not null)
+        {
+#if __IOS__
+            // This is a workaround for a bug in the current version of the iOS renderer for CollectionView
+            // where CollectionView throws `NullReferneceException` on changing span of GridItemsLayout.
+            // It won't be needed once we move MAUI version up to 8.0.10+.
+            // Will have to consider if toolkit can require a newer version (currently it's fixed to API).
+            ListView.ItemsLayout = new GridItemsLayout(span, ItemsLayoutOrientation.Vertical)
+            {
+                VerticalItemSpacing = verticalSpacing,
+                HorizontalItemSpacing = horizontalSpacing,
+            }; 
+#else
+            if (ListView.ItemsLayout is GridItemsLayout layout)
+            {
+                layout.Span = span;
+                layout.VerticalItemSpacing = verticalSpacing;
+                layout.HorizontalItemSpacing = horizontalSpacing;
+            }
+#endif
+        }
+    }
+
     /// <inheritdoc />
     protected override void OnSizeAllocated(double width, double height)
     {