Changed SmoothScrollIntoView method to truly asynchronous

Author: Vijay-Nirmal

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ListViewExtensions/ListViewExtensionsPage.xaml.cs

@@ -37,7 +37,7 @@ public void OnXamlRendered(FrameworkElement control)
 
         private void Load()
         {
-            SampleController.Current.RegisterNewCommand("Start Smooth Scroll", (sender, args) =>
+            SampleController.Current.RegisterNewCommand("Start Smooth Scroll", async (sender, args) =>
             {
                 var index = int.TryParse(IndexInput.Text, out var i) ? i : 0;
                 var itemPlacement = ItemPlacementInput.SelectedItem switch
@@ -55,7 +55,7 @@ private void Load()
                 var scrollIfVisibile = ScrollIfVisibileInput.IsChecked ?? true;
                 var additionalHorizontalOffset = int.TryParse(AdditionalHorizontalOffsetInput.Text, out var ho) ? ho : 0;
                 var additionalVerticalOffset = int.TryParse(AdditionalVerticalOffsetInput.Text, out var vo) ? vo : 0;
-                sampleListView.SmoothScrollIntoViewWithIndexAsync(index, itemPlacement, disableAnimation, scrollIfVisibile, additionalHorizontalOffset, additionalVerticalOffset);
+                await sampleListView.SmoothScrollIntoViewWithIndexAsync(index, itemPlacement, disableAnimation, scrollIfVisibile, additionalHorizontalOffset, additionalVerticalOffset);
             });
 
             if (sampleListView != null)

Microsoft.Toolkit.Uwp.UI/Extensions/ListViewBase/ListViewExtensions.SmoothScrollIntoView.cs

@@ -26,7 +26,7 @@ public static partial class ListViewExtensions
         /// <param name="scrollIfVisible">Set false to disable scrolling when the corresponding item is in view</param>
         /// <param name="additionalHorizontalOffset">Adds additional horizontal offset</param>
         /// <param name="additionalVerticalOffset">Adds additional vertical offset</param>
-        /// <returns>Note: Even though this return <see cref="Task"/>, it will not wait until the scrolling completes</returns>
+        /// <returns>Returns <see cref="Task"/> that completes after scrolling</returns>
         public static async Task SmoothScrollIntoViewWithIndexAsync(this ListViewBase listViewBase, int index, ScrollItemPlacement itemPlacement = ScrollItemPlacement.Default, bool disableAnimation = false, bool scrollIfVisible = true, int additionalHorizontalOffset = 0, int additionalVerticalOffset = 0)
         {
             if (index > (listViewBase.Items.Count - 1))
@@ -56,15 +56,15 @@ public static async Task SmoothScrollIntoViewWithIndexAsync(this ListViewBase li
                 previousXOffset = scrollViewer.HorizontalOffset;
                 previousYOffset = scrollViewer.VerticalOffset;
 
-                var tcs = new TaskCompletionSource<object>();
+                var tcs = new TaskCompletionSource<VoidResult>();
 
-                void ViewChanged(object obj, ScrollViewerViewChangedEventArgs args) => tcs.TrySetResult(result: null);
+                void ViewChanged(object obj, ScrollViewerViewChangedEventArgs args) => tcs.TrySetResult(result: default);
 
                 try
                 {
                     scrollViewer.ViewChanged += ViewChanged;
                     listViewBase.ScrollIntoView(listViewBase.Items[index], ScrollIntoViewAlignment.Leading);
-                    await tcs.Task;
+                    await tcs.Task.ConfigureAwait(true);
                 }
                 finally
                 {
@@ -80,20 +80,7 @@ public static async Task SmoothScrollIntoViewWithIndexAsync(this ListViewBase li
             // Scrolling back to previous position
             if (isVirtualizing)
             {
-                var tcs = new TaskCompletionSource<object>();
-
-                void ViewChanged(object obj, ScrollViewerViewChangedEventArgs args) => tcs.TrySetResult(result: null);
-
-                try
-                {
-                    scrollViewer.ViewChanged += ViewChanged;
-                    scrollViewer.ChangeView(previousXOffset, previousYOffset, zoomFactor: null, disableAnimation: true);
-                    await tcs.Task;
-                }
-                finally
-                {
-                    scrollViewer.ViewChanged -= ViewChanged;
-                }
+                await scrollViewer.ChangeViewAsync(previousXOffset, previousYOffset, zoomFactor: null, disableAnimation: true).ConfigureAwait(true);
             }
 
             var listViewBaseWidth = listViewBase.ActualWidth;
@@ -185,7 +172,7 @@ public static async Task SmoothScrollIntoViewWithIndexAsync(this ListViewBase li
                 }
             }
 
-            scrollViewer.ChangeView(finalXPosition, finalYPosition, zoomFactor: null, disableAnimation);
+            await scrollViewer.ChangeViewAsync(finalXPosition, finalYPosition, zoomFactor: null, disableAnimation).ConfigureAwait(true);
         }
 
         /// <summary>
@@ -198,10 +185,42 @@ public static async Task SmoothScrollIntoViewWithIndexAsync(this ListViewBase li
         /// <param name="scrollIfVisibile">Set true to disable scrolling when the corresponding item is in view</param>
         /// <param name="additionalHorizontalOffset">Adds additional horizontal offset</param>
         /// <param name="additionalVerticalOffset">Adds additional vertical offset</param>
-        /// <returns>Note: Even though this return <see cref="Task"/>, it will not wait until the scrolling completes</returns>
+        /// <returns>Returns <see cref="Task"/> that completes after scrolling</returns>
         public static async Task SmoothScrollIntoViewWithItemAsync(this ListViewBase listViewBase, object item, ScrollItemPlacement itemPlacement = ScrollItemPlacement.Default, bool disableAnimation = false, bool scrollIfVisibile = true, int additionalHorizontalOffset = 0, int additionalVerticalOffset = 0)
         {
-            await SmoothScrollIntoViewWithIndexAsync(listViewBase, listViewBase.Items.IndexOf(item), itemPlacement, disableAnimation, scrollIfVisibile, additionalHorizontalOffset, additionalVerticalOffset);
+            await SmoothScrollIntoViewWithIndexAsync(listViewBase, listViewBase.Items.IndexOf(item), itemPlacement, disableAnimation, scrollIfVisibile, additionalHorizontalOffset, additionalVerticalOffset).ConfigureAwait(true);
+        }
+
+        /// <summary>
+        /// Changes the view of <see cref="ScrollViewer"/> asynchronous.
+        /// </summary>
+        /// <param name="scrollViewer">The scroll viewer.</param>
+        /// <param name="horizontalOffset">The horizontal offset.</param>
+        /// <param name="verticalOffset">The vertical offset.</param>
+        /// <param name="zoomFactor">The zoom factor.</param>
+        /// <param name="disableAnimation">if set to <c>true</c> disable animation.</param>
+        private static async Task ChangeViewAsync(this ScrollViewer scrollViewer, double? horizontalOffset, double? verticalOffset, float? zoomFactor, bool disableAnimation)
+        {
+            var tcs = new TaskCompletionSource<VoidResult>();
+
+            void ViewChanged(object _, ScrollViewerViewChangedEventArgs __) => tcs.TrySetResult(result: default);
+
+            try
+            {
+                scrollViewer.ViewChanged += ViewChanged;
+                scrollViewer.ChangeView(horizontalOffset, verticalOffset, zoomFactor, disableAnimation);
+                await tcs.Task.ConfigureAwait(true);
+            }
+            finally
+            {
+                scrollViewer.ViewChanged -= ViewChanged;
+            }
         }
+
+        /// <summary>
+        /// Used as a placeholder TResult to indicate that a <![CDATA[Task<TResult>]]>  has a void TResult
+        /// </summary>
+        /// <see href="https://referencesource.microsoft.com/#System.Core/System/Threading/Tasks/TaskExtensions.cs,6e36a68760fb02e6,references"/>
+        private struct VoidResult { }
     }
 }