Add EmptyContent parameter to Virtualize component

src/Components/Web/src/PublicAPI.Unshipped.txt

@@ -74,3 +74,5 @@ static Microsoft.AspNetCore.Components.Web.RenderMode.Auto.get -> Microsoft.AspN
 static Microsoft.AspNetCore.Components.Web.RenderMode.Server.get -> Microsoft.AspNetCore.Components.Web.ServerRenderMode!
 static Microsoft.AspNetCore.Components.Web.RenderMode.WebAssembly.get -> Microsoft.AspNetCore.Components.Web.WebAssemblyRenderMode!
 virtual Microsoft.AspNetCore.Components.HtmlRendering.Infrastructure.StaticHtmlRenderer.WriteComponentHtml(int componentId, System.IO.TextWriter! output) -> void
+Microsoft.AspNetCore.Components.Web.Virtualization.Virtualize<TItem>.EmptyContent.get -> Microsoft.AspNetCore.Components.RenderFragment?
+Microsoft.AspNetCore.Components.Web.Virtualization.Virtualize<TItem>.EmptyContent.set -> void

src/Components/Web/src/Virtualization/Virtualize.cs

@@ -47,6 +47,10 @@ public sealed class Virtualize<TItem> : ComponentBase, IVirtualizeJsCallbacks, I
 
     private RenderFragment<PlaceholderContext>? _placeholder;
 
+    private RenderFragment? _emptyContent;
+
+    private bool _loading;
+
     [Inject]
     private IJSRuntime JSRuntime { get; set; } = default!;
 
@@ -68,6 +72,13 @@ public sealed class Virtualize<TItem> : ComponentBase, IVirtualizeJsCallbacks, I
     [Parameter]
     public RenderFragment<PlaceholderContext>? Placeholder { get; set; }
 
+    /// <summary>
+    /// Gets or sets the content to show when <see cref="Items"/> is empty
+    /// or when the <see cref="ItemsProviderResult&lt;TItem&gt;.TotalItemCount"/> is zero.
+    /// </summary>
+    [Parameter]
+    public RenderFragment? EmptyContent { get; set; }
+
     /// <summary>
     /// Gets the size of each item in pixels. Defaults to 50px.
     /// </summary>
@@ -167,6 +178,7 @@ protected override void OnParametersSet()
 
         _itemTemplate = ItemContent ?? ChildContent;
         _placeholder = Placeholder ?? DefaultPlaceholder;
+        _emptyContent = EmptyContent;
     }
 
     /// <inheritdoc />
@@ -213,15 +225,19 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
 
         _lastRenderedItemCount = 0;
 
-        // Render the loaded items.
-        if (_loadedItems != null && _itemTemplate != null)
+        if (_loadedItems != null && !_loading && _itemCount == 0 && _emptyContent != null)
+        {
+            builder.AddContent(4, _emptyContent);
+        }
+        else if (_loadedItems != null && _itemTemplate != null)
         {
             var itemsToShow = _loadedItems
                 .Skip(_itemsBefore - _loadedItemsStartIndex)
                 .Take(lastItemIndex - _loadedItemsStartIndex);
 
-            builder.OpenRegion(4);
+            builder.OpenRegion(5);
 
+            // Render the loaded items.
             foreach (var item in itemsToShow)
             {
                 _itemTemplate(item)(builder);
@@ -235,7 +251,7 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
 
         _lastRenderedPlaceholderCount = Math.Max(0, lastItemIndex - _itemsBefore - _lastRenderedItemCount);
 
-        builder.OpenRegion(5);
+        builder.OpenRegion(6);
 
         // Render the placeholders after the loaded items.
         for (; renderIndex < lastItemIndex; renderIndex++)
@@ -247,9 +263,9 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
 
         var itemsAfter = Math.Max(0, _itemCount - _visibleItemCapacity - _itemsBefore);
 
-        builder.OpenElement(6, SpacerElement);
-        builder.AddAttribute(7, "style", GetSpacerStyle(itemsAfter));
-        builder.AddElementReferenceCapture(8, elementReference => _spacerAfter = elementReference);
+        builder.OpenElement(7, SpacerElement);
+        builder.AddAttribute(8, "style", GetSpacerStyle(itemsAfter));
+        builder.AddElementReferenceCapture(9, elementReference => _spacerAfter = elementReference);
 
         builder.CloseElement();
     }
@@ -354,6 +370,7 @@ private async ValueTask RefreshDataCoreAsync(bool renderOnSuccess)
         {
             _refreshCts = new CancellationTokenSource();
             cancellationToken = _refreshCts.Token;
+            _loading = true;
         }
 
         var request = new ItemsProviderRequest(_itemsBefore, _visibleItemCapacity, cancellationToken);
@@ -368,6 +385,7 @@ private async ValueTask RefreshDataCoreAsync(bool renderOnSuccess)
                 _itemCount = result.TotalItemCount;
                 _loadedItems = result.Items;
                 _loadedItemsStartIndex = request.StartIndex;
+                _loading = false;
 
                 if (renderOnSuccess)
                 {

src/Components/test/E2ETest/Tests/VirtualizationTest.cs

@@ -489,6 +489,51 @@ public void CanHandleDataSetShrinkingWithExistingOffsetAlreadyBeyondNewListEnd(s
         Browser.True(() => GetPeopleNames(container).Contains("Person 25"));
     }
 
+    [Fact]
+    public void EmptyContentRendered_Sync()
+    {
+        Browser.MountTestComponent<VirtualizationComponent>();
+        Browser.Exists(By.Id("no-data-sync"));
+    }
+
+    [Fact]
+    public void EmptyContentRendered_Async()
+    {
+        Browser.MountTestComponent<VirtualizationComponent>();
+        var finishLoadingWithItemsButton = Browser.Exists(By.Id("finish-loading-button"));
+        var finishLoadingWithoutItemsButton = Browser.Exists(By.Id("finish-loading-button-empty"));
+        var refreshDataAsync = Browser.Exists(By.Id("refresh-data-async"));
+
+        // Check that no items or placeholders are visible.
+        // No data fetches have happened so we don't know how many items there are.
+        Browser.Equal(0, GetItemCount);
+        Browser.Equal(0, GetPlaceholderCount);
+
+        // Check that <EmptyContent> is not shown while loading
+        Browser.DoesNotExist(By.Id("no-data-async"));
+
+        // Load the initial set of items.
+        finishLoadingWithItemsButton.Click();
+
+        // Check that <EmptyContent> is still not shown (because there are items loaded)
+        Browser.DoesNotExist(By.Id("no-data-async"));
+
+        // Start loading
+        refreshDataAsync.Click();
+
+        // Check that <EmptyContent> is not shown
+        Browser.DoesNotExist(By.Id("no-data-async"));
+
+        // Simulate 0 items
+        finishLoadingWithoutItemsButton.Click();
+
+        // Check that <EmptyContent> is shown
+        Browser.Exists(By.Id("no-data-async"));
+
+        int GetItemCount() => Browser.FindElements(By.Id("async-item")).Count;
+        int GetPlaceholderCount() => Browser.FindElements(By.Id("async-placeholder")).Count;
+    }
+
     private string[] GetPeopleNames(IWebElement container)
     {
         var peopleElements = container.FindElements(By.CssSelector(".person span"));

src/Components/test/testassets/BasicTestApp/VirtualizationComponent.razor

@@ -14,20 +14,40 @@
 
 <p>
     Asynchronous:<br />
-    <button id="finish-loading-button" @onclick="FinishLoadingAsync">Finish loading</button><br />
+    <button id="finish-loading-button-empty" @onclick="() => FinishLoadingAsync(0)">Finish loading with total item count = 0</button><br />
+    <button id="finish-loading-button" @onclick="() => FinishLoadingAsync(200)">Finish loading with total item count = 200</button><br />
+    <button id="refresh-data-async" @onclick="() => asyncComponent.RefreshDataAsync()">Call RefreshDataAsync</button><br />
     Cancellation count: <span id="cancellation-count">@asyncCancellationCount</span><br />
     <div id="async-container" style="background-color: #eee; height: 500px; overflow-y: auto">
-        <Virtualize ItemsProvider="GetItemsAsync" ItemSize="itemSize">
+        <Virtualize @ref="asyncComponent" ItemsProvider="GetItemsAsync" ItemSize="itemSize">
             <ItemContent>
                 <div @key="context" id="async-item" style="height: @(itemSize)px; background-color: rgb(@((context % 2) * 255), @((1-(context % 2)) * 255), 255);">Item @context</div>
             </ItemContent>
             <Placeholder>
                 <div id="async-placeholder" style="height: @(context.Size)px; background-color: orange;">Loading item @context.Index...</div>
             </Placeholder>
+            <EmptyContent>
+                <p id="no-data-async">No data to show</p>
+            </EmptyContent>
         </Virtualize>
     </div>
 </p>
 
+<p>
+    Empty Content:<br />
+<div id="empty-container" style="background-color: #eee; height: 100px; overflow-y: auto">
+    <Virtualize Items="@emptyCollection" ItemSize="itemSize">
+        <ItemContent>
+            <div @key="context" style="height: @(itemSize)px; background-color: rgb(@((context % 2) * 255), @((1-(context % 2)) * 255), 255);">
+                Item @context</div>
+        </ItemContent>
+        <EmptyContent>
+            <p id="no-data-sync">No data to show</p>
+        </EmptyContent>
+    </Virtualize>
+</div>
+</p>
+
 <p>
     Slightly incorrect item size:<br />
     <div id="incorrect-size-container" style="background-color: #eee; height: 500px; overflow-y: auto">
@@ -47,12 +67,13 @@
 @code {
     float itemSize = 100;
     ICollection<int> fixedItems = Enumerable.Range(0, 1000).ToList();
+    ICollection<int> emptyCollection = Array.Empty<int>();
 
     int asyncTotalItemCount = 200;
     int asyncCancellationCount = 0;
     TaskCompletionSource asyncTcs = new TaskCompletionSource();
 
-    HashSet<int> cachedItems = new HashSet<int>();
+    Virtualize<int> asyncComponent;
 
     async ValueTask<ItemsProviderResult<int>> GetItemsAsync(ItemsProviderRequest request)
     {
@@ -67,8 +88,9 @@
         return new ItemsProviderResult<int>(Enumerable.Range(request.StartIndex, request.Count), asyncTotalItemCount);
     }
 
-    void FinishLoadingAsync()
+    void FinishLoadingAsync(int totalItemCount)
     {
+        asyncTotalItemCount = totalItemCount;
         asyncTcs.SetResult();
         asyncTcs = new TaskCompletionSource();
     }