Implement #4177

Author: vnbaaij

src/Core/Components/DataGrid/FluentDataGrid.razor

@@ -40,7 +40,11 @@
                 </thead>
             }
             <tbody>
-                @if (EffectiveLoadingValue)
+				@if (_lastError != null)
+				{
+					@_renderErrorContent
+				}
+				else if (EffectiveLoadingValue)
                 {
                     @_renderLoadingContent
                 }
@@ -83,7 +87,11 @@
     {
         var initialRowIndex = (GenerateHeader != DataGridGeneratedHeaderType.None) ? 2 : 1; // aria-rowindex is 1-based, plus 1 if there is a header
         var rowIndex = initialRowIndex;
-        if (_internalGridContext.Items.Any())
+		if (_lastError != null)
+		{
+			RenderErrorContent(__builder);
+		}
+		else if (_internalGridContext.Items.Any())
         {
             foreach (var item in _internalGridContext.Items)
             {
@@ -261,4 +269,36 @@
             </FluentDataGridCell>
         </FluentDataGridRow>
     }
+
+	private void RenderErrorContent(RenderTreeBuilder __builder)
+	{
+		if (_lastError == null)
+		{
+			return;
+		}
+
+		string? style = null;
+		string? colspan = null;
+		if (DisplayMode == DataGridDisplayMode.Grid)
+		{
+			style = $"grid-column: 1 / {_columns.Count + 1}";
+		}
+		else
+		{
+			colspan = _columns.Count.ToString();
+		}
+
+		<FluentDataGridRow Class="@ERROR_CONTENT_ROW_CLASS" TGridItem="TGridItem">
+			<FluentDataGridCell Class="empty-content-cell" Style="@style" colspan="@colspan">
+				@if (ErrorContent is null)
+				{
+					@("An error occurred while retrieving data.")
+				}
+				else
+				{
+					@ErrorContent(_lastError)
+				}
+			</FluentDataGridCell>
+		</FluentDataGridRow>
+	}
 }

src/Core/Components/DataGrid/FluentDataGrid.razor.cs

@@ -26,6 +26,7 @@ public partial class FluentDataGrid<TGridItem> : FluentComponentBase, IHandleEve
 
     internal const string EMPTY_CONTENT_ROW_CLASS = "empty-content-row";
     internal const string LOADING_CONTENT_ROW_CLASS = "loading-content-row";
+    internal const string ERROR_CONTENT_ROW_CLASS = "error-content-row";
 
     private ElementReference? _gridReference;
     private Virtualize<(int, TGridItem)>? _virtualizeComponent;
@@ -44,11 +45,13 @@ public partial class FluentDataGrid<TGridItem> : FluentComponentBase, IHandleEve
     private readonly RenderFragment _renderNonVirtualizedRows;
     private readonly RenderFragment _renderEmptyContent;
     private readonly RenderFragment _renderLoadingContent;
+    private readonly RenderFragment _renderErrorContent;
     private string? _internalGridTemplateColumns;
     private PaginationState? _lastRefreshedPaginationState;
     private IQueryable<TGridItem>? _lastAssignedItems;
     private GridItemsProvider<TGridItem>? _lastAssignedItemsProvider;
     private CancellationTokenSource? _pendingDataLoadCancellationTokenSource;
+    private Exception? _lastError;
     private GridItemsProviderRequest<TGridItem>? _lastRequest;
     private bool _forceRefreshData;
     private readonly EventCallbackSubscriber<PaginationState> _currentPageItemsChanged;
@@ -64,11 +67,12 @@ public FluentDataGrid(LibraryConfiguration configuration) : base(configuration)
         _renderNonVirtualizedRows = RenderNonVirtualizedRows;
         _renderEmptyContent = RenderEmptyContent;
         _renderLoadingContent = RenderLoadingContent;
+        _renderErrorContent = RenderErrorContent;
 
-        // As a special case, we don't issue the first data load request until we've collected the initial set of columns
-        // This is so we can apply default sort order (or any future per-column options) before loading data
-        // We use EventCallbackSubscriber to safely hook this async operation into the synchronous rendering flow
-        EventCallbackSubscriber<object?>? columnsFirstCollectedSubscriber = new(
+    // As a special case, we don't issue the first data load request until we've collected the initial set of columns
+    // This is so we can apply default sort order (or any future per-column options) before loading data
+    // We use EventCallbackSubscriber to safely hook this async operation into the synchronous rendering flow
+    EventCallbackSubscriber<object?>? columnsFirstCollectedSubscriber = new(
             EventCallback.Factory.Create<object?>(this, RefreshDataCoreAsync));
         columnsFirstCollectedSubscriber.SubscribeOrMove(_internalGridContext.ColumnsFirstCollected);
     }
@@ -323,6 +327,26 @@ public FluentDataGrid(LibraryConfiguration configuration) : base(configuration)
     [Parameter]
     public RenderFragment? LoadingContent { get; set; }
 
+    /// <summary>
+    /// Gets or sets the callback that is invoked when the asynchronous loading state of items changes and <see cref="IAsyncQueryExecutor"/> is used.
+    /// </summary>
+    /// <remarks>The callback receives a <see langword="true"/> value when items start loading
+    /// and a <see langword="false"/> value when the loading process completes.</remarks>
+    [Parameter]
+    public EventCallback<bool> OnItemsLoading { get; set; }
+
+    /// <summary>
+    /// Gets or sets a delegate that determines whether a given exception should be handled.
+    /// </summary>
+    [Parameter]
+    public Func<Exception, bool>? HandleLoadingError { get; set; }
+
+    /// <summary>
+    /// Gets or sets the content to render when an error occurs.
+    /// </summary>
+    [Parameter]
+    public RenderFragment<Exception>? ErrorContent { get; set; }
+
     /// <summary>
     /// Sets <see cref="GridTemplateColumns"/> to automatically fit the columns to the available width as best it can.
     /// </summary>
@@ -821,7 +845,7 @@ private async Task RefreshDataCoreAsync()
                 Pagination?.SetTotalItemCountAsync(_internalGridContext.TotalItemCount);
             }
 
-            if (_internalGridContext.TotalItemCount > 0 && Loading is null)
+            if ((_internalGridContext.TotalItemCount > 0 && Loading is null) || _lastError != null)
             {
                 Loading = false;
                 _ = InvokeAsync(StateHasChanged);
@@ -841,6 +865,7 @@ private async Task RefreshDataCoreAsync()
     // Normalizes all the different ways of configuring a data source so they have common GridItemsProvider-shaped API
     private async ValueTask<GridItemsProviderResult<TGridItem>> ResolveItemsRequestAsync(GridItemsProviderRequest<TGridItem> request)
     {
+        CheckAndResetLastError();
         try
         {
             if (ItemsProvider is not null)
@@ -857,6 +882,11 @@ private async ValueTask<GridItemsProviderResult<TGridItem>> ResolveItemsRequestA
 
             if (Items is not null)
             {
+                if (_asyncQueryExecutor is not null)
+                {
+                    await OnItemsLoading.InvokeAsync(true);
+                }
+
                 var totalItemCount = _asyncQueryExecutor is null ? Items.Count() : await _asyncQueryExecutor.CountAsync(Items, request.CancellationToken);
                 _internalGridContext.TotalItemCount = totalItemCount;
                 IQueryable<TGridItem>? result;
@@ -877,14 +907,43 @@ private async ValueTask<GridItemsProviderResult<TGridItem>> ResolveItemsRequestA
                 return GridItemsProviderResult.From(resultArray, totalItemCount);
             }
         }
-        catch (OperationCanceledException oce) when (oce.CancellationToken == request.CancellationToken)
+        catch (OperationCanceledException oce) when (oce.CancellationToken == request.CancellationToken) // No-op; we canceled the operation, so it's fine to suppress this exception.
         {
-            // No-op; we canceled the operation, so it's fine to suppress this exception.
+        }
+        catch (Exception ex) when (HandleLoadingError?.Invoke(ex) == true)
+        {
+            _lastError = ex.GetBaseException();
+        }
+        finally
+        {
+            if (Items is not null && _asyncQueryExecutor is not null)
+            {
+                CheckAndResetLoading();
+                await OnItemsLoading.InvokeAsync(false);
+            }
         }
 
         return GridItemsProviderResult.From(Array.Empty<TGridItem>(), 0);
     }
 
+    private void CheckAndResetLoading()
+    {
+        if (Loading == true)
+        {
+            Loading = false;
+            StateHasChanged();
+        }
+    }
+
+    private void CheckAndResetLastError()
+    {
+        if (_lastError != null)
+        {
+            _lastError = null;
+            StateHasChanged();
+        }
+    }
+
     private string AriaSortValue(ColumnBase<TGridItem> column)
          => _sortByColumn == column
              ? (_sortByAscending ? "ascending" : "descending")