fix(admin-panel): go back to the first page when a table refresh finds its page empty

src/Web/Shared/Components/Form/DataTable.razor

@@ -130,6 +130,7 @@
 
     private async Task GetPageAsync(int page)
     {
+        var restartFromFirstPage = false;
         this._isLoading = true;
         try
         {
@@ -151,12 +152,21 @@
             else
             {
                 this._hasMoreEntries = false;
+
+                // A refresh of a page which does not exist anymore (for example after a filter reduced the results) would
+                // show an empty table without a way to notice why. Start over from the first page instead.
+                restartFromFirstPage = isRefresh && page > 1;
             }
         }
         finally
         {
             this._isLoading = false;
             await this.InvokeAsync(this.StateHasChanged).ConfigureAwait(false);
         }
+
+        if (restartFromFirstPage)
+        {
+            await this.GetPageAsync(1);
+        }
     }
 }

tests/MUnique.OpenMU.Web.Tests/Components/DataTableTests.cs

@@ -0,0 +1,75 @@
+// <copyright file="DataTableTests.cs" company="MUnique">
+// Licensed under the MIT License. See LICENSE file in the project root for full license information.
+// </copyright>
+
+namespace MUnique.OpenMU.Web.Tests.Components;
+
+using Bunit;
+using Microsoft.AspNetCore.Components;
+using Microsoft.Extensions.DependencyInjection;
+using MUnique.OpenMU.Web.Shared.Components.Form;
+using MUnique.OpenMU.Web.Shared.Services;
+
+/// <summary>
+/// Tests for the <see cref="DataTable{TItem}"/> component.
+/// </summary>
+[TestFixture]
+public class DataTableTests
+{
+    /// <summary>
+    /// A refresh of a page which doesn't exist anymore, e.g. after a filter reduced the results,
+    /// starts over from the first page instead of showing an empty table.
+    /// </summary>
+    [Test]
+    public void RefreshOfEmptyPage_StartsOverFromFirstPage()
+    {
+        using var context = new BunitContext();
+        var dataService = new TestDataService(Enumerable.Range(1, 45).Select(i => new Item(i)));
+        context.Services.AddSingleton<IDataService<Item>>(dataService);
+        var component = context.Render<DataTable<Item>>(parameters => parameters
+            .Add(table => table.TableHeader, (RenderFragment)(_ => { }))
+            .Add(table => table.ItemTemplate, item => builder => builder.AddContent(0, item.Number)));
+
+        // The buttons are: previous page, page number, next page.
+        component.FindAll("button")[2].Click();
+        component.FindAll("button")[2].Click();
+        component.WaitForAssertion(() => Assert.That(component.Instance.Page, Is.EqualTo(3)));
+
+        dataService.SetItems(Enumerable.Range(1, 3).Select(i => new Item(i)));
+
+        component.WaitForAssertion(() =>
+        {
+            Assert.That(component.Instance.Page, Is.EqualTo(1));
+            Assert.That(component.FindAll("tbody tr"), Has.Count.EqualTo(3));
+        });
+    }
+
+    /// <summary>
+    /// An item of the table.
+    /// </summary>
+    /// <param name="Number">The number of the item.</param>
+    public sealed record Item(int Number);
+
+    private sealed class TestDataService : IDataService<Item>, ISupportDataChangedNotification
+    {
+        private List<Item> _items;
+
+        public TestDataService(IEnumerable<Item> items)
+        {
+            this._items = items.ToList();
+        }
+
+        public event EventHandler? DataChanged;
+
+        public Task<List<Item>> GetAsync(int offset, int count)
+        {
+            return Task.FromResult(this._items.Skip(offset).Take(count).ToList());
+        }
+
+        public void SetItems(IEnumerable<Item> items)
+        {
+            this._items = items.ToList();
+            this.DataChanged?.Invoke(this, EventArgs.Empty);
+        }
+    }
+}