Ensure persisted component state gets passed at the end of streaming updates even when no components change

Author: javiercn

src/Components/Samples/BlazorUnitedApp/Data/WeatherForecastService.cs

@@ -10,13 +10,14 @@ public class WeatherForecastService
         "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
     };
 
-    public Task<WeatherForecast[]> GetForecastAsync(DateOnly startDate)
+    public async Task<WeatherForecast[]> GetForecastAsync(DateOnly startDate)
     {
-        return Task.FromResult(Enumerable.Range(1, 5).Select(index => new WeatherForecast
+        await Task.Delay(1000);
+        return [.. Enumerable.Range(1, 5).Select(index => new WeatherForecast
         {
             Date = startDate.AddDays(index),
             TemperatureC = Random.Shared.Next(-20, 55),
             Summary = Summaries[Random.Shared.Next(Summaries.Length)]
-        }).ToArray());
+        })];
     }
 }

src/Components/Samples/BlazorUnitedApp/Pages/FetchData.razor

@@ -2,13 +2,15 @@
 @using BlazorUnitedApp.Data
 @inject WeatherForecastService ForecastService
 
-<PageTitle>Weather forecast</PageTitle>
+<FetchDataChildren @key="0" @rendermode="InteractiveServer"></FetchDataChildren>
+
+@* <PageTitle>Weather forecast</PageTitle>
 
 <h1>Weather forecast</h1>
 
 <p>This component demonstrates fetching data from a service.</p>
 
-@if (forecasts == null)
+@if (Forecasts is null)
 {
     <p><em>Loading...</em></p>
 }
@@ -24,7 +26,7 @@ else
             </tr>
         </thead>
         <tbody>
-            @foreach (var forecast in forecasts)
+            @foreach (var forecast in Forecasts)
             {
                 <tr>
                     <td>@forecast.Date.ToShortDateString()</td>
@@ -38,10 +40,11 @@ else
 }
 
 @code {
-    private WeatherForecast[]? forecasts;
+    private WeatherForecast[]? Forecasts { get; set; }
 
     protected override async Task OnInitializedAsync()
     {
-        forecasts = await ForecastService.GetForecastAsync(DateOnly.FromDateTime(DateTime.Now));
+        Forecasts ??= await ForecastService.GetForecastAsync(DateOnly.FromDateTime(DateTime.Now));
     }
 }
+ *@

src/Components/Samples/BlazorUnitedApp/Pages/FetchDataChildren.razor

@@ -1,6 +1,7 @@
 @using BlazorUnitedApp.Data
 @inject WeatherForecastService ForecastService
 @inject NavigationManager NavigationManager
+@attribute [StreamRendering]
 
 <h1>Weather forecast</h1>
 

src/Components/Web.JS/src/Services/WebRootComponentManager.ts

@@ -269,10 +269,16 @@ export class WebRootComponentManager implements DescriptorHandler, RootComponent
 
   private refreshRootComponents(components: Iterable<RootComponentInfo>) {
     const operationsByRendererId = new Map<WebRendererId, RootComponentOperation[]>();
-
     for (const component of components) {
       const operation = this.determinePendingOperation(component);
       if (!operation) {
+        if (component.assignedRendererId) {
+          // If there are operations we will update the persisted component state for that
+          // render mode. But if we detect a component that doesn't require any update we
+          // still want to add an entry so that we process any persisted state update.
+          // This case is specially common during streaming rendering.
+          operationsByRendererId.set(component.assignedRendererId, []);
+        }
         continue;
       }