Sections with other features

src/Components/Components/src/CascadingParameterState.cs

@@ -67,7 +67,7 @@ public static IReadOnlyList<CascadingParameterState> FindCascadingParameters(Com
                 return candidateSupplier;
             }
 
-            candidate = candidate.ParentComponentState;
+            candidate = candidate.LogicalParentComponentState;
         } while (candidate != null);
 
         // No match

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

@@ -23,6 +23,7 @@ Microsoft.AspNetCore.Components.ParameterView.ToDictionary() -> System.Collectio
 Microsoft.AspNetCore.Components.RenderHandle.DispatchExceptionAsync(System.Exception! exception) -> System.Threading.Tasks.Task!
 *REMOVED*Microsoft.AspNetCore.Components.NavigationManager.ToAbsoluteUri(string! relativeUri) -> System.Uri!
 Microsoft.AspNetCore.Components.NavigationManager.ToAbsoluteUri(string? relativeUri) -> System.Uri!
+Microsoft.AspNetCore.Components.Rendering.ComponentState.LogicalParentComponentState.get -> Microsoft.AspNetCore.Components.Rendering.ComponentState?
 Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder.SetEventHandlerName(string! eventHandlerName) -> void
 *REMOVED*Microsoft.AspNetCore.Components.RouteData.RouteData(System.Type! pageType, System.Collections.Generic.IReadOnlyDictionary<string!, object!>! routeValues) -> void
 *REMOVED*Microsoft.AspNetCore.Components.RouteData.RouteValues.get -> System.Collections.Generic.IReadOnlyDictionary<string!, object!>!

src/Components/Components/src/RenderTree/Renderer.cs

@@ -126,6 +126,9 @@ private static IComponentActivator GetComponentActivatorOrDefault(IServiceProvid
     protected ComponentState GetComponentState(int componentId)
         => GetRequiredComponentState(componentId);
 
+    internal ComponentState GetComponentState(IComponent component)
+        => _componentStateByComponent.GetValueOrDefault(component);
+
     private async void RenderRootComponentsOnHotReload()
     {
         // Before re-rendering the root component, also clear any well-known caches in the framework
@@ -1039,7 +1042,7 @@ private void HandleExceptionViaErrorBoundary(Exception error, ComponentState? er
                 return; // Handled successfully
             }
 
-            candidate = candidate.ParentComponentState;
+            candidate = candidate.LogicalParentComponentState;
         }
 
         // It's unhandled, so treat as fatal

src/Components/Components/src/Rendering/ComponentState.cs

@@ -3,6 +3,7 @@
 
 using System.Diagnostics;
 using Microsoft.AspNetCore.Components.RenderTree;
+using Microsoft.AspNetCore.Components.Sections;
 
 namespace Microsoft.AspNetCore.Components.Rendering;
 
@@ -34,6 +35,9 @@ public ComponentState(Renderer renderer, int componentId, IComponent component,
         ComponentId = componentId;
         ParentComponentState = parentComponentState;
         Component = component ?? throw new ArgumentNullException(nameof(component));
+        LogicalParentComponentState = component is SectionOutlet.SectionOutletContentRenderer
+            ? (GetSectionOutletLogicalParent(renderer, (SectionOutlet)parentComponentState!.Component) ?? parentComponentState)
+            : parentComponentState;
         _renderer = renderer ?? throw new ArgumentNullException(nameof(renderer));
         _cascadingParameters = CascadingParameterState.FindCascadingParameters(this);
         CurrentRenderTree = new RenderTreeBuilder();
@@ -46,6 +50,18 @@ public ComponentState(Renderer renderer, int componentId, IComponent component,
         }
     }
 
+    private static ComponentState? GetSectionOutletLogicalParent(Renderer renderer, SectionOutlet sectionOutlet)
+    {
+        // This will return null if the SectionOutlet is not currently rendering any content
+        if (sectionOutlet.CurrentLogicalParent is { } logicalParent
+            && renderer.GetComponentState(logicalParent) is { } logicalParentComponentState)
+        {
+            return logicalParentComponentState;
+        }
+
+        return null;
+    }
+
     /// <summary>
     /// Gets the component ID.
     /// </summary>
@@ -61,6 +77,11 @@ public ComponentState(Renderer renderer, int componentId, IComponent component,
     /// </summary>
     public ComponentState? ParentComponentState { get; }
 
+    /// <summary>
+    /// Gets the <see cref="ComponentState"/> of the logical parent component, or null if this is a root component.
+    /// </summary>
+    public ComponentState? LogicalParentComponentState { get; }
+
     internal RenderTreeBuilder CurrentRenderTree { get; set; }
 
     internal void RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment, out Exception? renderFragmentException)

src/Components/Components/src/Sections/SectionContent.cs

@@ -6,7 +6,7 @@ namespace Microsoft.AspNetCore.Components.Sections;
 /// <summary>
 /// Provides content to <see cref="SectionOutlet"/> components with matching <see cref="SectionId"/>s.
 /// </summary>
-public sealed class SectionContent : ISectionContentProvider, IComponent, IDisposable
+public sealed class SectionContent : IComponent, IDisposable
 {
     private object? _registeredIdentifier;
     private bool? _registeredIsDefaultContent;
@@ -35,8 +35,6 @@ public sealed class SectionContent : ISectionContentProvider, IComponent, IDispo
     /// </summary>
     [Parameter] public RenderFragment? ChildContent { get; set; }
 
-    RenderFragment? ISectionContentProvider.Content => ChildContent;
-
     void IComponent.Attach(RenderHandle renderHandle)
     {
         _registry = renderHandle.Dispatcher.SectionRegistry;
@@ -79,7 +77,7 @@ Task IComponent.SetParametersAsync(ParameterView parameters)
             _registeredIsDefaultContent = IsDefaultContent;
         }
 
-        _registry.NotifyContentChanged(identifier, this);
+        _registry.NotifyContentProviderChanged(identifier, this);
 
         return Task.CompletedTask;
     }

src/Components/Components/src/Sections/SectionOutlet.cs

@@ -1,20 +1,20 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Microsoft.AspNetCore.Components.Rendering;
+
 namespace Microsoft.AspNetCore.Components.Sections;
 
 /// <summary>
 /// Renders content provided by <see cref="SectionContent"/> components with matching <see cref="SectionId"/>s.
 /// </summary>
-[StreamRendering(true)] // Because the content may be provided by a streaming component
-public sealed class SectionOutlet : ISectionContentSubscriber, IComponent, IDisposable
+public sealed class SectionOutlet : IComponent, IDisposable
 {
     private static readonly RenderFragment _emptyRenderFragment = _ => { };
-
     private object? _subscribedIdentifier;
     private RenderHandle _renderHandle;
     private SectionRegistry _registry = default!;
-    private RenderFragment? _content;
+    private SectionContent? _currentContentProvider;
 
     /// <summary>
     /// Gets or sets the <see cref="string"/> ID that determines which <see cref="SectionContent"/> instances will provide
@@ -28,6 +28,8 @@ public sealed class SectionOutlet : ISectionContentSubscriber, IComponent, IDisp
     /// </summary>
     [Parameter] public object? SectionId { get; set; }
 
+    internal IComponent? CurrentLogicalParent => _currentContentProvider;
+
     void IComponent.Attach(RenderHandle renderHandle)
     {
         _renderHandle = renderHandle;
@@ -73,9 +75,9 @@ Task IComponent.SetParametersAsync(ParameterView parameters)
         return Task.CompletedTask;
     }
 
-    void ISectionContentSubscriber.ContentChanged(RenderFragment? content)
+    internal void ContentUpdated(SectionContent? provider)
     {
-        _content = content;
+        _currentContentProvider = provider;
         RenderContent();
     }
 
@@ -89,7 +91,17 @@ private void RenderContent()
             return;
         }
 
-        _renderHandle.Render(_content ?? _emptyRenderFragment);
+        _renderHandle.Render(BuildRenderTree);
+    }
+
+    private void BuildRenderTree(RenderTreeBuilder builder)
+    {
+        var fragment = _currentContentProvider?.ChildContent ?? _emptyRenderFragment;
+
+        builder.OpenComponent<SectionOutletContentRenderer>(0);
+        builder.SetKey(fragment);
+        builder.AddComponentParameter(1, SectionOutletContentRenderer.ContentParameterName, fragment);
+        builder.CloseComponent();
     }
 
     /// <inheritdoc/>
@@ -100,4 +112,33 @@ public void Dispose()
             _registry.Unsubscribe(_subscribedIdentifier);
         }
     }
+
+    // This component simply renders the RenderFragment it is given
+    // The reason for rendering SectionOutlet output via this component is so that
+    // [1] We can use @key to guarantee that we only preserve descendant component
+    //     instances when they come from the same SectionContent, not unrelated ones
+    // [2] We know that whenever the SectionContent is changed to another one, there
+    //     will be a new ComponentState established to represent this intermediate
+    //     component, and it will already have the correct LogicalParentComponentState
+    //     so anything computed from this (e.g., whether or not streaming rendering is
+    //     enabled) will be freshly re-evaluated, without that information having to
+    //     change in place on an existing ComponentState.
+    internal sealed class SectionOutletContentRenderer : IComponent
+    {
+        public const string ContentParameterName = "content";
+
+        private RenderHandle _renderHandle;
+
+        public void Attach(RenderHandle renderHandle)
+        {
+            _renderHandle = renderHandle;
+        }
+
+        public Task SetParametersAsync(ParameterView parameters)
+        {
+            var fragment = parameters.GetValueOrDefault<RenderFragment>(ContentParameterName)!;
+            _renderHandle.Render(fragment);
+            return Task.CompletedTask;
+        }
+    }
 }

src/Components/Components/src/Sections/SectionRegistry.cs

@@ -5,10 +5,10 @@ namespace Microsoft.AspNetCore.Components.Sections;
 
 internal sealed class SectionRegistry
 {
-    private readonly Dictionary<object, ISectionContentSubscriber> _subscribersByIdentifier = new();
-    private readonly Dictionary<object, List<ISectionContentProvider>> _providersByIdentifier = new();
+    private readonly Dictionary<object, SectionOutlet> _subscribersByIdentifier = new();
+    private readonly Dictionary<object, List<SectionContent>> _providersByIdentifier = new();
 
-    public void AddProvider(object identifier, ISectionContentProvider provider, bool isDefaultProvider)
+    public void AddProvider(object identifier, SectionContent provider, bool isDefaultProvider)
     {
         if (!_providersByIdentifier.TryGetValue(identifier, out var providers))
         {
@@ -26,7 +26,7 @@ public void AddProvider(object identifier, ISectionContentProvider provider, boo
         }
     }
 
-    public void RemoveProvider(object identifier, ISectionContentProvider provider)
+    public void RemoveProvider(object identifier, SectionContent provider)
     {
         if (!_providersByIdentifier.TryGetValue(identifier, out var providers))
         {
@@ -46,21 +46,21 @@ public void RemoveProvider(object identifier, ISectionContentProvider provider)
         {
             // We just removed the most recently added provider, meaning we need to change
             // the current content to that of second most recently added provider.
-            var content = GetCurrentProviderContentOrDefault(providers);
-            NotifyContentChangedForSubscriber(identifier, content);
+            var contentProvider = GetCurrentProviderContentOrDefault(providers);
+            NotifyContentChangedForSubscriber(identifier, contentProvider);
         }
     }
 
-    public void Subscribe(object identifier, ISectionContentSubscriber subscriber)
+    public void Subscribe(object identifier, SectionOutlet subscriber)
     {
         if (_subscribersByIdentifier.ContainsKey(identifier))
         {
             throw new InvalidOperationException($"There is already a subscriber to the content with the given section ID '{identifier}'.");
         }
 
         // Notify the new subscriber with any existing content.
-        var content = GetCurrentProviderContentOrDefault(identifier);
-        subscriber.ContentChanged(content);
+        var provider = GetCurrentProviderContentOrDefault(identifier);
+        subscriber.ContentUpdated(provider);
 
         _subscribersByIdentifier.Add(identifier, subscriber);
     }
@@ -73,7 +73,7 @@ public void Unsubscribe(object identifier)
         }
     }
 
-    public void NotifyContentChanged(object identifier, ISectionContentProvider provider)
+    public void NotifyContentProviderChanged(object identifier, SectionContent provider)
     {
         if (!_providersByIdentifier.TryGetValue(identifier, out var providers))
         {
@@ -84,25 +84,25 @@ public void NotifyContentChanged(object identifier, ISectionContentProvider prov
         // most recently added provider changes.
         if (providers.Count != 0 && providers[^1] == provider)
         {
-            NotifyContentChangedForSubscriber(identifier, provider.Content);
+            NotifyContentChangedForSubscriber(identifier, provider);
         }
     }
 
-    private static RenderFragment? GetCurrentProviderContentOrDefault(List<ISectionContentProvider> providers)
+    private static SectionContent? GetCurrentProviderContentOrDefault(List<SectionContent> providers)
         => providers.Count != 0
-            ? providers[^1].Content
+            ? providers[^1]
             : null;
 
-    private RenderFragment? GetCurrentProviderContentOrDefault(object identifier)
+    private SectionContent? GetCurrentProviderContentOrDefault(object identifier)
         => _providersByIdentifier.TryGetValue(identifier, out var existingList)
             ? GetCurrentProviderContentOrDefault(existingList)
             : null;
 
-    private void NotifyContentChangedForSubscriber(object identifier, RenderFragment? content)
+    private void NotifyContentChangedForSubscriber(object identifier, SectionContent? provider)
     {
         if (_subscribersByIdentifier.TryGetValue(identifier, out var subscriber))
         {
-            subscriber.ContentChanged(content);
+            subscriber.ContentUpdated(provider);
         }
     }
 }

src/Components/Endpoints/src/Rendering/EndpointComponentState.cs

@@ -28,7 +28,7 @@ public EndpointComponentState(Renderer renderer, int componentId, IComponent com
         }
         else
         {
-            var parentEndpointComponentState = (EndpointComponentState?)parentComponentState;
+            var parentEndpointComponentState = (EndpointComponentState?)LogicalParentComponentState;
             StreamRendering = parentEndpointComponentState?.StreamRendering ?? false;
         }
     }

src/Components/test/E2ETest/ServerRenderingTests/SectionsWithStreamingRenderingTest.cs

@@ -0,0 +1,68 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Components.TestServer.RazorComponents;
+using Microsoft.AspNetCore.Components.E2ETest.Infrastructure;
+using Microsoft.AspNetCore.Components.E2ETest.Infrastructure.ServerFixtures;
+using Microsoft.AspNetCore.E2ETesting;
+using OpenQA.Selenium;
+using TestServer;
+using Xunit.Abstractions;
+
+namespace Microsoft.AspNetCore.Components.E2ETests.ServerRenderingTests;
+
+public class SectionsWithStreamingRenderingTest : ServerTestBase<BasicTestAppServerSiteFixture<RazorComponentEndpointsStartup<App>>>
+{
+    public SectionsWithStreamingRenderingTest(
+        BrowserFixture browserFixture,
+        BasicTestAppServerSiteFixture<RazorComponentEndpointsStartup<App>> serverFixture,
+        ITestOutputHelper output)
+        : base(browserFixture, serverFixture, output)
+    {
+    }
+
+    public override Task InitializeAsync()
+        => InitializeAsync(BrowserFixture.StreamingContext);
+
+    [Fact]
+    public void StreamingRenderingForSectionOutletContentIsDeterminedByMatchingSectionContent()
+    {
+        Navigate($"{ServerPathBase}/sections-with-streaming");
+
+        Browser.Equal("Sections with Streaming", () => Browser.Exists(By.TagName("h1")).Text);
+
+        // Second SectionContent overrides the content and StreamingRendering attribute for SectionOutlet
+
+        Browser.DoesNotExist(By.Id("first-section-content"));
+
+        CanPerformStreamingRendering();
+    }
+
+    private void CanPerformStreamingRendering()
+    {
+        // Initial "waiting" state
+        var getStatusText = () => Browser.Exists(By.Id("status"));
+        var getDisplayedItems = () => Browser.FindElements(By.TagName("li"));
+        Assert.Equal("Waiting for more...", getStatusText().Text);
+        Assert.Empty(getDisplayedItems());
+
+        // Can add items
+        for (var i = 1; i <= 3; i++)
+        {
+            // Each time we click, there's another streaming render batch and the UI is updated
+            Browser.FindElement(By.Id("add-item-link")).Click();
+            Browser.Collection(getDisplayedItems, Enumerable.Range(1, i).Select<int, Action<IWebElement>>(index =>
+            {
+                return actualItem => Assert.Equal($"Item {index}", actualItem.Text);
+            }).ToArray());
+            Assert.Equal("Waiting for more...", getStatusText().Text);
+
+            // These are insta-removed so they don't pollute anything
+            Browser.DoesNotExist(By.TagName("blazor-ssr"));
+        }
+
+        // Can finish the response
+        Browser.FindElement(By.Id("end-response-link")).Click();
+        Browser.Equal("Finished", () => getStatusText().Text);
+    }
+}