Modify HtmlRenderer to update content when a component re-renders (#4…

…0512) Prior to this change HtmlRenderer would return a snapshot of a component immediately after it was initially rendered. This meant any updates to a component as a result of other root components rendering were lost. In this change, we update the HtmlRenderer to return a placeholder `IHtmlContent` that retrieves render frames only when `WriteTo` is invoked. Typically MVC will defer calling WriteTo until the content is about to be sent to the wire, which means any updates to a component (e.g. HeadOutlet being updated) are picked up. Also updates the template to consolidate `_Host` and `_Layout`. Fixes #38400
dotnet · Mar 3, 2022 · e9dddd6 · e9dddd6
1 parent 3817558
commit e9dddd6
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Showing 5 changed files with 71 additions and 56 deletions.
diff --git a/src/Mvc/Mvc.ViewFeatures/src/Buffers/ViewBuffer.cs b/src/Mvc/Mvc.ViewFeatures/src/Buffers/ViewBuffer.cs
@@ -12,7 +12,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers;
 /// An <see cref="IHtmlContentBuilder"/> that is backed by a buffer provided by <see cref="IViewBufferScope"/>.
 /// </summary>
 [DebuggerDisplay("{DebuggerToString()}")]
-internal class ViewBuffer : IHtmlContentBuilder
+internal sealed class ViewBuffer : IHtmlContentBuilder
 {
     public const int PartialViewPageSize = 32;
     public const int TagHelperPageSize = 32;

diff --git a/src/Mvc/Mvc.ViewFeatures/src/RazorComponents/HtmlRenderer.cs b/src/Mvc/Mvc.ViewFeatures/src/RazorComponents/HtmlRenderer.cs
@@ -3,7 +3,9 @@
 
 using System.Diagnostics;
 using System.Runtime.ExceptionServices;
+using System.Text.Encodings.Web;
 using Microsoft.AspNetCore.Components.RenderTree;
+using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Mvc.ViewFeatures.Buffers;
 using Microsoft.Extensions.Logging;
 
@@ -47,14 +49,11 @@ protected override Task UpdateDisplayAsync(in RenderBatch renderBatch)
 
     public async Task<ComponentRenderedText> RenderComponentAsync(Type componentType, ParameterView initialParameters)
     {
-        var (componentId, frames) = await CreateInitialRenderAsync(componentType, initialParameters);
-
-        var viewBuffer = new ViewBuffer(_viewBufferScope, nameof(HtmlRenderer), ViewBuffer.ViewPageSize);
+        var component = InstantiateComponent(componentType);
+        var componentId = AssignRootComponentId(component);
 
-        var context = new HtmlRenderingContext { HtmlContentBuilder = viewBuffer, };
-        var newPosition = RenderFrames(context, frames, 0, frames.Count);
-        Debug.Assert(newPosition == frames.Count);
-        return new ComponentRenderedText(componentId, context.HtmlContentBuilder);
+        await RenderRootComponentAsync(componentId, initialParameters);
+        return new ComponentRenderedText(componentId, new ComponentHtmlContent(this, componentId));
     }
 
     public Task<ComponentRenderedText> RenderComponentAsync<TComponent>(ParameterView initialParameters) where TComponent : IComponent
@@ -248,21 +247,50 @@ private static int RenderAttributes(
         return position + maxElements;
     }
 
-    private async Task<(int, ArrayRange<RenderTreeFrame>)> CreateInitialRenderAsync(Type componentType, ParameterView initialParameters)
+    private ViewBuffer GetRenderedHtmlContent(int componentId)
     {
-        var component = InstantiateComponent(componentType);
-        var componentId = AssignRootComponentId(component);
+        var viewBuffer = new ViewBuffer(_viewBufferScope, nameof(HtmlRenderer), ViewBuffer.ViewPageSize);
+        var context = new HtmlRenderingContext(viewBuffer);
 
-        await RenderRootComponentAsync(componentId, initialParameters);
+        var frames = GetCurrentRenderTreeFrames(componentId);
+        var newPosition = RenderFrames(context, frames, 0, frames.Count);
+        Debug.Assert(newPosition == frames.Count);
 
-        return (componentId, GetCurrentRenderTreeFrames(componentId));
+        return viewBuffer;
     }
 
     private sealed class HtmlRenderingContext
     {
-        public ViewBuffer HtmlContentBuilder { get; init; }
+        public HtmlRenderingContext(ViewBuffer viewBuffer)
+        {
+            HtmlContentBuilder = viewBuffer;
+        }
+
+        public ViewBuffer HtmlContentBuilder { get; }
 
         public string ClosestSelectValueAsString { get; set; }
     }
+
+    /// <summary>
+    /// A <see cref="IHtmlContent"/> that defers rendering component markup until <see cref="IHtmlContent.WriteTo(TextWriter, HtmlEncoder)"/>
+    /// is called.
+    /// </summary>
+    private sealed class ComponentHtmlContent : IHtmlContent
+    {
+        private readonly HtmlRenderer _renderer;
+        private readonly int _componentId;
+
+        public ComponentHtmlContent(HtmlRenderer renderer, int componentId)
+        {
+            _renderer = renderer;
+            _componentId = componentId;
+        }
+
+        public void WriteTo(TextWriter writer, HtmlEncoder encoder)
+        {
+            var actualHtmlContent = _renderer.GetRenderedHtmlContent(_componentId);
+            actualHtmlContent.WriteTo(writer, encoder);
+        }
+    }
 }
 
diff --git a/src/ProjectTemplates/Web.ProjectTemplates/content/BlazorServerWeb-CSharp/Pages/_Host.cshtml b/src/ProjectTemplates/Web.ProjectTemplates/content/BlazorServerWeb-CSharp/Pages/_Host.cshtml
@@ -1,8 +1,33 @@
 @page "/"
+@using Microsoft.AspNetCore.Components.Web
 @namespace BlazorServerWeb_CSharp.Pages
 @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
-@{
-    Layout = "_Layout";
-}
 
-<component type="typeof(App)" render-mode="ServerPrerendered" />
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <base href="~/" />
+    <link rel="stylesheet" href="css/bootstrap/bootstrap.min.css" />
+    <link href="css/site.css" rel="stylesheet" />
+    <link href="BlazorServerWeb-CSharp.styles.css" rel="stylesheet" />
+    <component type="typeof(HeadOutlet)" render-mode="ServerPrerendered" />
+</head>
+<body>
+    <component type="typeof(App)" render-mode="ServerPrerendered" />
+
+    <div id="blazor-error-ui">
+        <environment include="Staging,Production">
+            An error has occurred. This application may no longer respond until reloaded.
+        </environment>
+        <environment include="Development">
+            An unhandled exception has occurred. See browser dev tools for details.
+        </environment>
+        <a href="" class="reload">Reload</a>
+        <a class="dismiss">🗙</a>
+    </div>
+
+    <script src="_framework/blazor.server.js"></script>
+</body>
+</html>
diff --git a/...ProjectTemplates/Web.ProjectTemplates/content/BlazorServerWeb-CSharp/Pages/_Layout.cshtml b/...ProjectTemplates/Web.ProjectTemplates/content/BlazorServerWeb-CSharp/Pages/_Layout.cshtml
diff --git a/src/ProjectTemplates/test/template-baselines.json b/src/ProjectTemplates/test/template-baselines.json
@@ -1262,7 +1262,6 @@
         "Pages/FetchData.razor",
         "Pages/Index.razor",
         "Pages/_Host.cshtml",
-        "Pages/_Layout.cshtml",
         "Properties/launchSettings.json",
         "Shared/LoginDisplay.razor",
         "Shared/MainLayout.razor",
@@ -1302,7 +1301,6 @@
         "Pages/FetchData.razor",
         "Pages/Index.razor",
         "Pages/_Host.cshtml",
-        "Pages/_Layout.cshtml",
         "Properties/launchSettings.json",
         "Shared/LoginDisplay.razor",
         "Shared/MainLayout.razor",
@@ -1342,7 +1340,6 @@
         "Pages/FetchData.razor",
         "Pages/Index.razor",
         "Pages/_Host.cshtml",
-        "Pages/_Layout.cshtml",
         "Properties/launchSettings.json",
         "Shared/LoginDisplay.razor",
         "Shared/MainLayout.razor",
@@ -1382,7 +1379,6 @@
         "Pages/FetchData.razor",
         "Pages/Index.razor",
         "Pages/_Host.cshtml",
-        "Pages/_Layout.cshtml",
         "Properties/launchSettings.json",
         "Shared/MainLayout.razor",
         "Shared/MainLayout.razor.css",
@@ -1421,7 +1417,6 @@
         "Pages/FetchData.razor",
         "Pages/Index.razor",
         "Pages/_Host.cshtml",
-        "Pages/_Layout.cshtml",
         "Properties/launchSettings.json",
         "Shared/LoginDisplay.razor",
         "Shared/MainLayout.razor",
@@ -1461,7 +1456,6 @@
         "Pages/FetchData.razor",
         "Pages/Index.razor",
         "Pages/_Host.cshtml",
-        "Pages/_Layout.cshtml",
         "Properties/launchSettings.json",
         "Shared/LoginDisplay.razor",
         "Shared/MainLayout.razor",