Progressively enhanced navigation

src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs

@@ -34,6 +34,7 @@ public Task RenderComponent()
     private async Task RenderComponentCore()
     {
         _context.Response.ContentType = RazorComponentResultExecutor.DefaultContentType;
+        _renderer.InitializeStreamingRenderingFraming(_context);
 
         if (!await TryValidateRequestAsync(out var isPost, out var handler))
         {

src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Prerendering.cs

@@ -147,13 +147,33 @@ private static ValueTask<PrerenderedComponentHtmlContent> HandleNavigationExcept
                 "Navigation commands can not be issued during server-side prerendering after the response from the server has started. Applications must buffer the" +
                 "response and avoid using features like FlushAsync() before all components on the page have been rendered to prevent failed navigation commands.");
         }
+        else if (IsPossibleExternalDestination(httpContext.Request, navigationException.Location) && httpContext.Request.Headers.ContainsKey("blazor-enhanced-nav"))
+        {
+            // It's unsafe to do a 301/302/etc to an external destination when this was requested via fetch, because
+            // assuming it doesn't expose CORS headers, we won't be allowed to follow the redirection nor will
+            // we even find out what the destination URL would have been. But since it's our own JS code making this
+            // fetch request, we can have a custom protocol for describing the URL we wanted to redirect to.
+            httpContext.Response.Headers.Add("blazor-enhanced-nav-redirect-location", navigationException.Location);
+            return new ValueTask<PrerenderedComponentHtmlContent>(PrerenderedComponentHtmlContent.Empty);
+        }
         else
         {
             httpContext.Response.Redirect(navigationException.Location);
             return new ValueTask<PrerenderedComponentHtmlContent>(PrerenderedComponentHtmlContent.Empty);
         }
     }
 
+    private static bool IsPossibleExternalDestination(HttpRequest request, string destinationUrl)
+    {
+        if (!Uri.TryCreate(destinationUrl, UriKind.Absolute, out var absoluteUri))
+        {
+            return false;
+        }
+
+        return absoluteUri.Scheme != request.Scheme
+            || absoluteUri.Authority != request.Host.Value;
+    }
+
     internal static ServerComponentInvocationSequence GetOrCreateInvocationId(HttpContext httpContext)
     {
         if (!httpContext.Items.TryGetValue(ComponentSequenceKey, out var result))

src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.Streaming.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.InteropServices;
+using System.Text.Encodings.Web;
 using Microsoft.AspNetCore.Components.RenderTree;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
@@ -13,8 +14,25 @@ namespace Microsoft.AspNetCore.Components.Endpoints;
 
 internal partial class EndpointHtmlRenderer
 {
+    private const string _progressivelyEnhancedNavRequestHeaderName = "blazor-enhanced-nav";
+    private const string _streamingRenderingFramingHeaderName = "ssr-framing";
     private TextWriter? _streamingUpdatesWriter;
     private HashSet<int>? _visitedComponentIdsInCurrentStreamingBatch;
+    private string? _ssrFramingCommentMarkup;
+
+    public void InitializeStreamingRenderingFraming(HttpContext httpContext)
+    {
+        if (httpContext.Request.Headers.ContainsKey(_progressivelyEnhancedNavRequestHeaderName))
+        {
+            var id = Guid.NewGuid().ToString();
+            httpContext.Response.Headers.Add(_streamingRenderingFramingHeaderName, id);
+            _ssrFramingCommentMarkup = $"<!--{id}-->";
+        }
+        else
+        {
+            _ssrFramingCommentMarkup = string.Empty;
+        }
+    }
 
     public async Task SendStreamingUpdatesAsync(HttpContext httpContext, Task untilTaskCompleted, TextWriter writer)
     {
@@ -26,10 +44,16 @@ public async Task SendStreamingUpdatesAsync(HttpContext httpContext, Task untilT
             throw new InvalidOperationException($"{nameof(SendStreamingUpdatesAsync)} can only be called once.");
         }
 
+        if (_ssrFramingCommentMarkup is null)
+        {
+            throw new InvalidOperationException("Cannot begin streaming rendering because no framing header was set.");
+        }
+
         _streamingUpdatesWriter = writer;
 
         try
         {
+            await writer.WriteAsync(_ssrFramingCommentMarkup);
             await writer.FlushAsync(); // Make sure the initial HTML was sent
             await untilTaskCompleted;
         }
@@ -39,10 +63,12 @@ public async Task SendStreamingUpdatesAsync(HttpContext httpContext, Task untilT
         }
         catch (Exception ex)
         {
+            // Theoretically it might be possible to let the error middleware run, capture the output,
+            // then emit it in a special format so the JS code can display the error page. However
+            // for now we're not going to support that and will simply emit a message.
             HandleExceptionAfterResponseStarted(_httpContext, writer, ex);
-
-            // The rest of the pipeline can treat this as a regular unhandled exception
-            // TODO: Is this really right? I think we'll terminate the response in an invalid way.
+            await writer.FlushAsync(); // Important otherwise the client won't receive the error message, as we're about to fail the pipeline
+            await _httpContext.Response.CompleteAsync();
             throw;
         }
     }
@@ -115,6 +141,7 @@ private void SendBatchAsStreamingUpdate(in RenderBatch renderBatch, TextWriter w
             }
 
             writer.Write("</blazor-ssr>");
+            writer.Write(_ssrFramingCommentMarkup);
         }
     }
 
@@ -143,16 +170,16 @@ private static void HandleExceptionAfterResponseStarted(HttpContext httpContext,
             ? exception.ToString()
             : "There was an unhandled exception on the current request. For more details turn on detailed exceptions by setting 'DetailedErrors: true' in 'appSettings.Development.json'";
 
-        writer.Write("<template blazor-type=\"exception\">");
-        writer.Write(message);
-        writer.Write("</template>");
+        writer.Write("<blazor-ssr><template type=\"error\">");
+        writer.Write(HtmlEncoder.Default.Encode(message));
+        writer.Write("</template></blazor-ssr>");
     }
 
     private static void HandleNavigationAfterResponseStarted(TextWriter writer, string destinationUrl)
     {
-        writer.Write("<template blazor-type=\"redirection\">");
-        writer.Write(destinationUrl);
-        writer.Write("</template>");
+        writer.Write("<blazor-ssr><template type=\"redirection\">");
+        writer.Write(HtmlEncoder.Default.Encode(destinationUrl));
+        writer.Write("</template></blazor-ssr>");
     }
 
     protected override void WriteComponentHtml(int componentId, TextWriter output)

src/Components/Endpoints/src/Results/RazorComponentResultExecutor.cs

@@ -51,6 +51,8 @@ internal static Task RenderComponentToResponse(
         var endpointHtmlRenderer = httpContext.RequestServices.GetRequiredService<EndpointHtmlRenderer>();
         return endpointHtmlRenderer.Dispatcher.InvokeAsync(async () =>
         {
+            endpointHtmlRenderer.InitializeStreamingRenderingFraming(httpContext);
+
             // We could pool these dictionary instances if we wanted, and possibly even the ParameterView
             // backing buffers could come from a pool like they do during rendering.
             var hostParameters = ParameterView.FromDictionary(new Dictionary<string, object?>

src/Components/Endpoints/test/RazorComponentResultExecutorTest.cs

@@ -224,7 +224,7 @@ await RazorComponentResultExecutor.RenderComponentToResponse(
 
         // Assert
         Assert.Equal(
-            $"<!--bl:X-->Some output\n<!--/bl:X--><template blazor-type=\"redirection\">https://test/somewhere/else</template>",
+            $"<!--bl:X-->Some output\n<!--/bl:X--><blazor-ssr><template type=\"redirection\">https://test/somewhere/else</template></blazor-ssr>",
             MaskComponentIds(GetStringContent(responseBody)));
     }
 
@@ -269,18 +269,18 @@ public async Task OnUnhandledExceptionAfterResponseStarted_WithStreamingOn_Emits
         httpContext.Response.Body = responseBody;
 
         var expectedResponseExceptionInfo = isDevelopmentEnvironment
-            ? "System.InvalidTimeZoneException: Test message"
-            : "There was an unhandled exception on the current request. For more details turn on detailed exceptions by setting 'DetailedErrors: true' in 'appSettings.Development.json'";
+            ? "System.InvalidTimeZoneException: Test message with &lt;b&gt;markup&lt;/b&gt;"
+            : "There was an unhandled exception on the current request. For more details turn on detailed exceptions by setting &#x27;DetailedErrors: true&#x27; in &#x27;appSettings.Development.json&#x27;";
 
         // Act
         var ex = await Assert.ThrowsAsync<InvalidTimeZoneException>(() => RazorComponentResultExecutor.RenderComponentToResponse(
             httpContext, typeof(StreamingComponentThatThrowsAsynchronously),
             null, preventStreamingRendering: false));
 
         // Assert
-        Assert.Contains("Test message", ex.Message);
+        Assert.Contains("Test message with <b>markup</b>", ex.Message);
         Assert.Contains(
-            $"<!--bl:X-->Some output\n<!--/bl:X--><template blazor-type=\"exception\">{expectedResponseExceptionInfo}",
+            $"<!--bl:X-->Some output\n<!--/bl:X--><blazor-ssr><template type=\"error\">{expectedResponseExceptionInfo}",
             MaskComponentIds(GetStringContent(responseBody)));
     }
 

src/Components/Endpoints/test/TestComponents/StreamingComponentThatThrowsAsynchronously.razor

@@ -4,6 +4,6 @@ Some output
     protected override async Task OnInitializedAsync()
     {
         await Task.Yield();
-        throw new InvalidTimeZoneException("Test message");
+        throw new InvalidTimeZoneException("Test message with <b>markup</b>");
     }
 }

src/Components/Web.JS/src/Boot.Web.ts

@@ -15,31 +15,49 @@ import { shouldAutoStart } from './BootCommon';
 import { Blazor } from './GlobalExports';
 import { WebStartOptions } from './Platform/WebStartOptions';
 import { attachStreamingRenderingListener } from './Rendering/StreamingRendering';
+import { attachProgressivelyEnhancedNavigationListener, detachProgressivelyEnhancedNavigationListener } from './Services/NavigationEnhancement';
 import { WebAssemblyComponentDescriptor } from './Services/ComponentDescriptorDiscovery';
 import { ServerComponentDescriptor, discoverComponents } from './Services/ComponentDescriptorDiscovery';
 
 let started = false;
+let webStartOptions: Partial<WebStartOptions> | undefined;
 
 async function boot(options?: Partial<WebStartOptions>): Promise<void> {
   if (started) {
     throw new Error('Blazor has already started.');
   }
   started = true;
-  await activateInteractiveComponents(options);
+  webStartOptions = options;
 
   attachStreamingRenderingListener(options?.ssr);
+
+  if (!options?.ssr?.disableDomPreservation) {
+    attachProgressivelyEnhancedNavigationListener(activateInteractiveComponents);
+  }
+
+  await activateInteractiveComponents();
 }
 
-async function activateInteractiveComponents(options?: Partial<WebStartOptions>) {
+async function activateInteractiveComponents() {
   const serverComponents = discoverComponents(document, 'server') as ServerComponentDescriptor[];
   const webAssemblyComponents = discoverComponents(document, 'webassembly') as WebAssemblyComponentDescriptor[];
 
   if (serverComponents.length) {
-    await startCircuit(options?.circuit, serverComponents);
+    // TEMPORARY until https://github.com/dotnet/aspnetcore/issues/48763 is implemented
+    // As soon we we see you have interactive components, we'll stop doing enhanced nav even if you don't have an interactive router
+    // This is because, otherwise, we would need a way to add new interactive root components to an existing circuit and that's #48763
+    detachProgressivelyEnhancedNavigationListener();
+
+    await startCircuit(webStartOptions?.circuit, serverComponents);
   }
 
   if (webAssemblyComponents.length) {
-    await startWebAssembly(options?.webAssembly, webAssemblyComponents);
+    // TEMPORARY until https://github.com/dotnet/aspnetcore/issues/48763 is implemented
+    // As soon we we see you have interactive components, we'll stop doing enhanced nav even if you don't have an interactive router
+    // This is because, otherwise, we would need a way to add new interactive root components to an existing WebAssembly runtime and that's #48763
+    detachProgressivelyEnhancedNavigationListener();
+
+    await startWebAssembly(webStartOptions?.webAssembly, webAssemblyComponents);
   }
 }
 

src/Components/Web.JS/src/Rendering/DomMerging/AttributeSync.ts

@@ -9,7 +9,7 @@ export function synchronizeAttributes(destination: Element, source: Element) {
 
   // Optimize for the common case where all attributes are unchanged and are even still in the same order
   const destAttrsLength = destAttrs.length;
-  if (destAttrsLength === destAttrs.length) {
+  if (destAttrsLength === sourceAttrs.length) {
     let hasDifference = false;
     for (let i = 0; i < destAttrsLength; i++) {
       const sourceAttr = sourceAttrs.item(i)!;

src/Components/Web.JS/src/Rendering/DomMerging/DomSync.ts

@@ -5,13 +5,13 @@ import { applyAnyDeferredValue } from '../DomSpecialPropertyUtil';
 import { synchronizeAttributes } from './AttributeSync';
 import { UpdateCost, ItemList, Operation, computeEditScript } from './EditScript';
 
-export function synchronizeDomContent(destination: CommentBoundedRange | Element, newContent: DocumentFragment | Element) {
+export function synchronizeDomContent(destination: CommentBoundedRange | Node, newContent: Node) {
   let destinationParent: Node;
   let nextDestinationNode: Node | null;
   let originalNodesForDiff: ItemList<Node>;
 
   // Figure out how to interpret the 'destination' parameter, since it can come in two very different forms
-  if (destination instanceof Element) {
+  if (destination instanceof Node) {
     destinationParent = destination;
     nextDestinationNode = destination.firstChild;
     originalNodesForDiff = destination.childNodes;
@@ -70,7 +70,7 @@ export function synchronizeDomContent(destination: CommentBoundedRange | Element
 
     // Handle any common trailing items
     // These can only exist if there were some edits, otherwise everything would be in the set of common leading items
-    const endAtNodeExclOrNull = destination instanceof Element ? null : destination.endExclusive;
+    const endAtNodeExclOrNull = destination instanceof Node ? null : destination.endExclusive;
     while (nextDestinationNode !== endAtNodeExclOrNull) {
       treatAsMatch(nextDestinationNode!, nextNewContentNode!);
       nextDestinationNode = nextDestinationNode!.nextSibling;
@@ -94,6 +94,9 @@ function treatAsMatch(destination: Node, source: Node) {
       applyAnyDeferredValue(destination as Element);
       synchronizeDomContent(destination as Element, source as Element);
       break;
+    case Node.DOCUMENT_TYPE_NODE:
+      // See comment below about doctype nodes. We leave them alone.
+      break;
     default:
       throw new Error(`Not implemented: matching nodes of type ${destination.nodeType}`);
   }
@@ -130,6 +133,10 @@ function domNodeComparer(a: Node, b: Node): UpdateCost {
       //       For the converse (forcing retention, even if that means reordering), we could post-process the list of
       //       inserts/deletes to find matches based on key to treat those pairs as 'move' operations.
       return (a as Element).tagName === (b as Element).tagName ? UpdateCost.None : UpdateCost.Infinite;
+    case Node.DOCUMENT_TYPE_NODE:
+      // It's invalid to insert or delete doctype, and we have no use case for doing that. So just skip such
+      // nodes by saying they are always unchanged.
+      return UpdateCost.None;
     default:
       // For anything else we know nothing, so the risk-averse choice is to say we can't retain or update the old value
       return UpdateCost.Infinite;
@@ -169,4 +176,3 @@ class SiblingSubsetNodeList implements ItemList<Node> {
     this.length = this.endIndexExcl - this.startIndex;
   }
 }
-

src/Components/Web.JS/src/Rendering/StreamingRendering.ts

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 import { SsrStartOptions } from "../Platform/SsrStartOptions";
+import { performEnhancedPageLoad } from "../Services/NavigationEnhancement";
+import { isWithinBaseUriSpace } from "../Services/NavigationUtils";
 import { synchronizeDomContent } from "./DomMerging/DomSync";
 
 let enableDomPreservation = true;
@@ -34,6 +36,28 @@ class BlazorStreamingUpdate extends HTMLElement {
           const componentId = node.getAttribute('blazor-component-id');
           if (componentId) {
             insertStreamingContentIntoDocument(componentId, node.content);
+          } else {
+            switch (node.getAttribute('type')) {
+              case 'redirection':
+                // We use 'replace' here because it's closest to the non-progressively-enhanced behavior, and will make the most sense
+                // if the async delay was very short, as the user would not perceive having been on the intermediate page.
+                const destinationUrl = node.content.textContent!;
+                if (isWithinBaseUriSpace(destinationUrl)) {
+                  history.replaceState(null, '', destinationUrl);
+                  performEnhancedPageLoad(destinationUrl);
+                } else {
+                  location.replace(destinationUrl);
+                }
+                break;
+              case 'error':
+                // This is kind of brutal but matches what happens without progressive enhancement
+                document.documentElement.textContent = node.content.textContent;
+                const docStyle = document.documentElement.style;
+                docStyle.fontFamily = 'consolas, monospace';
+                docStyle.whiteSpace = 'pre-wrap';
+                docStyle.padding = '1rem';
+                break;
+            }
           }
         }
       });