[Blazor] Generate Link headers based on StaticWebAssets manifest properties

src/Components/Endpoints/src/Builder/ResourceCollectionConvention.cs

@@ -49,6 +49,7 @@ public void ApplyConvention(EndpointBuilder eb)
         if (_collection != null && _collectionImportMap != null)
         {
             eb.Metadata.Add(_collection);
+            eb.Metadata.Add(new ResourcePreloadCollection(_collection));
 
             if (_collectionUrl != null)
             {

src/Components/Endpoints/src/Builder/ResourcePreloadCollection.cs

@@ -0,0 +1,94 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Linq;
+using System.Text;
+using Microsoft.Extensions.Primitives;
+
+namespace Microsoft.AspNetCore.Components.Endpoints;
+
+internal class ResourcePreloadCollection
+{
+    private readonly Dictionary<string, StringValues> _storage = new();
+
+    public ResourcePreloadCollection(ResourceAssetCollection assets)
+    {
+        if (assets != null)
+        {
+            var headers = new List<(string? Group, int Order, string Value)>();
+            foreach (var asset in assets)
+            {
+                if (asset.Properties == null)
+                {
+                    continue;
+                }
+
+                // Use preloadgroup=webassembly to identify assets that should to be preloaded
+                string? group = null;
+                foreach (var property in asset.Properties)
+                {
+                    if (property.Name.Equals("preloadgroup", StringComparison.OrdinalIgnoreCase))
+                    {
+                        group = property.Value ?? string.Empty;
+                        break;
+                    }
+                }
+
+                if (group == null)
+                {
+                    continue;
+                }
+
+                var header = new StringBuilder();
+                header.Append('<');
+                header.Append(asset.Url);
+                header.Append('>');
+
+                int order = 0;
+                foreach (var property in asset.Properties)
+                {
+                    if (property.Name.Equals("preloadrel", StringComparison.OrdinalIgnoreCase))
+                    {
+                        header.Append("; rel=").Append(property.Value);
+                    }
+                    else if (property.Name.Equals("preloadas", StringComparison.OrdinalIgnoreCase))
+                    {
+                        header.Append("; as=").Append(property.Value);
+                    }
+                    else if (property.Name.Equals("preloadpriority", StringComparison.OrdinalIgnoreCase))
+                    {
+                        header.Append("; fetchpriority=").Append(property.Value);
+                    }
+                    else if (property.Name.Equals("preloadcrossorigin", StringComparison.OrdinalIgnoreCase))
+                    {
+                        header.Append("; crossorigin=").Append(property.Value);
+                    }
+                    else if (property.Name.Equals("integrity", StringComparison.OrdinalIgnoreCase))
+                    {
+                        header.Append("; integrity=\"").Append(property.Value).Append('"');
+                    }
+                    else if (property.Name.Equals("preloadorder", StringComparison.OrdinalIgnoreCase))
+                    {
+                        if (!int.TryParse(property.Value, out order))
+                        {
+                            order = 0;
+                        }
+                    }
+                }
+
+                if (header != null)
+                {
+                    headers.Add((group, order, header.ToString()));
+                }
+            }
+
+            foreach (var group in headers.GroupBy(h => h.Group))
+            {
+                _storage[group.Key ?? string.Empty] = group.OrderBy(h => h.Order).Select(h => h.Value).ToArray();
+            }
+        }
+    }
+
+    public bool TryGetLinkHeaders(string group, out StringValues linkHeaders)
+        => _storage.TryGetValue(group, out linkHeaders);
+}

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

@@ -12,6 +12,7 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Options;
+using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNetCore.Components.Endpoints;
 
@@ -275,6 +276,12 @@ private void WriteComponentHtml(int componentId, TextWriter output, bool allowBo
             {
                 if (_httpContext.RequestServices.GetRequiredService<WebAssemblySettingsEmitter>().TryGetSettingsOnce(out var settings))
                 {
+                    if (marker.Type is ComponentMarker.WebAssemblyMarkerType)
+                    {
+                        // Preload WebAssembly assets when using WebAssembly (not Auto) mode
+                        AppendWebAssemblyPreloadHeaders();
+                    }
+
                     var settingsJson = JsonSerializer.Serialize(settings, ServerComponentSerializationSettings.JsonSerializationOptions);
                     output.Write($"<!--Blazor-WebAssembly:{settingsJson}-->");
                 }
@@ -311,6 +318,15 @@ private void WriteComponentHtml(int componentId, TextWriter output, bool allowBo
         }
     }
 
+    private void AppendWebAssemblyPreloadHeaders()
+    {
+        var preloads = _httpContext.GetEndpoint()?.Metadata.GetMetadata<ResourcePreloadCollection>();
+        if (preloads != null && preloads.TryGetLinkHeaders("webassembly", out var linkHeaders))
+        {
+            _httpContext.Response.Headers.Link = StringValues.Concat(_httpContext.Response.Headers.Link, linkHeaders);
+        }
+    }
+
     private static bool IsProgressivelyEnhancedNavigation(HttpRequest request)
     {
         // For enhanced nav, the Blazor JS code controls the "accept" header precisely, so we can be very specific about the format

src/Components/Endpoints/test/EndpointHtmlRendererTest.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Net.Http;
 using System.Text.Encodings.Web;
 using System.Text.Json;
 using System.Text.RegularExpressions;
@@ -71,6 +70,78 @@ public async Task CanRender_ParameterlessComponent_ClientMode()
         Assert.Empty(httpContext.Items);
     }
 
+    [Fact]
+    public async Task CanPreload_WebAssembly_ResourceAssets()
+    {
+        // Arrange
+        var httpContext = GetHttpContext();
+        var writer = new StringWriter();
+
+        httpContext.SetEndpoint(
+            new Endpoint(
+                ctx => Task.CompletedTask,
+                new EndpointMetadataCollection([
+                    new ResourcePreloadCollection(
+                        new ResourceAssetCollection([
+                            new ResourceAsset("second.js", [
+                                new ResourceAssetProperty("preloadrel", "preload"),
+                                new ResourceAssetProperty("preloadas", "script"),
+                                new ResourceAssetProperty("preloadpriority", "high"),
+                                new ResourceAssetProperty("preloadcrossorigin", "anonymous"),
+                                new ResourceAssetProperty("integrity", "abcd"),
+                                new ResourceAssetProperty("preloadorder", "2"),
+                                new ResourceAssetProperty("preloadgroup", "webassembly")
+                            ]),
+                            new ResourceAsset("first.js", [
+                                new ResourceAssetProperty("preloadrel", "preload"),
+                                new ResourceAssetProperty("preloadas", "script"),
+                                new ResourceAssetProperty("preloadpriority", "high"),
+                                new ResourceAssetProperty("preloadcrossorigin", "anonymous"),
+                                new ResourceAssetProperty("integrity", "abcd"),
+                                new ResourceAssetProperty("preloadorder", "1"),
+                                new ResourceAssetProperty("preloadgroup", "webassembly")
+                            ]),
+                            new ResourceAsset("preload-nowebassembly.js", [
+                                new ResourceAssetProperty("preloadrel", "preload"),
+                                new ResourceAssetProperty("preloadas", "script"),
+                                new ResourceAssetProperty("preloadpriority", "high"),
+                                new ResourceAssetProperty("preloadcrossorigin", "anonymous"),
+                                new ResourceAssetProperty("integrity", "abcd"),
+                                new ResourceAssetProperty("preloadorder", "1"),
+                                new ResourceAssetProperty("preloadgroup", "abcd")
+                            ]),
+                            new ResourceAsset("nopreload.js", [
+                                new ResourceAssetProperty("integrity", "abcd")
+                            ])
+                        ])
+                    )
+                ]),
+                "TestEndpoint"
+            )
+        );
+
+        // Act
+        var result = await renderer.PrerenderComponentAsync(httpContext, typeof(SimpleComponent), new InteractiveWebAssemblyRenderMode(prerender: false), ParameterView.Empty);
+        await renderer.Dispatcher.InvokeAsync(() => result.WriteTo(writer, HtmlEncoder.Default));
+
+        // Assert
+        Assert.Equal(2, httpContext.Response.Headers.Link.Count);
+
+        var firstPreloadLink = httpContext.Response.Headers.Link[0];
+        Assert.Contains("<first.js>", firstPreloadLink);
+        Assert.Contains("rel=preload", firstPreloadLink);
+        Assert.Contains("as=script", firstPreloadLink);
+        Assert.Contains("fetchpriority=high", firstPreloadLink);
+        Assert.Contains("integrity=\"abcd\"", firstPreloadLink);
+
+        var secondPreloadLink = httpContext.Response.Headers.Link[1];
+        Assert.Contains("<second.js>", secondPreloadLink);
+        Assert.Contains("rel=preload", secondPreloadLink);
+        Assert.Contains("as=script", secondPreloadLink);
+        Assert.Contains("fetchpriority=high", secondPreloadLink);
+        Assert.Contains("integrity=\"abcd\"", secondPreloadLink);
+    }
+
     [Fact]
     public async Task CanPrerender_ParameterlessComponent_ClientMode()
     {

src/Shared/Components/ResourceCollectionResolver.cs

@@ -42,9 +42,21 @@ public ResourceAssetCollection ResolveResourceCollection()
 #if !MVC_VIEWFEATURES
             string? label = null;
             string? integrity = null;
+            string? preloadRel = null;
+            string? preloadAs = null;
+            string? preloadPriority = null;
+            string? preloadCrossorigin = null;
+            string? preloadOrder = null;
+            string? preloadGroup = null;
 #else
             string label = null;
             string integrity = null;
+            string preloadRel = null;
+            string preloadAs = null;
+            string preloadPriority = null;
+            string preloadCrossorigin = null;
+            string preloadOrder = null;
+            string preloadGroup = null;
 #endif
 
             // If there's a selector this means that this is an alternative representation for a resource, so skip it.
@@ -59,15 +71,44 @@ public ResourceAssetCollection ResolveResourceCollection()
                         label = property.Value;
                         foundProperties++;
                     }
-
                     else if (property.Name.Equals("integrity", StringComparison.OrdinalIgnoreCase))
                     {
                         integrity = property.Value;
                         foundProperties++;
                     }
+                    else if (property.Name.Equals("preloadrel", StringComparison.OrdinalIgnoreCase))
+                    {
+                        preloadRel = property.Value;
+                        foundProperties++;
+                    }
+                    else if (property.Name.Equals("preloadas", StringComparison.OrdinalIgnoreCase))
+                    {
+                        preloadAs = property.Value;
+                        foundProperties++;
+                    }
+                    else if (property.Name.Equals("preloadpriority", StringComparison.OrdinalIgnoreCase))
+                    {
+                        preloadPriority = property.Value;
+                        foundProperties++;
+                    }
+                    else if (property.Name.Equals("preloadcrossorigin", StringComparison.OrdinalIgnoreCase))
+                    {
+                        preloadCrossorigin = property.Value;
+                        foundProperties++;
+                    }
+                    else if (property.Name.Equals("preloadorder", StringComparison.OrdinalIgnoreCase))
+                    {
+                        preloadOrder = property.Value;
+                        foundProperties++;
+                    }
+                    else if (property.Name.Equals("preloadgroup", StringComparison.OrdinalIgnoreCase))
+                    {
+                        preloadGroup = property.Value;
+                        foundProperties++;
+                    }
                 }
 
-                AddResource(resources, descriptor, label, integrity, foundProperties);
+                AddResource(resources, descriptor, label, integrity, preloadRel, preloadAs, preloadPriority, preloadCrossorigin, preloadOrder, preloadGroup, foundProperties);
             }
         }
 
@@ -97,11 +138,23 @@ private static void AddResource(
 #if !MVC_VIEWFEATURES
         string? label,
         string? integrity,
+        string? preloadRel,
+        string? preloadAs,
+        string? preloadPriority,
+        string? preloadCrossorigin,
+        string? preloadOrder,
+        string? preloadGroup,
 #else
         string label,
         string integrity,
+        string preloadRel,
+        string preloadAs,
+        string preloadPriority,
+        string preloadCrossorigin,
+        string preloadOrder,
+        string preloadGroup,
 #endif
-        int foundProperties)
+    int foundProperties)
     {
         if (label != null || integrity != null)
         {
@@ -115,6 +168,30 @@ private static void AddResource(
             {
                 properties[index++] = new ResourceAssetProperty("integrity", integrity);
             }
+            if (preloadRel != null)
+            {
+                properties[index++] = new ResourceAssetProperty("preloadrel", preloadRel);
+            }
+            if (preloadAs != null)
+            {
+                properties[index++] = new ResourceAssetProperty("preloadas", preloadAs);
+            }
+            if (preloadPriority != null)
+            {
+                properties[index++] = new ResourceAssetProperty("preloadpriority", preloadPriority);
+            }
+            if (preloadCrossorigin != null)
+            {
+                properties[index++] = new ResourceAssetProperty("preloadcrossorigin", preloadCrossorigin);
+            }
+            if (preloadOrder != null)
+            {
+                properties[index++] = new ResourceAssetProperty("preloadorder", preloadOrder);
+            }
+            if (preloadGroup != null)
+            {
+                properties[index++] = new ResourceAssetProperty("preloadgroup", preloadGroup);
+            }
 
             resources.Add(new ResourceAsset(descriptor.Route, properties));
         }