[StaticWebAssets] Generate Link elements based on StaticWebAssets manifest properties

src/StaticWebAssetsSdk/Targets/Microsoft.NET.Sdk.StaticWebAssets.HtmlImportMap.targets

@@ -35,12 +35,12 @@ Copyright (c) .NET Foundation. All rights reserved.
       ResolveHtmlImportMapBuildStaticWebAssets
     </ResolveBuildServiceWorkerStaticWebAssetsDependsOn>
     <ResolveHtmlImportMapBuildStaticWebAssetsDependsOn>
-      GenerateHtmlImportMapBuildStaticWebAssets;
-      $(ResolveHtmlImportMapBuildStaticWebAssetsDependsOn)
+      $(ResolveHtmlImportMapBuildStaticWebAssetsDependsOn);
+      GenerateHtmlImportMapBuildStaticWebAssets
     </ResolveHtmlImportMapBuildStaticWebAssetsDependsOn>
     <GenerateHtmlImportMapBuildStaticWebAssetsDependsOn>
-      ResolveHtmlImportMapBuildConfiguration;
-      $(GenerateHtmlImportMapBuildStaticWebAssetsDependsOn)
+      $(GenerateHtmlImportMapBuildStaticWebAssetsDependsOn);
+      ResolveHtmlImportMapBuildConfiguration
     </GenerateHtmlImportMapBuildStaticWebAssetsDependsOn>
 
     <!--
@@ -62,12 +62,12 @@ Copyright (c) .NET Foundation. All rights reserved.
       ResolveHtmlImportMapPublishStaticWebAssets
     </ResolvePublishServiceWorkerStaticWebAssetsDependsOn>
     <ResolveHtmlImportMapPublishStaticWebAssetsDependsOn>
-      GenerateHtmlImportMapPublishStaticWebAssets;
-      $(ResolveHtmlImportMapPublishStaticWebAssetsDependsOn)
+      $(ResolveHtmlImportMapPublishStaticWebAssetsDependsOn);
+      GenerateHtmlImportMapPublishStaticWebAssets
     </ResolveHtmlImportMapPublishStaticWebAssetsDependsOn>
     <GenerateHtmlImportMapPublishStaticWebAssetsDependsOn>
-      ResolveHtmlImportMapPublishConfiguration;
-      $(GenerateHtmlImportMapPublishStaticWebAssetsDependsOn)
+      $(GenerateHtmlImportMapPublishStaticWebAssetsDependsOn);
+      ResolveHtmlImportMapPublishConfiguration
     </GenerateHtmlImportMapPublishStaticWebAssetsDependsOn>
 
   </PropertyGroup>

src/StaticWebAssetsSdk/Tasks/WriteImportMapToHtml.cs

@@ -1,13 +1,13 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Text.RegularExpressions;
+using Microsoft.AspNetCore.StaticWebAssets.Tasks.Utils;
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
 using System.Text.Encodings.Web;
 using System.Text.Json.Serialization;
 using System.Text.Json;
-using Microsoft.AspNetCore.StaticWebAssets.Tasks.Utils;
+using System.Text.RegularExpressions;
 
 namespace Microsoft.AspNetCore.StaticWebAssets.Tasks;
 
@@ -42,9 +42,11 @@ public partial class WriteImportMapToHtml : Task
     // 2.group = fingerprint placeholder
     // 3.group = file extension
     // wrapped in quotes
-    private static readonly Regex _assetsRegex = new Regex(@"""([^""]+)(#\[\.{fingerprint}\])([^""]+)""");
+    internal static readonly Regex _assetsRegex = new Regex(@"""([^""]+)(#\[\.{fingerprint}\])([^""]+)""");
+
+    internal static readonly Regex _importMapRegex = new Regex(@"<script\s+type=""importmap""\s*>\s*</script>");
 
-    private static readonly Regex _importMapRegex = new Regex(@"<script\s+type=""importmap""\s*>\s*</script>");
+    internal static readonly Regex _preloadRegex = new Regex(@"<link\s+rel=""preload""(\sid=""(?<group>[^""]+)"")?\s*[/]?>");
 
     public override bool Execute()
     {
@@ -75,6 +77,13 @@ public override bool Execute()
                     return $"<script type=\"importmap\">{JsonSerializer.Serialize(importMap, ImportMapSerializerContext.CustomEncoder.Options)}</script>";
                 });
 
+                // Generate import map
+                outputContent = _preloadRegex.Replace(outputContent, e =>
+                {
+                    Log.LogMessage("Writing preload links to '{0}'", item.ItemSpec);
+                    return GeneratePreloadLinks(resources, e.Groups["group"]?.Value);
+                });
+
                 // Fingerprint all assets used in html
                 outputContent = _assetsRegex.Replace(outputContent, e =>
                 {
@@ -105,6 +114,48 @@ public override bool Execute()
         return true;
     }
 
+    private static string GeneratePreloadLinks(List<ResourceAsset> assets, string? group)
+    {
+        var links = new List<(string? Order, string Value)>();
+        foreach (var asset in assets)
+        {
+            if (asset.PreloadRel == null)
+            {
+                continue;
+            }
+
+            if (group != null && asset.PreloadGroup != group)
+            {
+                continue;
+            }
+
+            var link = new StringBuilder();
+            link.Append($"<link href=\"").Append(asset.Url).Append("\" rel=\"").Append(asset.PreloadRel).Append('"');
+            if (!string.IsNullOrEmpty(asset.PreloadAs))
+            {
+                link.Append(" as=\"").Append(asset.PreloadAs).Append('"');
+            }
+            if (!string.IsNullOrEmpty(asset.PreloadPriority))
+            {
+                link.Append(" fetchpriority=\"").Append(asset.PreloadPriority).Append('"');
+            }
+            if (!string.IsNullOrEmpty(asset.PreloadCrossorigin))
+            {
+                link.Append(" crossorigin=\"").Append(asset.PreloadCrossorigin).Append('"');
+            }
+            if (!string.IsNullOrEmpty(asset.Integrity))
+            {
+                link.Append(" integrity=\"").Append(asset.Integrity).Append('"');
+            }
+
+            link.Append(" />");
+            links.Add((asset.PreloadOrder, link.ToString()));
+        }
+
+        links.Sort((a, b) => string.Compare(a.Order, b.Order, StringComparison.InvariantCulture));
+        return String.Join(Environment.NewLine, links.Select(l => l.Value));
+    }
+
     private string GetFingerprintedAssetPath(Dictionary<string, ResourceAsset> urlMappings, string assetPath)
     {
         if (urlMappings.TryGetValue(assetPath, out var asset) && (!IncludeOnlyHardFingerprintedModules || asset.IsHardFingerprinted))
@@ -125,33 +176,54 @@ internal List<ResourceAsset> CreateResourcesFromEndpoints(IEnumerable<StaticWebA
         // subresource integrity to things like images, script tags, etc.
         foreach (var endpoint in endpoints)
         {
-            string? label = null;
-            string? integrity = null;
-
             // If there's a selector this means that this is an alternative representation for a resource, so skip it.
             if (endpoint.Selectors?.Length == 0)
             {
+                var resourceAsset = new ResourceAsset(endpoint.Route);
                 for (var i = 0; i < endpoint.EndpointProperties?.Length; i++)
                 {
                     var property = endpoint.EndpointProperties[i];
                     if (property.Name.Equals("label", StringComparison.OrdinalIgnoreCase))
                     {
-                        label = property.Value;
+                        resourceAsset.Label = property.Value;
                     }
                     else if (property.Name.Equals("integrity", StringComparison.OrdinalIgnoreCase))
                     {
-                        integrity = property.Value;
+                        resourceAsset.Integrity = property.Value;
+                    }
+                    else if (property.Name.Equals("preloadgroup", StringComparison.OrdinalIgnoreCase))
+                    {
+                        resourceAsset.PreloadGroup = property.Value;
+                    }
+                    else if (property.Name.Equals("preloadrel", StringComparison.OrdinalIgnoreCase))
+                    {
+                        resourceAsset.PreloadRel = property.Value;
+                    }
+                    else if (property.Name.Equals("preloadas", StringComparison.OrdinalIgnoreCase))
+                    {
+                        resourceAsset.PreloadAs = property.Value;
+                    }
+                    else if (property.Name.Equals("preloadpriority", StringComparison.OrdinalIgnoreCase))
+                    {
+                        resourceAsset.PreloadPriority = property.Value;
+                    }
+                    else if (property.Name.Equals("preloadcrossorigin", StringComparison.OrdinalIgnoreCase))
+                    {
+                        resourceAsset.PreloadCrossorigin = property.Value;
+                    }
+                    else if (property.Name.Equals("preloadorder", StringComparison.OrdinalIgnoreCase))
+                    {
+                        resourceAsset.PreloadOrder = property.Value;
                     }
                 }
 
-                bool isHardFingerprinted = true;
                 var asset = Assets.FirstOrDefault(a => a.ItemSpec == endpoint.AssetFile);
                 if (asset != null)
                 {
-                    isHardFingerprinted = asset.GetMetadata("RelativePath").Contains("#[.{fingerprint}]!");
+                    resourceAsset.IsHardFingerprinted = asset.GetMetadata("RelativePath").Contains("#[.{fingerprint}]!");
                 }
 
-                resources.Add(new ResourceAsset(endpoint.Route, label, integrity, isHardFingerprinted));
+                resources.Add(resourceAsset);
             }
         }
 
@@ -207,12 +279,18 @@ private static Dictionary<string, ResourceAsset> GroupResourcesByLabel(List<Reso
     }
 }
 
-internal sealed class ResourceAsset(string url, string? label, string? integrity, bool isHardFingerprinted)
+internal sealed class ResourceAsset(string url)
 {
     public string Url { get; } = url;
-    public string? Label { get; set; } = label;
-    public string? Integrity { get; set; } = integrity;
-    public bool IsHardFingerprinted { get; set; } = isHardFingerprinted;
+    public string? Label { get; set; }
+    public string? Integrity { get; set; }
+    public string? PreloadGroup { get; set; }
+    public string? PreloadRel { get; set; }
+    public string? PreloadAs { get; set; }
+    public string? PreloadPriority { get; set; }
+    public string? PreloadCrossorigin { get; set; }
+    public string? PreloadOrder { get; set; }
+    public bool IsHardFingerprinted { get; set; } = true;
 }
 
 internal class ImportMap(Dictionary<string, string> imports, Dictionary<string, Dictionary<string, string>> scopes, Dictionary<string, string> integrity)