[StaticWebAssets] Avoid generating dependent manifests if original manifest is up to date (#43709)

Author: javiercn

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

@@ -628,14 +628,16 @@ Copyright (c) .NET Foundation. All rights reserved.
       ManifestType="Build"
       Assets="@(StaticWebAsset)"
       Endpoints="@(StaticWebAssetEndpoint)"
-      ManifestPath="$(StaticWebAssetEndpointsBuildManifestPath)">
+      ManifestPath="$(StaticWebAssetEndpointsBuildManifestPath)"
+      CacheFilePath="$(StaticWebAssetBuildManifestPath)">
     </GenerateStaticWebAssetEndpointsManifest>
 
     <GenerateStaticWebAssetsDevelopmentManifest
       DiscoveryPatterns="@(StaticWebAssetDiscoveryPattern)"
       Assets="@(StaticWebAsset)"
       Source="$(PackageId)"
-      ManifestPath="$(StaticWebAssetDevelopmentManifestPath)">
+      ManifestPath="$(StaticWebAssetDevelopmentManifestPath)"
+      CacheFilePath="$(StaticWebAssetBuildManifestPath)">
     </GenerateStaticWebAssetsDevelopmentManifest>
 
     <ItemGroup>

src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetEndpointsManifest.cs

@@ -23,8 +23,16 @@ public class GenerateStaticWebAssetEndpointsManifest : Task
     [Required]
     public string ManifestPath { get; set; }
 
+    public string CacheFilePath { get; set; }
+
     public override bool Execute()
     {
+        if (!string.IsNullOrEmpty(CacheFilePath) && File.Exists(ManifestPath) && File.GetLastWriteTimeUtc(ManifestPath) > File.GetLastWriteTimeUtc(CacheFilePath))
+        {
+            Log.LogMessage(MessageImportance.Low, "Skipping manifest generation because manifest file '{0}' is up to date.", ManifestPath);
+            return true;
+        }
+
         try
         {
             // Get the list of the asset that need to be part of the manifest (this is similar to GenerateStaticWebAssetsDevelopmentManifest)

src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsDevelopmentManifest.cs

@@ -25,8 +25,17 @@ public class GenerateStaticWebAssetsDevelopmentManifest : Task
         [Required]
         public string ManifestPath { get; set; }
 
+        [Required]
+        public string CacheFilePath { get; set; }
+
         public override bool Execute()
         {
+            if (File.Exists(ManifestPath) && File.GetLastWriteTimeUtc(ManifestPath) > File.GetLastWriteTimeUtc(CacheFilePath))
+            {
+                Log.LogMessage(MessageImportance.Low, "Skipping manifest generation because manifest file '{0}' is up to date.", ManifestPath);
+                return true;
+            }
+
             try
             {
                 if (Assets.Length == 0 && DiscoveryPatterns.Length == 0)