Fix Nuget Solution Package manager UI is slow because restore is run for every project instead of after all projects are updated

src/NuGet.Clients/NuGet.PackageManagement.UI/Actions/UIActionEngine.cs

@@ -835,20 +835,17 @@ private async Task<IReadOnlyList<ResolvedAction>> GetActionsAsync(
             var results = new List<ResolvedAction>();
 
             Debug.Assert(userAction.PackageId != null, "Package id can never be null in a User action");
+
             if (userAction.Action == NuGetProjectActionType.Install)
             {
-                foreach (var target in targets)
-                {
-                    var actions = await _packageManager.PreviewInstallPackageAsync(
-                        target,
-                        new PackageIdentity(userAction.PackageId, userAction.Version),
-                        resolutionContext,
-                        projectContext,
-                        uiService.ActiveSources,
-                        null,
-                        token);
-                    results.AddRange(actions.Select(a => new ResolvedAction(target, a)));
-                }
+                results.AddRange(await _packageManager.PreviewProjectsInstallPackageAsync(
+                    targets?.ToList(),
+                    new PackageIdentity(userAction.PackageId, userAction.Version),
+                    resolutionContext,
+                    projectContext,
+                    uiService.ActiveSources?.ToList(),
+                    token
+                ));
             }
             else
             {

src/NuGet.Core/NuGet.PackageManagement/BuildIntegration/DependencyGraphRestoreUtility.cs

@@ -136,7 +136,7 @@ public static string GetDefaultDGSpecFileName()
         }
 
         /// <summary>
-        /// Restore without writing the lock file
+        /// Restore a project without writing the lock file
         /// </summary>
         internal static async Task<RestoreResultPair> PreviewRestoreAsync(
             ISolutionManager solutionManager,
@@ -150,6 +150,8 @@ internal static async Task<RestoreResultPair> PreviewRestoreAsync(
             ILogger log,
             CancellationToken token)
         {
+            token.ThrowIfCancellationRequested();
+
             // Restoring packages
             var logger = context.Logger;
 
@@ -185,6 +187,59 @@ internal static async Task<RestoreResultPair> PreviewRestoreAsync(
             }
         }
 
+        /// <summary>
+        /// Restore many projects without writing the lock file
+        /// SourceRepositories(sources) is only used for the CachingSourceProvider, the project-specific sources will still be resolved in RestoreRunner.
+        /// </summary>
+        internal static async Task<IEnumerable<RestoreResultPair>> PreviewRestoreProjectsAsync(
+            ISolutionManager solutionManager,
+            IEnumerable<BuildIntegratedNuGetProject> projects,
+            IEnumerable<PackageSpec> updatedNugetPackageSpecs,
+            DependencyGraphCacheContext context,
+            RestoreCommandProvidersCache providerCache,
+            Action<SourceCacheContext> cacheContextModifier,
+            IEnumerable<SourceRepository> sources,
+            Guid parentId,
+            ILogger log,
+            CancellationToken token)
+        {
+            token.ThrowIfCancellationRequested();
+
+            // Add the new spec to the dg file and fill in the rest.
+            var dgFile = await GetSolutionRestoreSpec(solutionManager, context);
+
+            dgFile = dgFile.WithoutRestores()
+                .WithPackageSpecs(updatedNugetPackageSpecs);
+
+            foreach (var project in projects)
+            {
+                dgFile.AddRestore(project.MSBuildProjectPath);
+            }
+
+            using (var sourceCacheContext = new SourceCacheContext())
+            {
+                // Update cache context
+                cacheContextModifier(sourceCacheContext);
+
+                // Settings passed here will be used to populate the restore requests.
+                var restoreContext = GetRestoreContext(
+                    context,
+                    providerCache,
+                    sourceCacheContext,
+                    sources,
+                    dgFile,
+                    parentId,
+                    forceRestore: true,
+                    isRestoreOriginalAction: false,
+                    restoreForceEvaluate: true,
+                    additionalMessasges: null);
+
+                var requests = await RestoreRunner.GetRequests(restoreContext);
+                var results = await RestoreRunner.RunWithoutCommit(requests, restoreContext);
+                return results;
+            }
+        }
+
         /// <summary>
         /// Restore a build integrated project(PackageReference and Project.Json only) and update the lock file
         /// </summary>