Revert "Merge pull request #23364 from tmat/DisableBackgroundCompiler"

This reverts commit 80d79c40, reversing
changes made to 37ef0cde.

src/EditorFeatures/TestUtilities/Workspaces/TestWorkspace.cs

@@ -39,6 +39,9 @@ public partial class TestWorkspace : Workspace
         public IList<TestHostDocument> AdditionalDocuments { get; }
         public IList<TestHostDocument> ProjectionDocuments { get; }
 
+        private readonly BackgroundCompiler _backgroundCompiler;
+        private readonly BackgroundParser _backgroundParser;
+
         public TestWorkspace()
             : this(TestExportProvider.ExportProviderWithCSharpAndVisualBasic, WorkspaceKind.Test)
         {
@@ -57,6 +60,10 @@ public TestWorkspace(ExportProvider exportProvider, string workspaceKind = null,
             this.ProjectionDocuments = new List<TestHostDocument>();
 
             this.CanApplyChangeDocument = true;
+
+            _backgroundCompiler = new BackgroundCompiler(this);
+            _backgroundParser = new BackgroundParser(this);
+            _backgroundParser.Start();
         }
 
         /// <summary>
@@ -83,9 +90,30 @@ internal static void ResetThreadAffinity(ForegroundThreadData foregroundThreadDa
             }
         }
 
+        protected internal override bool PartialSemanticsEnabled
+        {
+            get { return _backgroundCompiler != null; }
+        }
+
         public TestHostDocument DocumentWithCursor 
             => Documents.Single(d => d.CursorPosition.HasValue && !d.IsLinkFile);
 
+        protected override void OnDocumentTextChanged(Document document)
+        {
+            if (_backgroundParser != null)
+            {
+                _backgroundParser.Parse(document);
+            }
+        }
+
+        protected override void OnDocumentClosing(DocumentId documentId)
+        {
+            if (_backgroundParser != null)
+            {
+                _backgroundParser.CancelParse(documentId);
+            }
+        }
+
         public new void RegisterText(SourceTextContainer text)
         {
             base.RegisterText(text);
@@ -153,6 +181,11 @@ protected override void Dispose(bool finalize)
                 Dispatcher.CurrentDispatcher.DoEvents();
             }
 
+            if (_backgroundParser != null)
+            {
+                _backgroundParser.CancelAllParses();
+            }
+
             base.Dispose(finalize);
         }