Make document tracker resilient to multiple subscribes.

- Added tests to verify new functionality.

#2314

src/Microsoft.VisualStudio.Editor.Razor/DefaultVisualStudioDocumentTracker.cs

@@ -29,6 +29,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
         private readonly Workspace _workspace;
         private bool _isSupportedProject;
         private ProjectSnapshot _projectSnapshot;
+        private int _subscribeCount;
 
         // Only allow a single tag helper computation task at a time.
         private (ProjectSnapshot project, Task task) _computingTagHelpers;
@@ -177,12 +178,18 @@ namespace Microsoft.VisualStudio.Editor.Razor
         {
             _foregroundDispatcher.AssertForegroundThread();
 
+            if (_subscribeCount++ > 0)
+            {
+                return;
+            }
+
             _projectSnapshot = _projectManager.GetOrCreateProject(_projectPath);
             _isSupportedProject = true;
 
             _projectManager.Changed += ProjectManager_Changed;
             _workspaceEditorSettings.Changed += EditorSettingsManager_Changed;
             _importDocumentManager.Changed += Import_Changed;
+
             _importDocumentManager.OnSubscribed(this);
 
             OnContextChanged(ContextChangeKind.ProjectChanged);
@@ -192,6 +199,11 @@ namespace Microsoft.VisualStudio.Editor.Razor
         {
             _foregroundDispatcher.AssertForegroundThread();
 
+            if (_subscribeCount == 0 || _subscribeCount-- > 1)
+            {
+                return;
+            }
+
             _importDocumentManager.OnUnsubscribed(this);
 
             _projectManager.Changed -= ProjectManager_Changed;

test/Microsoft.VisualStudio.Editor.Razor.Test/DefaultVisualStudioDocumentTrackerTest.cs

@@ -98,6 +98,72 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
         private DefaultVisualStudioDocumentTracker DocumentTracker { get; }
 
+        [ForegroundFact]
+        public void Subscribe_NoopsIfAlreadySubscribed()
+        {
+            // Arrange
+            var callCount = 0;
+            DocumentTracker.ContextChanged += (sender, args) =>
+            {
+                callCount++;
+            };
+            DocumentTracker.Subscribe();
+
+            // Call count is 2 right now:
+            // 1 trigger for initial subscribe context changed.
+            // 1 trigger for TagHelpers being changed (computed).
+
+            // Act
+            DocumentTracker.Subscribe();
+
+            // Assert
+            Assert.Equal(2, callCount);
+        }
+
+        [ForegroundFact]
+        public void Unsubscribe_NoopsIfAlreadyUnsubscribed()
+        {
+            // Arrange
+            var callCount = 0;
+            DocumentTracker.Subscribe();
+            DocumentTracker.ContextChanged += (sender, args) =>
+            {
+                callCount++;
+            };
+            DocumentTracker.Unsubscribe();
+
+            // Act
+            DocumentTracker.Unsubscribe();
+
+            // Assert
+            Assert.Equal(1, callCount);
+        }
+
+        [ForegroundFact]
+        public void Unsubscribe_NoopsIfSubscribeHasBeenCalledMultipleTimes()
+        {
+            // Arrange
+            var callCount = 0;
+            DocumentTracker.Subscribe();
+            DocumentTracker.Subscribe();
+            DocumentTracker.ContextChanged += (sender, args) =>
+            {
+                callCount++;
+            };
+
+            // Act - 1
+            DocumentTracker.Unsubscribe();
+
+            // Assert - 1
+            Assert.Equal(0, callCount);
+
+            // Act - 2
+            DocumentTracker.Unsubscribe();
+
+            // Assert - 2
+            Assert.Equal(1, callCount);
+        }
+
         [ForegroundFact]
         public void EditorSettingsManager_Changed_TriggersContextChanged()
         {