Merge pull request #11658 from CyrusNajmabadi/fewerAccurateTags

Don't classify the entire document when we're accurate tagging.

src/EditorFeatures/Core/Implementation/Classification/SemanticClassificationBufferTaggerProvider.Tagger.cs

@@ -24,7 +24,7 @@ private class Tagger : ForegroundThreadAffinitizedObject, IAccurateTagger<IClass
             private readonly ITaggerEventSource _eventSource;
 
             private TagSpanIntervalTree<IClassificationTag> _cachedTags_doNotAccessDirectly;
-            private ITextSnapshot _cachedSnapshot_doNotAccessDirectly;
+            private SnapshotSpan? _cachedTaggedSpan_doNotAccessDirectly;
 
             private IEditorClassificationService _classificationService;
 
@@ -62,18 +62,18 @@ private TagSpanIntervalTree<IClassificationTag> CachedTags
                 }
             }
 
-            private ITextSnapshot CachedSnapshot
+            private SnapshotSpan? CachedTaggedSpan
             {
                 get
                 {
                     this.AssertIsForeground();
-                    return _cachedSnapshot_doNotAccessDirectly;
+                    return _cachedTaggedSpan_doNotAccessDirectly;
                 }
 
                 set
                 {
                     this.AssertIsForeground();
-                    _cachedSnapshot_doNotAccessDirectly = value;
+                    _cachedTaggedSpan_doNotAccessDirectly = value;
                 }
             }
 
@@ -94,7 +94,7 @@ private void OnEventSourceChanged()
 
                 // When something changes, clear the cached data we have.
                 this.CachedTags = null;
-                this.CachedSnapshot = null;
+                this.CachedTaggedSpan = null;
 
                 // And notify any concerned parties that we have new tags.
                 this.TagsChanged?.Invoke(this, new SnapshotSpanEventArgs(_subjectBuffer.CurrentSnapshot.GetFullSpan()));
@@ -122,9 +122,19 @@ public IEnumerable<ITagSpan<IClassificationTag>> GetAllTags(NormalizedSnapshotSp
                 var snapshot = firstSpan.Snapshot;
                 Debug.Assert(snapshot.TextBuffer == _subjectBuffer);
 
-                var cachedSnapshot = this.CachedSnapshot;
+                // We want to classify from the start of the first requested span to the end of the 
+                // last requested span.
+                var spanToTag = new SnapshotSpan(snapshot,
+                    Span.FromBounds(spans.First().Start, spans.Last().End));
 
-                if (snapshot != cachedSnapshot)
+                // We don't need to actually classify if what we're being asked for is a subspan
+                // of the last classification we performed.
+                var cachedTaggedSpan = this.CachedTaggedSpan;
+                var canReuseCache =
+                    cachedTaggedSpan?.Snapshot == snapshot &&
+                    cachedTaggedSpan.Value.Contains(spanToTag);
+
+                if (!canReuseCache)
                 {
                     // Our cache is not there, or is out of date.  We need to compute the up to date 
                     // results.
@@ -138,11 +148,13 @@ public IEnumerable<ITagSpan<IClassificationTag>> GetAllTags(NormalizedSnapshotSp
                     _classificationService = _classificationService ?? document.Project.LanguageServices.GetService<IEditorClassificationService>();
 
                     var context = new TaggerContext<IClassificationTag>(document, snapshot, cancellationToken: cancellationToken);
-                    var spanToTag = new DocumentSnapshotSpan(document, snapshot.GetFullSpan());
-                    var task = SemanticClassificationUtilities.ProduceTagsAsync(context, spanToTag, _classificationService, _owner._typeMap);
+
+                    var task = SemanticClassificationUtilities.ProduceTagsAsync(
+                        context, new DocumentSnapshotSpan(document, spanToTag), 
+                        _classificationService, _owner._typeMap);
                     task.Wait(cancellationToken);
 
-                    CachedSnapshot = snapshot;
+                    CachedTaggedSpan = spanToTag;
                     CachedTags = new TagSpanIntervalTree<IClassificationTag>(snapshot.TextBuffer, SpanTrackingMode.EdgeExclusive, context.tagSpans);
                 }