Don't schedule timer if knownFilesSetChanges stream is not requested.

pkg/analyzer/lib/src/dart/analysis/file_state.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 13 matching lines @@
 import 'package:analyzer/src/source/source_resource.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/name_filter.dart';
 import 'package:analyzer/src/summary/package_bundle_reader.dart';
 import 'package:analyzer/src/summary/summarize_ast.dart';
 import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
 import 'package:front_end/src/base/api_signature.dart';
 import 'package:front_end/src/base/performace_logger.dart';
+import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:front_end/src/fasta/builder/builder.dart' as fasta;
 import 'package:front_end/src/fasta/parser/parser.dart' as fasta;
 import 'package:front_end/src/fasta/scanner.dart' as fasta;
-import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:meta/meta.dart';
 
 /**
  * [FileContentOverlay] is used to temporary override content of files.
  */
 class FileContentOverlay {
   final _map = <String, String>{};
 
   /**
    * Return the paths currently being overridden.
@@ skipping 650 matching lines @@
   Duration _knownFilesSetChangesDelay;
 
   /**
    * The instance of timer that is scheduled to send a new update to the
    * [knownFilesSetChanges] stream, or `null` if there are no changes to the
    * set of known files to notify the stream about.
    */
   Timer _knownFilesSetChangesTimer;
 
   /**
+   * Whether the [knownFilesSetChanges] stream is requested.
+   */
+  bool _knownFilesSetChangesRequested = false;
+
+  /**
    * The controller for the [knownFilesSetChanges] stream.
    */
   final StreamController<KnownFilesSetChange> _knownFilesSetChangesController =
       new StreamController<KnownFilesSetChange>();
 
   /**
    * Mapping from a path to the flag whether there is a URI for the path.
    */
   final Map<String, bool> _hasUriForPath = {};
 
@@ skipping 34 matching lines @@
   /**
    * Return the known files.
    */
   List<FileState> get knownFiles =>
       _pathToFiles.values.map((files) => files.first).toList();
 
   /**
    * Return the [Stream] that is periodically notified about changes to the
    * known files set.
    */
-  Stream<KnownFilesSetChange> get knownFilesSetChanges =>
-      _knownFilesSetChangesController.stream;
+  Stream<KnownFilesSetChange> get knownFilesSetChanges {
+    // If this is the first (and actually the only) time when the stream is
+    // requested, schedule the timer to send updates.
+    if (!_knownFilesSetChangesRequested) {
+      _knownFilesSetChangesRequested = true;
+      _scheduleKnownFilesSetChange();
+    }
+    return _knownFilesSetChangesController.stream;
+  }
 
   @visibleForTesting
   FileSystemStateTestView get test => _testView;
 
   /**
    * Return the [FileState] instance that correspond to an unresolved URI.
    */
   FileState get unresolvedFile {
     if (_unresolvedFile == null) {
       _unresolvedFile = new FileState._(this, null, null, null);
@@ skipping 125 matching lines @@
       files = <FileState>[];
       _pathToFiles[path] = files;
       // Schedule the stream update.
       _addedKnownFiles.add(path);
       _scheduleKnownFilesSetChange();
     }
     files.add(file);
   }
 
   void _scheduleKnownFilesSetChange() {
+    // Schedule the timer only if there is a client who listens the stream.
+    if (!_knownFilesSetChangesRequested) {
+      return;
+    }
+
     Duration delay = _knownFilesSetChangesDelay ?? new Duration(seconds: 1);
     _knownFilesSetChangesTimer ??= new Timer(delay, () {
       Set<String> addedFiles = _addedKnownFiles.toSet();
       Set<String> removedFiles = new Set<String>();
       _knownFilesSetChangesController
           .add(new KnownFilesSetChange(addedFiles, removedFiles));
       _addedKnownFiles.clear();
       _knownFilesSetChangesTimer = null;
     });
   }
@@ skipping 24 matching lines @@
 
 /**
  * Information about changes to the known file set.
  */
 class KnownFilesSetChange {
   final Set<String> added;
   final Set<String> removed;
 
   KnownFilesSetChange(this.added, this.removed);
 }