Streamline AnalysisDriver status tracking.

pkg/analyzer/lib/src/dart/analysis/status.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';
 
 /**
  * The status of analysis.
  */
 class AnalysisStatus {
@@ skipping 71 matching lines @@
    * Return the last status sent to the [stream].
    */
   AnalysisStatus get currentStatus => _currentStatus;
 
   /**
    * Return the stream that produces [AnalysisStatus] events.
    */
   Stream<AnalysisStatus> get stream => _statusController.stream;
 
   /**
+   * Prepare for the driver to start analyzing, but do not notify the [stream]
+   * yet.
+   *
+   * A call to [preTransitionToAnalyzing] has the same effect on [waitForIdle]
+   * as a call to [transitionToAnalyzing], but it has no effect on the [stream].
+   */
+  void preTransitionToAnalyzing() {
+    _idleCompleter ??= new Completer<Null>();
+  }
+
+  /**
    * Send a notifications to the [stream] that the driver started analyzing.

[scheglov, 2017/02/01 00:35:58]

Now probably that "a driver" started analyzing.
Or maybe don't mention drivers here and elsewhere in this class.

[Paul Berry, 2017/02/01 17:17:34]

Done.

    */
   void transitionToAnalyzing() {
     if (_currentStatus != AnalysisStatus.ANALYZING) {
+      preTransitionToAnalyzing();
       _currentStatus = AnalysisStatus.ANALYZING;
       _statusController.add(AnalysisStatus.ANALYZING);
     }
   }
 
   /**
    * Send a notifications to the [stream] stream that the driver is idle.
    */
   void transitionToIdle() {
     if (_currentStatus != AnalysisStatus.IDLE) {
       _currentStatus = AnalysisStatus.IDLE;
       _statusController.add(AnalysisStatus.IDLE);
-      _idleCompleter?.complete();
-      _idleCompleter = null;
     }
+    _idleCompleter?.complete();
+    _idleCompleter = null;
   }
 
   /**
    * Return a future that will be completed the next time the status is idle.
    *
    * If the status is currently idle, the returned future will be signaled
    * immediately.
    */
   Future<Null> waitForIdle() {
-    if (_currentStatus == AnalysisStatus.IDLE) {
-      return new Future.value();
-    } else {
-      _idleCompleter ??= new Completer<Null>();
-      return _idleCompleter.future;
-    }
+    return _idleCompleter?.future ?? new Future.value();
   }
 }