Make StreamSubscription be the active part of a stream.

sdk/lib/mdv_observe_impl/mdv_observe_impl.dart

 // Copyright (c) 2013, 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.
 
 // This library itself is undocumented and not supported for end use.
 // Because dart:html must use some of this functionality, it has to be available
 // via a dart:* library. The public APIs are reexported via package:mdv_observe.
 // Generally we try to keep this library minimal, with utility types and
 // functions in the package.
 library dart.mdv_observe_impl;
@@ skipping 45 matching lines @@
  */
 typedef ObservableBase = Object with ObservableMixin;
 
 /**
  * Mixin for implementing [Observable] objects.
  *
  * When a field, property, or indexable item is changed, a derived class should
  * call [notifyPropertyChange]. See that method for an example.
  */
 abstract class ObservableMixin implements Observable {
-  StreamController<List<ChangeRecord>> _observers;
+  Set<StreamController<List<ChangeRecord>>> _observers;
   Stream<List<ChangeRecord>> _stream;
   List<ChangeRecord> _changes;
 
   Stream<List<ChangeRecord>> get changes {
     if (_observers == null) {
-      _observers = new StreamController<List<ChangeRecord>>();
-      _stream = _observers.stream.asBroadcastStream();
+      _observers = new Set<StreamController<List<ChangeRecord>>>();

[Jennifer Messerly, 2013/05/24 23:28:08]

fwiw... this seems awfully complicated for something that's essentially just to
exposing a simple multi-subscription event.

Is there anything we can provide to make it easier to handle this case?

Perhaps StreamController.multiplex? :)

[Lasse Reichstein Nielsen, 2013/05/27 08:04:53]

Yes, we introduced StreamController.multiplex after having had to do this in two
places. I should ofcourse go back and make those two places use it :)

     }
-    return _stream;
+    StreamController controller;
+    controller = new StreamController(
+      onListen: () {
+        _observers.add(controller);
+      },
+      onCancel: () {
+        _observers.remove(controller);
+      }
+    );
+    return controller.stream;
   }
 
   void _deliverChanges() {
     var changes = _changes;
     _changes = null;
     if (hasObservers && changes != null) {
+      List<StreamController<List<ChangeRecord>>> observers =

[Jennifer Messerly, 2013/05/24 23:28:08]

style nit, I'd use "var" here and below:

http://www.dartlang.org/articles/style-guide/#prefer-using-var-without-a-type...

[Lasse Reichstein Nielsen, 2013/05/27 08:04:53]

It's your code, so ok.

+          _observers.toList();
+      for (StreamController<List<ChangeRecord>> observer in observers) {
+        if (_observers.contains(observer)) {
+          observer.add(changes);
+        }
+      }
       // TODO(jmesserly): make "changes" immutable

[Jennifer Messerly, 2013/05/24 18:47:55]

keep this comment by the "observer.add(changes);" line?

[Lasse Reichstein Nielsen, 2013/05/27 08:04:53]

Ofcourse :)

-      _observers.add(changes);
     }
   }
 
   /**
    * True if this object has any observers, and should call
    * [notifyPropertyChange] for changes.
    */
-  bool get hasObservers => _observers != null && _observers.hasListener;
+  bool get hasObservers => _observers != null && !_observers.isEmpty;
 
   /**
    * Notify that the field [name] of this object has been changed.
    *
    * The [oldValue] and [newValue] are also recorded. If the two values are
    * identical, no change will be recorded.
    *
    * For convenience this returns [newValue]. This makes it easy to use in a
    * setter:
    *
@@ skipping 115 matching lines @@
 /** Queues an action to happen during the [deliverChangeRecords] timeslice. */
 void queueChangeRecords(void deliverChanges()) {
   if (_deliverCallbacks == null) {
     _deliverCallbacks = new Queue<Function>();
     runAsync(deliverChangeRecords);
   }
   _deliverCallbacks.add(deliverChanges);
 }
 
 Queue _deliverCallbacks;