Make StreamSubscription be the active part of a stream.

sdk/lib/mdv_observe_impl/mdv_observe_impl.dart

Index: sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
diff --git a/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart b/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
index e56554bce82d3cbac789836e9b70d9b126b372f3..9f477aa6fa4353fb2e9137109350a611b3292713 100644
--- a/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
+++ b/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
@@ -63,24 +63,38 @@ typedef ObservableBase = Object with ObservableMixin;
  * 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);
     }
   }
 
@@ -88,7 +102,7 @@ abstract class ObservableMixin implements Observable {
    * 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.