Add groupBy to Stream.

sdk/lib/async/stream.dart

Index: sdk/lib/async/stream.dart
diff --git a/sdk/lib/async/stream.dart b/sdk/lib/async/stream.dart
index 27c0ff78371de2cbc433f1dccbd837fac9a9f155..a43b9c38aaeaa80b9f62a2aa8a917e6817c3b87d 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -386,6 +386,76 @@ abstract class Stream<T> {
     return new _MapStream<T, S>(this, convert);
   }
 
+  /// Groups events by a computed key.
+  ///
+  /// A key is extracted from incoming events.
+  /// The first time a key is seen, a stream is created for it, and emitted
+  /// on the returned stream, along with the key, as a [StreamGroup] object.
+  /// Then the event is emitted on the stream ([StreamGroup.values])
+  /// corresponding to the key.
+  ///
+  /// An error on the source stream, or when calling the `key` functions,
+  /// will emit the error on the returned stream.
+  ///
+  /// Canceling the subscription on the returned stream will stop processing
+  /// and close the streams for all groups.
+  ///
+  /// Pausing the subscription on the returned stream will pause processing
+  /// and no further events are added to streams for the individual groups.
+  ///
+  /// Pausing or canceling an individual group stream has no effect other than
+  /// on that stream. Events will be queued while the group stream
+  /// is paused and until it is first listened to.
+  /// If the [StreamGroup.values] stream is never listened to,
+  /// it will enqueue all the events unnecessarily.

[floitsch, 2017/05/04 13:32:09]

One should therefore not ignore streams of keys that aren't used, but rather
[StreamGroup.cancel] the group.

+  Stream<StreamGroup<K, T>> groupBy<K>(K key(T event)) {
+    var controller;
+    controller = new StreamController<Group<K, V>>(
+        sync: true,
+        onListen: () {
+          var groupControllers = new HashMap<K, StreamController<T>>();
+
+          void closeAll() {
+            for (var groupController in groupControllers.values) {
+              groupController.close();
+            }
+          }
+
+          var subscription = this.listen(
+              (data) {
+                K theKey;
+                try {
+                  theKey = key(data);
+                } catch (error, stackTrace) {
+                  controller.addError(error, stackTrace);
+                  return;
+                }
+                var groupController = groupControllers[theKey];
+                if (groupController == null) {
+                  groupController =
+                      new StreamController<T>.broadcast(sync: true);
+                  groupControllers[theKey] = groupController;
+                  controller.add(
+                      new StreamGroup<K, T>(theKey, groupController.stream));
+                }
+                groupController.add(data);
+              },
+              onError: controller.addError,
+              onDone: () {
+                controller.close();
+                closeAll();
+              });
+          controller.onPause = subscription.pause;
+          controller.onResume = subscription.resume;
+          controller.onCancel = () {
+            subscription.cancel();
+            // Don't fire sync events in response to a callback.
+            scheduleMicrotask(closeAll);
+          };
+        });
+    return controller.stream;
+  }
+
   /**
    * Creates a new stream with each data event of this stream asynchronously
    * mapped to a new event.
@@ -1796,3 +1866,31 @@ class _ControllerEventSinkWrapper<T> implements EventSink<T> {
     _sink.close();
   }
 }
+
+/// A group created by [Stream.groupBy] or [Stream.groupByMapped].
+///
+/// The stream created by `groupBy` emits a `StreamGroup` for each distinct key
+/// it encounters.
+/// This group contains the [key] itself, along with a stream of the [values]
+/// associated with that key.
+class StreamGroup<K, V> {
+  /// The key that identifiers the values emitted by [values].
+  final K key;
+
+  /// The [values] that [GroupBy] have grouped by the common [key].
+  final Stream<V> values;
+
+  StreamGroup(this.key, this.values);
+
+  /// Tells [values] to discard values instead of retaining them.
+  ///
+  /// Must only be used instead of listening to the [values] stream.
+  /// If the stream has been listened to, this call fails.
+  /// After calling this method, listening on the [values] stream fails.
+  Future cancel() {
+    // If values has been listened to,
+    // this throws a StateError saying that stream has already been listened to,
+    // which is a correct error message for this call too.
+    return values.listen(null).cancel();
+  }
+}