Add SingleSubscriptionTransformer. This transforms a broadcast stream to a single-subscription stream, adding internal buffering. R=lrn@google.com Review URL: https://codereview.chromium.org//1584023002 .

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8cb7880..d7142da 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 1.7.0
+
+- Added `SingleSubscriptionTransformer`, a `StreamTransformer` that converts a
+  broadcast stream into a single-subscription stream.
+
 ## 1.6.0
 
 - Added `CancelableOperation.valueOrCancellation()`, which allows users to be

diff --git a/lib/async.dart b/lib/async.dart
index 8f14bac..ab630dd 100644
--- a/lib/async.dart
+++ b/lib/async.dart

@@ -17,6 +17,7 @@
 export "src/lazy_stream.dart";
 export "src/restartable_timer.dart";
 export "src/result_future.dart";
+export "src/single_subscription_transformer.dart";
 export "src/stream_completer.dart";
 export "src/stream_group.dart";
 export "src/stream_queue.dart";

diff --git a/lib/src/single_subscription_transformer.dart b/lib/src/single_subscription_transformer.dart
new file mode 100644
index 0000000..28fd512
--- /dev/null
+++ b/lib/src/single_subscription_transformer.dart

@@ -0,0 +1,25 @@
+// 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.
+
+library async.single_subscription_transformer;
+
+import 'dart:async';
+
+/// A transformer that converts a broadcast stream into a single-subscription
+/// stream.
+///
+/// This buffers the broadcast stream's events, which means that it starts
+/// listening to a stream as soon as it's bound.
+class SingleSubscriptionTransformer<S, T> implements StreamTransformer<S, T> {
+  const SingleSubscriptionTransformer();
+
+  Stream<T> bind(Stream<S> stream) {
+    var subscription;
+    var controller = new StreamController(sync: true,
+        onCancel: () => subscription.cancel());
+    subscription = stream.listen(controller.add,
+        onError: controller.addError, onDone: controller.close);
+    return controller.stream;
+  }
+}

diff --git a/pubspec.yaml b/pubspec.yaml
index e13f6ab..423f3ce 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml

@@ -1,5 +1,5 @@
 name: async
-version: 1.6.0
+version: 1.7.0
 author: Dart Team <misc@dartlang.org>
 description: Utility functions and classes related to the 'dart:async' library.
 homepage: https://www.github.com/dart-lang/async

diff --git a/test/single_subscription_transformer_test.dart b/test/single_subscription_transformer_test.dart
new file mode 100644
index 0000000..f21d4e9
--- /dev/null
+++ b/test/single_subscription_transformer_test.dart

@@ -0,0 +1,50 @@
+// 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 'package:async/async.dart';
+import 'package:test/test.dart';
+
+import 'utils.dart';
+
+void main() {
+  test("buffers events as soon as it's bound", () async {
+    var controller = new StreamController.broadcast();
+    var stream = controller.stream.transform(
+        const SingleSubscriptionTransformer());
+
+    // Add events before [stream] has a listener to be sure it buffers them.
+    controller.add(1);
+    controller.add(2);
+    await flushMicrotasks();
+
+    expect(stream.toList(), completion(equals([1, 2, 3, 4])));
+    await flushMicrotasks();
+
+    controller.add(3);
+    controller.add(4);
+    controller.close();
+  });
+
+  test("cancels the subscription to the broadcast stream when it's canceled",
+      () async {
+    var canceled = false;
+    var controller = new StreamController.broadcast(onCancel: () {
+      canceled = true;
+    });
+    var stream = controller.stream.transform(
+        const SingleSubscriptionTransformer());
+    await flushMicrotasks();
+    expect(canceled, isFalse);
+
+    var subscription = stream.listen(null);
+    await flushMicrotasks();
+    expect(canceled, isFalse);
+
+    subscription.cancel();
+    await flushMicrotasks();
+    expect(canceled, isTrue);
+  });
+}