Let StreamSubscription.cancel return a Future.

sdk/lib/async/stream_pipe.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 /**
  * Utility function to attach a stack trace to an [error]  if it doesn't have
  * one already.
  */
 _asyncError(Object error, Object stackTrace) {
   if (stackTrace == null) return error;
   if (getAttachedStackTrace(error) != null) return error;
   _attachStackTrace(error, stackTrace);
   return error;
 }
 
 /** Runs user code and takes actions depending on success or failure. */
 _runUserCode(userCode(), onSuccess(value), onError(error)) {
   try {
     onSuccess(userCode());
   } catch (e, s) {
     onError(_asyncError(e, s));
   }
 }
 
 /** Helper function to make an onError argument to [_runUserCode]. */
 _cancelAndError(StreamSubscription subscription, _Future future) =>
   (error) {

[floitsch, 2013/10/12 18:53:57]

You will need to update this for the stack-trace argument.

[Lasse Reichstein Nielsen, 2013/10/14 11:32:33]

And not make it curried.

[Anders Johnsen, 2013/10/16 11:52:21]

Done.

[Anders Johnsen, 2013/10/16 11:52:21]

Done.

-    subscription.cancel();
-    future._completeError(error);
+    var cancelFuture = subscription.cancel();
+    if (cancelFuture != null) {

[floitsch, 2013/10/12 18:53:57]

if (cancelFuture is Future)

Users are used to write: onCancel: () => foo()
If this thing returns something else than a future it should not be an error.

[Anders Johnsen, 2013/10/16 11:52:21]

Done.

+      cancelFuture.whenComplete(() => future._completeError(error));
+    } else {
+      future._completeError(error);
+    }
+  };
+
+/** Helper function to make an onError argument to [_runUserCode]. */
+_cancelAndValue(StreamSubscription subscription, _Future future) =>
+  (value) {
+    var cancelFuture = subscription.cancel();
+    if (cancelFuture != null) {

[floitsch, 2013/10/12 18:53:57]

ditto.

[Anders Johnsen, 2013/10/16 11:52:21]

Done.

+      cancelFuture.whenComplete(() => future._complete(value));
+    } else {
+      future._complete(value);
+    }
   };
 
 
 /**
  * A [Stream] that forwards subscriptions to another stream.
  *
  * This stream implements [Stream], but forwards all subscriptions
  * to an underlying stream, and wraps the returned subscription to
  * modify the events on the way.
  *
@@ skipping 407 matching lines @@
 
   void handleError(error, EventSink<T> sink) {
     _handleError(error, sink);
   }
 
   void handleDone(EventSink<T> sink) {
     _handleDone(sink);
   }
 }