Adding streams to dart:html.

tools/dom/src/HtmlStreamController.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.
+
+part of html;
+
+/**
+ * Adapter for exposing DOM events as Dart streams.
+ */
+class _HtmlStreamController<T extends Event> extends StreamController<T> {

[Jennifer Messerly, 2013/01/11 21:57:17]

High level question: do we want to subclass StreamController?
a problem with StreamController is that it implements StreamSink, meaning "add"
and "close" are visible. That is the meaning of calling these?

I worry about "add" because, since these StreamControllers aren't cached for a
particular element, someone might call "add" and expect to be dispatching an
event. But it won't actually work.

Another thing I noticed, StreamController has an extra level of indirection. I
wonder if it would be better to inherit from Stream directly. You could
potentially avoid some bookeeping, at the cost of extra implementation work.

[floitsch, 2013/01/11 22:46:11]

If I'm not wrong we can directly forward the registration request to the DOM,
and just return a subscription object that knows how to remove the event
listener.
Normally there shouldn't be a need for a list of subscribers.

If there is something that makes this impossible, we should see if we can fix
it.

[blois, 2013/01/11 22:51:30]

Will take a look at this now.

[Jennifer Messerly, 2013/01/11 23:30:14]

I hope it works -- that would be awesome :)

+  final EventTarget _target;
+  final String _eventType;
+  final bool _useCapture;
+  EventListener _eventHandler;
+
+  _HtmlStreamController(this._target, this._eventType, this._useCapture) {
+    // Stash reference to the method to allow removeEventListener to function.
+    _eventHandler = _handleEvent;

[Jennifer Messerly, 2013/01/11 21:57:17]

Can this just be:

    _eventHandler = this.add;

in other words, do you need _handleEvent?

[blois, 2013/01/11 22:51:30]

Unfortunately, this fails in dart2js checked mode.
http://code.google.com/p/dart/issues/detail?id=7754

Added a comment to this extent.

+  }
+
+  void onSubscriptionStateChange() {
+    super.onSubscriptionStateChange();
+    if (hasSubscribers) {
+      _target.$dom_addEventListener(_eventType, _eventHandler, _useCapture);
+    } else {
+      _target.$dom_removeEventListener(_eventType, _eventHandler, _useCapture);
+    }
+  }
+
+  void _handleEvent(e) {
+    this.add(e);
+  }
+}
+
+
+/**
+ * A factory to expose DOM events as Streams.
+ */
+class HtmlStreamProvider<T extends Event> {
+  final String _eventType;
+
+  const HtmlStreamProvider(this._eventType);
+
+  /**
+   * Gets a [Stream] for this event type, on the specified target.
+   *
+   * This may be used to capture DOM events:
+   *
+   *     Element.keyDownEvent.forTarget(element, useCapture: true).listen(...);
+   *
+   * Or for listening to an event which will bubble through the DOM tree:
+   *
+   *     MediaElement.pauseEvent.forTarget(document.body).listen(...);
+   *
+   * See also:
+   *
+   * [addEventListener](http://docs.webplatform.org/wiki/dom/methods/addEventListener)
+   */
+  Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
+    return new _HtmlStreamController(e, _eventType, useCapture);
+  }
+}