Implement measurement using futures

client/html/src/Measurement.dart

+// Copyright (c) 2011, 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.
+
+typedef Object ComputeValue();
+
+class MeasurementRequest<T> {
+  final ComputeValue computeValue;
+  final Completer<T> completer;
+  Object value;
+  bool exception = false;
+  MeasurementRequest(this.computeValue, this.completer);
+}
+
+final _MEASUREMENT_MESSAGE = "DART-MEASURE";
+List<MeasurementRequest> _pendingRequests;
+List<TimeoutHandler> _pendingMeasurementFrameCallbacks;
+bool _nextMeasurementFrameScheduled = false;
+bool _firstMeasurementRequest = true;
+
+void _maybeScheduleMeasurementFrame() {
+  if (_nextMeasurementFrameScheduled == false) {

[arv (Not doing code reviews), 2011/10/27 05:50:24]

if (!_nextMeasurementFrameScheduled) {

[Jacob, 2011/10/27 20:59:25]

Done.

+    _nextMeasurementFrameScheduled = true;
+    // postMessage gives us a way to receive a callback after the current
+    // event listener has unwound but before the browser has repainted.
+    if (_firstMeasurementRequest) {
+      // Messages from other windows do not cause a security risk as

[arv (Not doing code reviews), 2011/10/27 05:50:24]

as as

[Jacob, 2011/10/27 20:59:25]

as as ==> as all
done done

+      // as we care about is that _onCompleteMeasurementRequests is called
+      // after the current event loop is unwound and calling the function is
+      // a noop if no requests are pending.
+      window.on.message.add(Isolate.bind(

[arv (Not doing code reviews), 2011/10/27 05:50:24]

If anyone else is listening to on message they will get this. I wonder if there
is an alternative that does not expose this message? 

[Jacob, 2011/10/27 20:59:25]

Do you have any ideas? Creating an iframe just for this seems like overkill and
I can't think of any other ideas.
I'm not too worried as pretty much everyone listening for post messages needs to
be examining the message data to determine whether they should handle the
message or ignore it.

+        (e) => _onCompleteMeasurementFutures()
+        ));
+      _firstMeasurementRequest = false;
+    }
+
+    // TODO(jacobr): other mechanisms such as setImmediate and
+    // requestAnimationFrame may work better of platforms that support them.
+    // The key is we need a way to execute code immediately after the current
+    // event listener queue unwinds.
+    window.postMessage(_MEASUREMENT_MESSAGE, "*");
+  }
+}
+
+void _addMeasurementFrameCallback(TimeoutHandler callback) {
+  if (null == _pendingMeasurementFrameCallbacks) {
+    _pendingMeasurementFrameCallbacks = <TimeoutHandler>[];
+    _maybeScheduleMeasurementFrame();
+  }
+  _pendingMeasurementFrameCallbacks.add(callback);
+}
+
+Future _createMeasurementFuture(ComputeValue computeValue, Completer completer) {

[arv (Not doing code reviews), 2011/10/27 05:50:24]

long line

[Jacob, 2011/10/27 20:59:25]

Done.

+  if (null == _pendingRequests) {
+    _pendingRequests = <MeasurementRequest>[];
+    _maybeScheduleMeasurementFrame();
+  }
+  _pendingRequests.add(new MeasurementRequest(computeValue, completer));
+  return completer.future;
+}
+
+void _onCompleteMeasurementFutures() {
+  if (_nextMeasurementFrameScheduled == false) {
+    // Ignore spurious call to this function.
+    return;
+  }
+
+  _nextMeasurementFrameScheduled = false;
+  // We must compute all new values before fulfilling the futures as
+  // the onComplete callbacks for the futures could modify the DOM making
+  // subsequent measurement calculations expensive to compute.
+  for (MeasurementRequest request in _pendingRequests) {
+    try {
+      request.value = request.computeValue();
+    } catch(var e) {
+      request.value = e;
+      request.exception = true;
+    }
+  }
+
+  final completedRequests = _pendingRequests;
+  final readyMeasurementFrameCallbacks = _pendingMeasurementFrameCallbacks;
+  _pendingRequests = null;
+  _pendingMeasurementFrameCallbacks = null;
+  for (MeasurementRequest request in completedRequests) {
+    if (request.exception) {
+      request.completer.completeException(request.value);
+    } else {
+      request.completer.complete(request.value);
+    }
+  }
+
+  if (null != readyMeasurementFrameCallbacks) {
+    for (TimeoutHandler handler in readyMeasurementFrameCallbacks) {
+      // TODO(jacobr): wrap each call to a handler in a try-catch block.
+      handler();
+    }
+  }
+}