Example showing alternate async measurement solution

client/html/src/Measurement.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
-typedef Object ComputeValue();
-
-class _MeasurementRequest<T> {
-  final ComputeValue computeValue;
-  final Completer<T> completer;
-  Object value;
-  bool exception = false;
-  _MeasurementRequest(this.computeValue, this.completer);
-}
+bool _inMeasurementFrame = false;
 
 final _MEASUREMENT_MESSAGE = "DART-MEASURE";
-List<_MeasurementRequest> _pendingRequests;
-List<TimeoutHandler> _pendingMeasurementFrameCallbacks;
+
+Queue<MeasurementCallback> _pendingMeasurementFrameCallbacks;
 bool _nextMeasurementFrameScheduled = false;
 bool _firstMeasurementRequest = true;
 
 void _maybeScheduleMeasurementFrame() {
   if (_nextMeasurementFrameScheduled) return;
 
   _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
     // all we care about is that _onCompleteMeasurementRequests is called
-    // after the current event loop is unwound and calling the function is
-    // a noop when zero requests are pending.
-    window.on.message.add((e) => _completeMeasurementFutures());
+    // after the current event loop is unwound and calling
+    // _runMeasurementFrames is a noop when zero requests are pending.
+    window.on.message.add((e) => _runMeasurementFrames());
     _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, "*");
 }
 
 /**
  * Registers a [callback] which is called after the next batch of measurements
  * completes. Even if no measurements completed, the callback is triggered
  * when they would have completed to avoid confusing bugs if it happened that
  * no measurements were actually requested.
  */
-void _addMeasurementFrameCallback(TimeoutHandler callback) {
+void _addMeasurementFrameCallback(MeasurementCallback callback) {
+  assert(callback != null);
   if (_pendingMeasurementFrameCallbacks === null) {
-    _pendingMeasurementFrameCallbacks = <TimeoutHandler>[];
-    _maybeScheduleMeasurementFrame();
+    _pendingMeasurementFrameCallbacks = new Queue<MeasurementCallback>();
   }
+  _maybeScheduleMeasurementFrame();
   _pendingMeasurementFrameCallbacks.add(callback);
 }
 
 /**
- * Returns a [Future] whose value will be the result of evaluating
- * [computeValue] during the next safe measurement interval.
- * The next safe measurement interval is after the current event loop has
- * unwound but before the browser has rendered the page.
- * It is important that the [computeValue] function only queries the html
- * layout and html in any way.
- */
-Future _createMeasurementFuture(ComputeValue computeValue,
-                                Completer completer) {
-  if (_pendingRequests === null) {
-    _pendingRequests = <_MeasurementRequest>[];
-    _maybeScheduleMeasurementFrame();
-  }
-  _pendingRequests.add(new _MeasurementRequest(computeValue, completer));
-  return completer.future;
-}
-
-/**
- * Complete all pending measurement futures evaluating them in a single batch
+ * Run all pending measurement frames evaluating them in a single batch
  * so that the the browser is guaranteed to avoid multiple layouts.
  */
-void _completeMeasurementFutures() {
-  if (_nextMeasurementFrameScheduled == false) {
+void _runMeasurementFrames() {
+  if (_nextMeasurementFrameScheduled == false || _inMeasurementFrame) {
     // 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.
-  if (_pendingRequests !== null) {
-    for (_MeasurementRequest request in _pendingRequests) {
-      try {
-        request.value = request.computeValue();
-      } catch(var e) {
-        request.value = e;
-        request.exception = true;
+  _inMeasurementFrame = true;
+
+  final layoutCallbacks = <LayoutCallback>[];
+  while (!_pendingMeasurementFrameCallbacks.isEmpty()) {
+    MeasurementCallback measurementCallback =
+        _pendingMeasurementFrameCallbacks.removeFirst();
+    try {
+      final layoutCallback = measurementCallback();
+      if (layoutCallback != null) {
+        layoutCallbacks.add(layoutCallback);
       }
+    } catch (Object e) {
+      window.console.error(
+          'Caught exception in measurement frame callback: ${e}');
+      // TODO(jacobr): throw this exception again in the correct way.
     }
   }
 
-  final completedRequests = _pendingRequests;
-  final readyMeasurementFrameCallbacks = _pendingMeasurementFrameCallbacks;
-  _pendingRequests = null;
-  _pendingMeasurementFrameCallbacks = null;
-  if (completedRequests !== null) {
-    for (_MeasurementRequest request in completedRequests) {
-      if (request.exception) {
-        request.completer.completeException(request.value);
-      } else {
-        request.completer.complete(request.value);
-      }
-    }
-  }
+  _inMeasurementFrame = false;
+  _nextMeasurementFrameScheduled = false;
 
-  if (readyMeasurementFrameCallbacks !== null) {
-    for (TimeoutHandler handler in readyMeasurementFrameCallbacks) {
-      // TODO(jacobr): wrap each call to a handler in a try-catch block.
-      handler();
+  for (LayoutCallback layoutCallback in layoutCallbacks) {
+    try {
+      layoutCallback();
+    } catch (Object e) {
+      window.console.error('Caught exception in layout callback: ${e}');
+      // TODO(jacobr): throw this exception again in the correct way.
     }
   }
 }