Changing window.requestLayoutFrame into a microtask API

tools/dom/src/Microtask.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 html;
+
+typedef void _MicrotaskCallback();
+
+/**
+ * This class attempts to invoke a callback as soon as the current event stack
+ * unwinds, but before the browser repaints.
+ */
+abstract class _MicrotaskScheduler {
+  bool _nextMicrotaskFrameScheduled = false;
+  _MicrotaskCallback _callback;
+
+  _MicrotaskScheduler(this._callback);
+
+  /**
+   * Creates the best possible measurement scheduler for the current platform.
+   */
+  factory _MicrotaskScheduler.best(_MicrotaskCallback callback) {
+    if (Window._supportsSetImmediate) {
+      return new _SetImmediateScheduler(callback);
+    } else if (MutationObserver.supported) {
+      return new _MutationObserverScheduler(callback);
+    }
+    return new _PostMessageScheduler(callback);
+  }
+
+  /**
+   * Schedules a measurement callback if one has not been scheduled already.
+   */
+  void maybeSchedule() {
+    if (this._nextMicrotaskFrameScheduled) {
+      return;
+    }
+    this._nextMicrotaskFrameScheduled = true;
+    this._schedule();
+  }
+
+  /**
+   * Does the actual scheduling of the callback.
+   */
+  void _schedule();
+
+  /**
+   * Handles the measurement callback and forwards it if necessary.
+   */
+  void _onCallback() {
+    // Ignore spurious messages.
+    if (!_nextMicrotaskFrameScheduled) {
+      return;
+    }
+    _nextMicrotaskFrameScheduled = false;
+    this._callback();
+  }
+}
+
+/**
+ * Scheduler which uses window.postMessage to schedule events.
+ */
+class _PostMessageScheduler extends _MicrotaskScheduler {
+  const _MEASUREMENT_MESSAGE = "DART-MEASURE";
+
+  _PostMessageScheduler(_MicrotaskCallback callback): super(callback) {
+      // Messages from other windows do not cause a security risk as
+      // all we care about is that _handleMessage is called
+      // after the current event loop is unwound and calling the function is
+      // a noop when zero requests are pending.
+      window.onMessage.listen(this._handleMessage);
+  }
+
+  void _schedule() {
+    window.postMessage(_MEASUREMENT_MESSAGE, "*");
+  }
+
+  void _handleMessage(e) {
+    this._onCallback();
+  }
+}
+
+/**
+ * Scheduler which uses a MutationObserver to schedule events.
+ */
+class _MutationObserverScheduler extends _MicrotaskScheduler {
+  MutationObserver _observer;
+  Element _dummy;
+
+  _MutationObserverScheduler(_MicrotaskCallback callback): super(callback) {
+    // Mutation events get fired as soon as the current event stack is unwound
+    // so we just make a dummy event and listen for that.
+    _observer = new MutationObserver(this._handleMutation);
+    _dummy = new DivElement();
+    _observer.observe(_dummy, attributes: true);
+  }
+
+  void _schedule() {
+    // Toggle it to trigger the mutation event.
+    _dummy.hidden = !_dummy.hidden;
+  }
+
+  _handleMutation(List<MutationRecord> mutations, MutationObserver observer) {
+    this._onCallback();
+  }
+}
+
+/**
+ * Scheduler which uses window.setImmediate to schedule events.
+ */
+class _SetImmediateScheduler extends _MicrotaskScheduler {
+  _SetImmediateScheduler(_MicrotaskCallback callback): super(callback);
+
+  void _schedule() {
+    window._setImmediate(_handleImmediate);
+  }
+
+  void _handleImmediate() {
+    this._onCallback();
+  }
+}
+
+List<TimeoutHandler> _pendingMicrotasks;
+_MicrotaskScheduler _microtaskScheduler = null;
+
+void _maybeScheduleMicrotaskFrame() {
+  if (_microtaskScheduler == null) {
+    _microtaskScheduler =
+      new _MicrotaskScheduler.best(_completeMicrotasks);
+  }
+  _microtaskScheduler.maybeSchedule();
+}
+
+/**
+ * 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 _addMicrotaskCallback(TimeoutHandler callback) {
+  if (_pendingMicrotasks == null) {
+    _pendingMicrotasks = <TimeoutHandler>[];
+    _maybeScheduleMicrotaskFrame();
+  }
+  _pendingMicrotasks.add(callback);
+}
+
+
+/**
+ * Complete all pending measurement futures evaluating them in a single batch
+ * so that the the browser is guaranteed to avoid multiple layouts.
+ */
+void _completeMicrotasks() {
+  var callbacks = _pendingMicrotasks;
+  _pendingMicrotasks = null;
+  for (var callback in callbacks) {
+    callback();
+  }
+}