Initial ResizeObserver implementation

third_party/WebKit/Source/core/observer/ResizeObserverController.cpp

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/observer/ResizeObserverController.h"
+
+#include "core/observer/ResizeObserver.h"
+
+
+namespace blink {
+
+ResizeObserverController::ResizeObserverController()
+{
+}
+
+void ResizeObserverController::addObserver(ResizeObserver& observer)
+{
+    m_observers.add(&observer);
+}
+
+bool ResizeObserverController::gatherObservations()
+{
+    bool hasObservations = false;
+    for (auto& observer : m_observers)
+        hasObservations = hasObservations || observer->gatherObservations();

[szager1, 2016/07/11 18:33:47]

This will skip calling gatherObservations() if hasObservations is already true. 
Either reverse the operands, or (better):

hasObservations |= observer->gatherObservations();

... and you should probably add a test for this.

[atotic1, 2016/07/11 23:18:58]

Good catch. I've reversed the operands, since |= could be optimizing my
gatherObservations away.

+    return hasObservations;
+}
+
+void ResizeObserverController::deliverObservations()
+{
+    // Copy is needed because m_observers might get modified during deliverObservations.
+    ObserverSet observers;

[szager1, 2016/07/11 18:33:47]

The idiomatic way to do this is:

HeapVector<Member<ResizeObserver>> observers;
copyToVector(m_observers, observers);

[atotic1, 2016/07/11 23:18:58]

Done. A question:
m_observers is a collection of WeakMembers
observers is a collection of Members

This code will prevent Observers from being GCd until the end of this routine. 
I assume this does not matter?

[szager1, 2016/07/11 23:33:34]

Correct: a GC can never happen during the execution of this method; and even if
it did, you would still want to deliver pending notifications, even if that
meant keeping objects alive a bit longer.

+    for (auto it = m_observers.begin(); it != m_observers.end(); ++it)
+        observers.add(*it);
+
+    for (auto& observer : observers) {
+        if (observer)
+            observer->deliverObservations();
+    }
+}
+
+void ResizeObserverController::clearObservations()
+{
+    for (auto& observer : m_observers)
+        observer->clearObservations();
+}
+
+DEFINE_TRACE(ResizeObserverController)
+{
+    visitor->trace(m_observers);
+}
+
+} // namespace blink