IntersectionObserver: second cut.

third_party/WebKit/Source/core/dom/ElementIntersectionObserverData.h

+// Copyright 2015 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.
+
+#ifndef ElementIntersectionObserverData_h
+#define ElementIntersectionObserverData_h
+
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class Element;
+class IntersectionObservation;
+class IntersectionObserver;
+
+class ElementIntersectionObserverData : public GarbageCollectedFinalized<ElementIntersectionObserverData> {
+public:
+    DECLARE_TRACE();
+    ElementIntersectionObserverData();
+    ~ElementIntersectionObserverData();
+
+    bool hasIntersectionObserver() const;
+    bool hasIntersectionObservation() const;
+    bool hasObservationFor(IntersectionObserver&) const;
+    void addObservation(IntersectionObservation&);
+    void removeObservation(IntersectionObserver&);
+    void activateValidIntersectionObservers(Element&);
+    void deactivateAllIntersectionObservers(Element&);
+
+#if !ENABLE(OILPAN)
+    void dispose();
+#endif
+
+    WeakPtr<Element> createWeakPtr(Element*);
+
+private:
+    // IntersectionObservers for which the Element owning this data is root.

[haraken, 2015/12/22 01:13:29]

Help me understand: Is it possible that a non-Document Element becomes a root of
an IntersectionObserver? I was assuming that a target can be any Element but a
root is a Document.

Another question is: why do we need both
ElementIntersectionObserverData::m_intersectionObservers and
IntersectionObseverController::m_trackedObservers? I don't understand why we
need to keep track of a hash set of weak members in two places.

[esprehn, 2015/12/22 07:50:32]

root can be any element that's an ancestor.

[szager1, 2015/12/22 07:53:41]

The root can be any container element; see the spec:

http://rawgit.com/slightlyoff/IntersectionObserver/master/index.html#intersec...
IntersectionObserverController is associated with a Document, and it is
responsible for running the intersection algorithm and generating notifications.
 The reason it is associated with a document is because the algorithm runs
during FrameView::updateAllLifecyclePhases, at a point where the document is
known to be layout and compositing clean.

ElementIntersectionObserverData is associated with an Element, and it tracks
IntersectionObservers for which the given element is either root or target.

+    HeapHashSet<WeakMember<IntersectionObserver>> m_intersectionObservers;
+    // IntersectionObservations for which the Element owning this data is target.
+    HeapHashMap<Member<IntersectionObserver>, Member<IntersectionObservation>> m_intersectionObservations;

[haraken, 2015/12/22 01:13:29]

I don't fully understand why we need this mapping. Wouldn't it be more
straightforward if we have the following two HashSets?

- ElementIntersectionObserverData retains HashSet<Member<IntersectionObserver>>.
- IntersectionObserver retains HashSet<Member<IntersectionObservation>>.

[szager1, 2015/12/22 07:53:41]

Please take a look at the design doc linked in CL description:

https://docs.google.com/document/d/1hLK0eyT5_BzyNS4OkjsnoqqFQDYCbKfyBinj94OnL...

An IntersectionObservation must be kept alive by its target Element, hence the
hard reference.  If the target Element goes away, then the
IntersectionObservation should go away (i.e., the IntersectionObserver should
*not* keep it alive).

+
+#if !ENABLE(OILPAN)
+    OwnPtrWillBeMember<WeakPtrFactory<Element>> m_weakPointerFactory;

[haraken, 2015/12/22 01:13:30]

This can simply be:

  WeakPtrFactory<Element>

OwnPtrWillBeMember is not needed.

[szager1, 2015/12/22 07:53:41]

WeakPtrFactory must be constructed with a pointer to the object it generates
WeakPtr's for, which isn't available in the ElementIntersectionObserverData
constructor.  Rather than plumbing the object pointer all the way down (which
overly-complicates the methods), I just construct it lazily at a time when the
object pointer is available.

I went ahead and changed OwnPtrWillBeMember to OwnPtr, since this code will be
compiled when oilpan is enabled.

+#endif
+};
+
+} // namespace blink {
+
+#endif // ElementIntersectionObserverData_h