Compute snap offsets (both repeat and element based)

third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h

Index: third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h b/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h
new file mode 100644
index 0000000000000000000000000000000000000000..21f8f793d60fae74b997e3d15186aea40eb73d5b
--- /dev/null
+++ b/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.h
@@ -0,0 +1,48 @@
+// 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.
+
+#ifndef SnapCoordinator_h
+#define SnapCoordinator_h
+
+#include "core/layout/LayoutBox.h"
+#include "platform/heap/Handle.h"
+#include "wtf/Vector.h"
+
+namespace blink {
+
+class ComputedStyle;
+class Element;
+struct LengthPoint;
+
+// Snap Coordinator keeps track of snap containers and all of their associated

[skobes, 2016/03/07 03:42:27]

This comment should either describe what snap containers and snap elements are,
or link to something that does.

[majidvp, 2016/05/03 17:23:36]

Done.

+// snap elements. It also contains the logic to generate the final list of snap
+// offsets for any snap container.
+class CORE_EXPORT SnapCoordinator final : public NoBaseWillBeGarbageCollectedFinalized<SnapCoordinator> {
+    WTF_MAKE_NONCOPYABLE(SnapCoordinator);
+
+public:
+    static PassOwnPtrWillBeRawPtr<SnapCoordinator> create();
+    ~SnapCoordinator();
+    DEFINE_INLINE_TRACE() {}
+
+    void snapContainerDidChange(LayoutBox&, ScrollSnapType);
+    void snapAreaDidChange(LayoutBox&, const Vector<LengthPoint>& snapCoordinates);
+
+#ifndef NDEBUG
+    void showSnapAreaMap();
+    void showSnapAreasFor(const LayoutBox*);
+#endif
+
+private:
+    friend class SnapCoordinatorTest;
+    explicit SnapCoordinator();
+
+    Vector<double> snapOffsets(const Element&, ScrollbarOrientation);
+
+    HashSet<const LayoutBox*> m_snapContainers;

[skobes, 2016/03/07 03:42:27]

Why do we need this exactly?  Is it just for showSnapAreaMap?  If so, guard with
#ifndef NDEBUG.

[majidvp, 2016/05/03 17:23:36]

Here is the rseponse I gave to esprehn@ on this issue earlier.
If you prefer, I can move this to debug-only until it actually used.

On 2015/12/01 18:31:11, esprehn wrote:
At the moment it is for debug only but I am planning to use it in follow up
patches. In particular, after layout we need to traverse over all snap
containers and recompute their snap points if necessary and send updates to
CW. I suspect walking the whole layout tree to do this is going to be
expensive given that snap containers are going to be sparse which is why
I am keeping a set.

If it turns out there is a better way to do this, then I can get rid of it.

+};
+
+} // namespace blink
+
+#endif // SnapCoordinator_h