Compute snap offsets (both repeat and element based)

Source/core/page/scrolling/SnapCoordinator.h

Index: Source/core/page/scrolling/SnapCoordinator.h
diff --git a/Source/core/page/scrolling/SnapCoordinator.h b/Source/core/page/scrolling/SnapCoordinator.h
new file mode 100644
index 0000000000000000000000000000000000000000..0f461738ec1d174aae1e05c0f2cac027ade9b983
--- /dev/null
+++ b/Source/core/page/scrolling/SnapCoordinator.h
@@ -0,0 +1,68 @@
+// 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 SnapCoordinator_h
+#define SnapCoordinator_h
+
+#include "core/dom/Element.h"
+#include "platform/heap/Handle.h"
+#include "platform/scroll/ScrollTypes.h"
+#include "wtf/PassRefPtr.h"
+#include "wtf/RefCounted.h"
+#include "wtf/Vector.h"
+
+namespace blink {
+
+using SnapElementSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<const Element>>;
+using SnapElementMap = WillBeHeapHashMap<RawPtrWillBeWeakMember<const ContainerNode>, OwnPtrWillBeMember<SnapElementSet>>;
+// using SnapContainerMap = WillBeHeapHashMap<RawPtrWillBeWeakMember<const Element>, RawPtrWillBeWeakMember<const ContainerNode>>;
+
+class ContainerNode;
+class ComputedStyle;
+
+// Snap Coordinator keeps track of snap containers and all of their associated
+// 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 NoBaseWillBeGarbageCollected<SnapCoordinator> {
+    WTF_MAKE_NONCOPYABLE(SnapCoordinator);
+
+public:
+    static PassOwnPtrWillBeRawPtr<SnapCoordinator> create(LocalFrame&);
+    ~SnapCoordinator();
+    DEFINE_INLINE_TRACE() {
+        visitor->trace(m_frame);
+    }
+
+    void styleChanged(const Node*, const ComputedStyle*);

[esprehn, 2015/09/24 17:37:41]

I don't want general "styleChanged" hooks, please put the actual comparison in
Element.cpp's recalcOwnStyle

[majidvp, 2015/10/15 21:47:03]

Done. Now only exposing snap{Element,Container}DidChange.

+
+    void detach(const ContainerNode&);
+    void attach(const ContainerNode&);

[esprehn, 2015/09/24 17:37:41]

Expose a real API, don't generically hook attach/detach. Also this should take
Element&, not ContainerNode.

[majidvp, 2015/10/15 21:47:03]

Done. Now only exposing snap{Element,Container}DidChange.

+
+#ifndef NDEBUG
+    void showSnapElementMap();
+    void showSnapElementsFor(const ContainerNode*);
+#endif
+
+private:
+    friend class SnapCoordinatorTest;
+    explicit SnapCoordinator(LocalFrame&);
+
+    Vector<double> snapOffsets(const ContainerNode&, ScrollbarOrientation);
+    const ContainerNode* findSnapContainer(const Element&);
+    void snapContainerDidChange(const ContainerNode&);
+    void snapElementDidChange(const Element&);
+    void addSnapContainer(const ContainerNode&);
+    void removeSnapContainer(const ContainerNode&);
+    void addSnapElement(const ContainerNode&, const Element&);
+    void removeSnapElement(const Element&);
+
+    SnapElementSet& ensureSnapElementSet(const ContainerNode&);
+
+    SnapElementMap m_elementMap;
+    RawPtrWillBeMember<LocalFrame> m_frame;

[esprehn, 2015/09/24 17:37:41]

You never actually use Frame, why does it have this member?

This thing belongs on FrameView or Document, not on the LocalFrame.

[majidvp, 2015/10/15 21:47:03]

Done. Moved it to document.

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