Unify the calculation of main thread offset of sticky element

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

 // 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 StickyPositionScrollingConstraints_h
 #define StickyPositionScrollingConstraints_h
 
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/FloatSize.h"
+#include "platform/wtf/HashMap.h"
 
 namespace blink {
 
 class LayoutBoxModelObject;
+class PaintLayer;
+class StickyPositionScrollingConstraints;
+
+typedef WTF::HashMap<PaintLayer*, StickyPositionScrollingConstraints>
+    StickyConstraintsMap;
 
 class StickyPositionScrollingConstraints final {
  public:
   enum AnchorEdgeFlags {
     kAnchorEdgeLeft = 1 << 0,
     kAnchorEdgeRight = 1 << 1,
     kAnchorEdgeTop = 1 << 2,
     kAnchorEdgeBottom = 1 << 3
   };
   typedef unsigned AnchorEdges;
@@ skipping 82 matching lines @@
     return nearest_sticky_box_shifting_containing_block_;
   }
 
   const FloatSize& GetTotalStickyBoxStickyOffset() const {
     return total_sticky_box_sticky_offset_;
   }
   const FloatSize& GetTotalContainingBlockStickyOffset() const {
     return total_containing_block_sticky_offset_;
   }
 
+  FloatSize GetLocalStickyOffset(const StickyConstraintsMap&) const;

[chrishtr, 2017/06/05 17:17:34]

Document clearly what this returns.

Not your fault, but: actually, on further inspection
of this file, each of these public accessors and setters needs documentation.
I suggest a class-level comment explaining each of the different types of
information, with examples.

[yigu, 2017/06/05 18:16:34]

Added a TODO for adding the class-level comment with examples.

+
   const LayoutBoxModelObject* NearestStickyAncestor() const {
     // If we have one or more sticky ancestor elements between ourselves and our
     // containing block, |m_nearestStickyBoxShiftingStickyBox| points to the
     // closest. Otherwise, |m_nearestStickyBoxShiftingContainingBlock| points
     // to the the first sticky ancestor between our containing block (inclusive)
     // and our scroll ancestor (exclusive). Therefore our nearest sticky
     // ancestor is the former if it exists, or the latter otherwise.
     //
     // If both are null, then we have no sticky ancestors before our scroll
     // ancestor, so the correct action is to return null.
@@ skipping 51 matching lines @@
   // constraining block constriant rect for different sticky descendants is
   // quite complex. See the StickyPositionComplexTableNesting test in
   // LayoutBoxModelObjectTest.cpp.
   FloatSize total_sticky_box_sticky_offset_;
   FloatSize total_containing_block_sticky_offset_;
 };
 
 }  // namespace blink
 
 #endif  // StickyPositionScrollingConstraints_h