Move LayoutBoxModelObject's PaintLayer member to rare paint data

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 63 matching lines @@
 //
 // All CSS objects follow the box model object. See THE BOX MODEL section in
 // LayoutBox for more information.
 //
 // This class actually doesn't have the box model but it exposes some common
 // functions or concepts that sub-classes can extend upon. For example, there
 // are accessors for margins, borders, paddings and borderBoundingBox().
 //
 // The reason for this partial implementation is that the 2 classes inheriting
 // from it (LayoutBox and LayoutInline) have different requirements but need to
-// have a PaintLayer.
-// For a full implementation of the box model, see LayoutBox.
+// have a PaintLayer. For a full implementation of the box model, see LayoutBox.
 //
-// An important member of this class is PaintLayer. This class is
-// central to painting and hit-testing (see its class comment).
-// PaintLayers are instantiated for several reasons based on the
-// return value of layerTypeRequired().
+// An important member of this class is PaintLayer, which is stored in a rare-
+// data pattern (see: Layer()). PaintLayers are instantiated for several reasons
+// based on the return value of layerTypeRequired().
 // Interestingly, most SVG objects inherit from LayoutSVGModelObject and thus
 // can't have a PaintLayer. This is an unfortunate artifact of our
 // design as it limits code sharing and prevents hardware accelerating SVG
 // (the current design require a PaintLayer for compositing).
 //
 //
 // ***** COORDINATE SYSTEMS *****
 //
 // In order to fully understand LayoutBoxModelObject and the inherited classes,
 // we need to introduce the concept of coordinate systems.
@@ skipping 70 matching lines @@
   int PixelSnappedOffsetLeft(const Element* parent) const {
     return RoundToInt(OffsetLeft(parent));
   }
   int PixelSnappedOffsetTop(const Element* parent) const {
     return RoundToInt(OffsetTop(parent));
   }
   virtual int PixelSnappedOffsetWidth(const Element*) const;
   virtual int PixelSnappedOffsetHeight(const Element*) const;
 
   bool HasSelfPaintingLayer() const;
-  PaintLayer* Layer() const { return layer_.get(); }
+  PaintLayer* Layer() const {
+    return GetRarePaintData() ? GetRarePaintData()->Layer() : nullptr;
+  }
+  // The type of PaintLayer to instantiate. Any value returned from this
+  // function other than NoPaintLayer will lead to a PaintLayer being created.
+  virtual PaintLayerType LayerTypeRequired() const = 0;
   PaintLayerScrollableArea* GetScrollableArea() const;
 
   virtual void UpdateFromStyle();
 
-  // The type of PaintLayer to instantiate. Any value returned from this
-  // function other than NoPaintLayer will populate |m_layer|.
-  virtual PaintLayerType LayerTypeRequired() const = 0;
-
   // This will work on inlines to return the bounding box of all of the lines'
   // border boxes.
   virtual IntRect BorderBoundingBox() const = 0;
 
   virtual LayoutRect VisualOverflowRect() const = 0;
 
   // Checks if this box, or any of it's descendants, or any of it's
   // continuations, will take up space in the layout of the page.
   bool HasNonEmptyLayoutSize() const;
   bool UsesCompositedScrolling() const;
@@ skipping 328 matching lines @@
     MoveChildrenTo(to_box_model_object, start_child, end_child, 0,
                    full_remove_insert);
   }
   virtual void MoveChildrenTo(LayoutBoxModelObject* to_box_model_object,
                               LayoutObject* start_child,
                               LayoutObject* end_child,
                               LayoutObject* before_child,
                               bool full_remove_insert = false);
 
  private:
-  void CreateLayer();
+  void CreateLayerAfterStyleChange();
 
   LayoutUnit ComputedCSSPadding(const Length&) const;
   bool IsBoxModelObject() const final { return true; }
 
   LayoutBoxModelObjectRareData& EnsureRareData() {
     if (!rare_data_)
       rare_data_ = WTF::MakeUnique<LayoutBoxModelObjectRareData>();
     return *rare_data_.get();
   }
 
-  // The PaintLayer associated with this object. |m_layer| can be nullptr
-  // depending on the return value of layerTypeRequired().
-  std::unique_ptr<PaintLayer> layer_;
-
   std::unique_ptr<LayoutBoxModelObjectRareData> rare_data_;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBoxModelObject, IsBoxModelObject());
 
 }  // namespace blink
 
 #endif  // LayoutBoxModelObject_h