[New Multicolumn] Create RenderMultiColumnSet during renderer creation, not during layout.

Source/core/rendering/RenderMultiColumnFlowThread.h

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 13 matching lines @@
  */
 
 
 #ifndef RenderMultiColumnFlowThread_h
 #define RenderMultiColumnFlowThread_h
 
 #include "core/rendering/RenderFlowThread.h"
 
 namespace WebCore {
 
+class RenderMultiColumnSet;
+
+// Flow thread implementation for CSS multicol. This will be inserted as an anonymous child block of
+// the actual multicol container (i.e. the RenderBlockFlow whose style computes to non-auto
+// column-count and/or column-width). RenderMultiColumnFlowThread is the heart of the multicol
+// implementation, and there is only one instance per multicol container. Child content of the
+// multicol container is parented into the flow thread at the time of renderer insertion.
+//
+// Apart from this flow thread child, the multicol container will also have RenderMultiColumnSet
+// "region" children, which are used to position the columns visually. The flow thread is in charge
+// of layout, and, after having calculated the column width, it lays out content as if everything
+// were in one tall single column, except that there will typically be some amount of blank space
+// (also known as pagination struts) at the offsets where the actual column boundaries are. This
+// way, content that needs to be preceded by a break will appear at the top of the next
+// column. Content needs to be preceded by a break when there's a forced break or when the content
+// is unbreakable and cannot fully fit in the same column as the preceding piece of
+// content. Although a RenderMultiColumnFlowThread is laid out, it does not take up any space in its
+// container. It's the RenderMultiColumnSet objects that take up the necessary amount of space, and
+// make sure that the columns are painted and hit-tested correctly.
 class RenderMultiColumnFlowThread FINAL : public RenderFlowThread {
 public:
     virtual ~RenderMultiColumnFlowThread();
 
     static RenderMultiColumnFlowThread* createAnonymous(Document&, RenderStyle* parentStyle);
 
     RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
 
+    RenderMultiColumnSet* firstMultiColumnSet() const;
+    RenderMultiColumnSet* lastMultiColumnSet() const;
+
+    virtual void addChild(RenderObject* newChild, RenderObject* beforeChild = 0) OVERRIDE;
+
     // Populate the flow thread with what's currently its siblings. Called when a regular block
     // becomes a multicol container.
     void populate();
 
     // Empty the flow thread by moving everything to the parent. Remove all multicol specific
     // renderers. Then destroy the flow thread. Called when a multicol container becomes a regular
     // block.
     void evacuateAndDestroy();
 
     unsigned columnCount() const { return m_columnCount; }
     LayoutUnit columnWidth() const { return m_columnWidth; }
     LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
     void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
     bool requiresBalancing() const { return !columnHeightAvailable() || multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance; }
 
     void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
     bool computeColumnCountAndWidth();
     bool recalculateColumnHeights();
 
 private:
     RenderMultiColumnFlowThread();
 
     virtual const char* renderName() const OVERRIDE;
+    virtual void addRegionToThread(RenderRegion*) OVERRIDE;
+    virtual void willBeRemovedFromTree() OVERRIDE;
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const OVERRIDE;
-    virtual void autoGenerateRegionsToBlockOffset(LayoutUnit) OVERRIDE;
     virtual LayoutUnit initialLogicalWidth() const OVERRIDE;
     virtual void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) OVERRIDE;
     virtual void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) OVERRIDE;
+    virtual RenderRegion* regionAtBlockOffset(LayoutUnit) const OVERRIDE;
     virtual bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) OVERRIDE;
     virtual bool isPageLogicalHeightKnown() const OVERRIDE;
 
     unsigned m_columnCount; // The used value of column-count
     LayoutUnit m_columnWidth; // The used value of column-width
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsRebalancing;
 };
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnFlowThread_h