[New Multicolumn] Support for paged overflow. (By mstensho).

Source/core/rendering/RenderBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 20 matching lines @@
 #include "config.h"
 #include "core/rendering/RenderBlockFlow.h"
 
 #include "core/accessibility/AXObjectCache.h"
 #include "core/frame/FrameView.h"
 #include "core/rendering/FastTextAutosizer.h"
 #include "core/rendering/HitTestLocation.h"
 #include "core/rendering/RenderFlowThread.h"
 #include "core/rendering/RenderLayer.h"
 #include "core/rendering/RenderMultiColumnFlowThread.h"
+#include "core/rendering/RenderPagedFlowThread.h"
 #include "core/rendering/RenderText.h"
 #include "core/rendering/RenderView.h"
 #include "core/rendering/line/LineWidth.h"
 #include "core/rendering/svg/SVGTextRunRenderingContext.h"
 #include "platform/text/BidiTextRun.h"
 
 namespace WebCore {
 
 bool RenderBlockFlow::s_canPropagateFloatIntoSibling = false;
 
@@ skipping 1769 matching lines @@
                     }
                 }
             }
         }
 
         parentBlockFlow->markAllDescendantsWithFloatsForLayout();
         parentBlockFlow->markSiblingsWithFloatsForLayout();
     }
 
     if (diff.needsFullLayout() || !oldStyle)
-        createOrDestroyMultiColumnFlowThreadIfNeeded();
+        createOrDestroyMultiColumnFlowThreadIfNeeded(oldStyle);
 }
 
 void RenderBlockFlow::updateStaticInlinePositionForChild(RenderBox* child, LayoutUnit logicalTop)
 {
     if (child->style()->isOriginalDisplayInlineType())
         setStaticInlinePositionForChild(child, logicalTop, startAlignedOffsetForLine(logicalTop, false));
     else
         setStaticInlinePositionForChild(child, logicalTop, startOffsetForContent());
 }
 
@@ skipping 886 matching lines @@
     bool hasStrongDirectionality;
     run.setDirection(directionForRun(run, hasStrongDirectionality));
     return run;
 }
 
 RootInlineBox* RenderBlockFlow::createRootInlineBox()
 {
     return new RootInlineBox(*this);
 }
 
-void RenderBlockFlow::createOrDestroyMultiColumnFlowThreadIfNeeded()
+bool RenderBlockFlow::isPagedOverflow(const RenderStyle* style)
+{
+    return style->isOverflowPaged() && node() != document().viewportDefiningElement();
+}
+
+RenderBlockFlow::FlowThreadType RenderBlockFlow::flowThreadType(const RenderStyle* style)
+{
+    if (isPagedOverflow(style))
+        return PagedFlowThread;
+    if (style->specifiesColumns())
+        return MultiColumnFlowThread;
+    return NoFlowThread;
+}
+
+void RenderBlockFlow::createOrDestroyMultiColumnFlowThreadIfNeeded(const RenderStyle* oldStyle)
 {
     if (!document().regionBasedColumnsEnabled())
         return;
 
-    bool needsFlowThread = style()->specifiesColumns();
-    if (needsFlowThread != static_cast<bool>(multiColumnFlowThread())) {
-        if (needsFlowThread) {
-            RenderMultiColumnFlowThread* flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
-            addChild(flowThread);
-            flowThread->populate();
-            RenderBlockFlowRareData& rareData = ensureRareData();
-            ASSERT(!rareData.m_multiColumnFlowThread);
-            rareData.m_multiColumnFlowThread = flowThread;
-        } else {
+    // Paged overflow trumps multicol in this implementation. Ideally, it should be possible to have
+    // both paged overflow and multicol on the same element, but then we need two flow
+    // threads. Anyway, this is nothing to worry about until we can actually nest multicol properly
+    // inside other fragmentation contexts.
+    FlowThreadType type = flowThreadType(style());
+
+    if (multiColumnFlowThread()) {
+        ASSERT(oldStyle);
+        if (type != flowThreadType(oldStyle)) {
+            // If we're no longer to be multicol/paged, destroy the flow thread. Also destroy it
+            // when switching between multicol and paged, since that affects the column set
+            // structure (multicol containers may have spanners, paged containers may not).
             multiColumnFlowThread()->evacuateAndDestroy();
             ASSERT(!multiColumnFlowThread());
         }
     }
+    if ((type != NoFlowThread) && !multiColumnFlowThread()) {

[rune, 2014/07/04 09:44:25]

The blink code often use early returns. How about:

if (type == NoFlowThread || multiColumnFlowThread())
    return;

Gets rid of the double negation, less indentation, and more readable perhaps.

+        RenderMultiColumnFlowThread* flowThread;
+        if (type == PagedFlowThread) {
+            // Paged overflow is currently done using the multicol implementation.
+            flowThread = RenderPagedFlowThread::createAnonymous(document(), style());
+        } else {
+            flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
+        }
+        addChild(flowThread);
+        flowThread->populate();
+        RenderBlockFlowRareData& rareData = ensureRareData();
+        ASSERT(!rareData.m_multiColumnFlowThread);
+        rareData.m_multiColumnFlowThread = flowThread;
+    }
 }
 
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtr(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace WebCore