Compute snap offsets (both repeat and element based)

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index 4a4f729789e74e74629b83a48ea724f4c655a1f7..49d2bb7edf98d0337b4c1d2623f64c2c774cc1d0 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -55,6 +55,7 @@
 #include "core/layout/shapes/ShapeOutsideInfo.h"
 #include "core/page/AutoscrollController.h"
 #include "core/page/Page.h"
+#include "core/page/scrolling/SnapCoordinator.h"
 #include "core/paint/BackgroundImageGeometry.h"
 #include "core/paint/BoxPainter.h"
 #include "core/paint/PaintLayer.h"
@@ -118,6 +119,7 @@ void LayoutBox::willBeDestroyed()
     clearOverrideSize();
     clearContainingBlockOverrideSize();
     clearExtraInlineAndBlockOffests();
+    clearScrollSnapMapping();
 
     LayoutBlock::removePercentHeightDescendantIfNeeded(this);
 
@@ -126,6 +128,12 @@ void LayoutBox::willBeDestroyed()
     LayoutBoxModelObject::willBeDestroyed();
 }
 
+void LayoutBox::insertedIntoTree()
+{
+    LayoutObject::insertedIntoTree();
+    addScrollSnapMapping();
+}
+
 void LayoutBox::removeFloatingOrPositionedChildFromBlockLists()
 {
     ASSERT(isFloatingOrOutOfFlowPositioned());
@@ -260,6 +268,8 @@ void LayoutBox::styleDidChange(StyleDifference diff, const ComputedStyle* oldSty
         LayoutFlowThread* flowThread = flowThreadContainingBlock();
         if (flowThread && flowThread != this)
             flowThread->flowThreadDescendantStyleDidChange(this, diff, *oldStyle);
+
+        updateScrollSnapMappingAfterStyleChange(&newStyle, oldStyle);

[esprehn, 2015/12/01 18:31:11]

newStyle should be a reference, don't do & here.

[majidvp, 2015/12/03 17:54:16]

Good point. I have now changed the code to make |addScrollSnapMapping|
and |clearScrollSnapMapping| more symmetrical and both using 
|updateScrollSnapMappingAfterStyleChange|. This is cleaner and 
addresses an small issue I noticed too.

To do this, I am using the fact that newStyle is a pointer and can be null.

     }
 }
 
@@ -330,6 +340,40 @@ void LayoutBox::updateGridPositionAfterStyleChange(const ComputedStyle* oldStyle
     toLayoutGrid(parent())->dirtyGrid();
 }
 
+void LayoutBox::updateScrollSnapMappingAfterStyleChange(const ComputedStyle* newStyle, const ComputedStyle* oldStyle)
+{
+    SnapCoordinator* snapCoordinator = document().snapCoordinator();
+    if (!snapCoordinator)
+        return;
+
+    ScrollSnapType oldSnapType = oldStyle ? oldStyle->scrollSnapType() :  ScrollSnapTypeNone;
+    ScrollSnapType newSnapType = newStyle ? newStyle->scrollSnapType() :  ScrollSnapTypeNone;
+    if (oldSnapType != newSnapType)
+        snapCoordinator->snapContainerDidChange(*this, newSnapType);
+
+    Vector<LengthPoint> emptyVector;
+    const Vector<LengthPoint>& oldSnapCoordinate = oldStyle ? oldStyle->scrollSnapCoordinate() :  emptyVector;
+    const Vector<LengthPoint>& newSnapCoordinate = newStyle ? newStyle->scrollSnapCoordinate() :  emptyVector;

[esprehn, 2015/12/01 18:31:11]

newStyle can't be null, you don't need the null check.

[majidvp, 2015/12/03 17:54:16]

See above.

+    if (oldSnapCoordinate != newSnapCoordinate)
+        snapCoordinator->snapAreaDidChange(*this, newSnapCoordinate);
+}
+
+void LayoutBox::addScrollSnapMapping()
+{
+    updateScrollSnapMappingAfterStyleChange(style(), nullptr);
+}
+
+void LayoutBox::clearScrollSnapMapping()
+{
+    if (SnapAreaSet* areas = snapAreas()) {
+        for (auto& snapArea : *areas)
+            snapArea->m_rareData->m_snapContainer = nullptr;
+        areas->clear();
+    }
+
+    this->setSnapContainer(nullptr);

[esprehn, 2015/12/01 18:31:11]

Why do you need this-> ?

[majidvp, 2015/12/03 17:54:16]

Done.

+}
+
 void LayoutBox::updateFromStyle()
 {
     LayoutBoxModelObject::updateFromStyle();
@@ -4845,4 +4889,38 @@ ShapeOutsideInfo* LayoutBox::shapeOutsideInfo() const
     return ShapeOutsideInfo::isEnabledFor(*this) ? ShapeOutsideInfo::info(*this) : nullptr;
 }
 
+LayoutBox* LayoutBox::snapContainer() const
+{
+    return m_rareData ? m_rareData->m_snapContainer : nullptr;
+}
+
+void LayoutBox::setSnapContainer(LayoutBox* container)
+{
+    if (LayoutBox* currentContainer = snapContainer())
+        currentContainer->removeSnapArea(*this);
+
+    ensureRareData().m_snapContainer = container;
+
+    if (container)
+        container->addSnapArea(*this);
+}
+
+
+void LayoutBox::addSnapArea(const LayoutBox& snapArea)
+{
+    ensureRareData().ensureSnapAreas().add(&snapArea);
+}
+
+void LayoutBox::removeSnapArea(const LayoutBox& snapArea)
+{
+    if (m_rareData && m_rareData->m_snapAreas) {
+        m_rareData->m_snapAreas->remove(&snapArea);
+    }
+}
+
+SnapAreaSet* LayoutBox::snapAreas() const
+{
+    return m_rareData ? m_rareData->m_snapAreas.get() : nullptr;
+}
+
 } // namespace blink