Compute snap offsets according to CSS Scroll Snap Spec Level 1

third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.cpp

Index: third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.cpp b/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.cpp
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index 0000000000000000000000000000000000000000..022f7a2a272cf758ab4af7fe93731a8109ff8664
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+++ b/third_party/WebKit/Source/core/page/scrolling/SnapCoordinator.cpp
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+// 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.
+
+#include "SnapCoordinator.h"
+
+#include "core/dom/Element.h"
+#include "core/dom/Node.h"
+#include "core/dom/NodeComputedStyle.h"
+#include "core/layout/LayoutBlock.h"
+#include "core/layout/LayoutBox.h"
+#include "core/layout/LayoutView.h"
+#include "core/style/ComputedStyle.h"
+#include "platform/LengthFunctions.h"
+
+namespace blink {
+
+SnapCoordinator::SnapCoordinator()
+    : m_snapContainers()
+{
+}
+
+SnapCoordinator::~SnapCoordinator() { }
+
+SnapCoordinator* SnapCoordinator::create()
+{
+    return new SnapCoordinator();
+}
+
+// Returns the scroll container that can be affected by this snap area.
+static LayoutBox* findSnapContainer(const LayoutBox& snapArea)
+{
+    // According to the new spec https://drafts.csswg.org/css-scroll-snap/#snap-model
+    // "Snap positions must only affect the nearest ancestor (on the element’s
+    // containing block chain) scroll container".
+    Element* viewportDefiningElement = snapArea.node()->document().viewportDefiningElement();
+    LayoutBox* box = snapArea.containingBlock();
+    while (box && !box->hasOverflowClip() && !box->isLayoutView() && box->node() != viewportDefiningElement)
+        box = box->containingBlock();
+
+    // If we reach to viewportDefiningElement then we dispatch to viewport
+    if (box->node() == viewportDefiningElement)
+        return snapArea.node()->document().layoutView();
+
+    return box;
+}
+
+void SnapCoordinator::snapAreaDidChange(LayoutBox& snapArea, const Vector<LengthPoint>& snapCoordinates)
+{
+    if (snapCoordinates.isEmpty()) {
+        snapArea.setSnapContainer(nullptr);
+        return;
+    }
+
+    if (LayoutBox* snapContainer = findSnapContainer(snapArea)) {
+        snapArea.setSnapContainer(snapContainer);
+    } else {
+        // TODO(majidvp): keep track of snap areas that do not have any
+        // container so that we check them again when a new container is
+        // added to the page.
+    }
+}
+
+void SnapCoordinator::snapContainerDidChange(LayoutBox& snapContainer, ScrollSnapType scrollSnapType)
+{
+    if (scrollSnapType == ScrollSnapTypeNone) {
+        // TODO(majidvp): Track and report these removals to CompositorWorker
+        // instance responsible for snapping
+        m_snapContainers.remove(&snapContainer);
+        snapContainer.clearSnapAreas();
+    } else {
+        m_snapContainers.add(&snapContainer);
+    }
+
+    // TODO(majidvp): Add logic to correctly handle orphaned snap areas here.
+    // 1. Removing container: find a new snap container for its orphan snap
+    // areas (most likely nearest ancestor of current container) otherwise add
+    // them to an orphan list.
+    // 2. Adding container: may take over snap areas from nearest ancestor snap
+    // container or from existing areas in orphan pool.
+}
+
+// Translate local snap coordinates into snap container's scrolling content
+// coordinate space.
+static Vector<FloatPoint> localToContainerSnapCoordinates(const LayoutBox& containerBox, const LayoutBox& snapArea)
+{
+    Vector<FloatPoint> result;
+    LayoutPoint scrollOffset(containerBox.scrollLeft(), containerBox.scrollTop());
+
+    const Vector<LengthPoint>& snapCoordinates = snapArea.style()->scrollSnapCoordinate();
+    for (auto& coordinate : snapCoordinates) {
+        FloatPoint localPoint = floatPointForLengthPoint(coordinate, FloatSize(snapArea.size()));
+        FloatPoint containerPoint = snapArea.localToAncestorPoint(localPoint, &containerBox);
+        containerPoint.moveBy(scrollOffset);
+        result.append(containerPoint);
+    }
+    return result;
+}
+
+Vector<double> SnapCoordinator::snapOffsets(const ContainerNode& element, ScrollbarOrientation orientation)
+{
+    const ComputedStyle* style = element.computedStyle();
+    const LayoutBox* snapContainer = element.layoutBox();
+    ASSERT(style);
+    ASSERT(snapContainer);
+
+    Vector<double> result;
+
+    if (style->getScrollSnapType() == ScrollSnapTypeNone)
+        return result;
+
+    const ScrollSnapPoints& snapPoints = (orientation == HorizontalScrollbar) ? style->scrollSnapPointsX() : style->scrollSnapPointsY();
+
+    LayoutUnit clientSize = (orientation == HorizontalScrollbar) ? snapContainer->clientWidth() : snapContainer->clientHeight();
+    LayoutUnit scrollSize = (orientation == HorizontalScrollbar) ? snapContainer->scrollWidth() : snapContainer->scrollHeight();
+
+    if (snapPoints.hasRepeat) {
+        LayoutUnit repeat = valueForLength(snapPoints.repeatOffset, clientSize);
+
+        // calc() values may be negative or zero in which case we clamp them to 1px.
+        // See: https://lists.w3.org/Archives/Public/www-style/2015Jul/0075.html
+        repeat = std::max<LayoutUnit>(repeat, LayoutUnit(1));
+        for (LayoutUnit offset = repeat; offset <= (scrollSize - clientSize); offset += repeat) {
+            result.append(offset.toFloat());
+        }
+    }
+
+    // Compute element-based snap points by mapping the snap coordinates from
+    // snap areas to snap container.
+    bool didAddSnapAreaOffset = false;
+    if (SnapAreaSet* snapAreas = snapContainer->snapAreas()) {
+        for (auto& snapArea : *snapAreas) {
+            Vector<FloatPoint> snapCoordinates = localToContainerSnapCoordinates(*snapContainer, *snapArea);
+            for (const FloatPoint& snapCoordinate : snapCoordinates) {
+                float snapOffset = (orientation == HorizontalScrollbar) ? snapCoordinate.x() : snapCoordinate.y();
+                if (snapOffset > scrollSize - clientSize)
+                    continue;
+                result.append(snapOffset);
+                didAddSnapAreaOffset = true;
+            }
+        }
+    }
+
+    if (didAddSnapAreaOffset)
+        std::sort(result.begin(), result.end());
+
+    return result;
+}
+
+#ifndef NDEBUG
+
+void SnapCoordinator::showSnapAreaMap()
+{
+    for (auto& container : m_snapContainers)
+        showSnapAreasFor(container);
+}
+
+void SnapCoordinator::showSnapAreasFor(const LayoutBox* container)
+{
+    const char* prefix = "    ";
+    container->node()->showNode();
+    if (SnapAreaSet* snapAreas = container->snapAreas()) {
+        for (auto& snapArea : *snapAreas) {
+            snapArea->node()->showNode(prefix);
+        }
+    }
+}
+
+#endif
+
+} // namespace blink