chromium/src/third_party/blink/renderer/core/page/scrolling/snap_coordinator.cc

// 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 "third_party/blink/renderer/core/page/scrolling/snap_coordinator.h"

#include "third_party/blink/renderer/core/dom/dom_node_ids.h"
#include "third_party/blink/renderer/core/dom/element.h"
#include "third_party/blink/renderer/core/dom/node.h"
#include "third_party/blink/renderer/core/frame/local_frame_view.h"
#include "third_party/blink/renderer/core/frame/web_feature.h"
#include "third_party/blink/renderer/core/layout/layout_block.h"
#include "third_party/blink/renderer/core/layout/layout_box.h"
#include "third_party/blink/renderer/core/layout/layout_view.h"
#include "third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h"
#include "third_party/blink/renderer/platform/geometry/length_functions.h"
#include "third_party/blink/renderer/platform/instrumentation/use_counter.h"

namespace blink {
namespace {
// This is experimentally determined and corresponds to the UA decided
// parameter as mentioned in spec.
constexpr float kProximityRatio = 1.0 / 3.0;
}  // namespace
// TODO(sunyunjia): Move the static functions to an anonymous namespace.

SnapCoordinator::SnapCoordinator() : snap_containers_() {}

SnapCoordinator::~SnapCoordinator() = default;

// Returns the layout box's next ancestor that can be a snap container.
// The origin may be either a snap area or a snap container.
LayoutBox* FindSnapContainer(const LayoutBox& origin_box) {
  // 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* document_element = origin_box.GetDocument().documentElement();
  LayoutBox* box = origin_box.ContainingBlock();
  while (box && !box->IsScrollContainer() && !IsA<LayoutView>(box) &&
         box->GetNode() != document_element) {
    box = box->ContainingBlock();
  }

  // If we reach to document element then we dispatch to layout view.
  if (box && box->GetNode() == document_element)
    return origin_box.GetDocument().GetLayoutView();

  return box;
}

// Snap types are categorized according to the spec
// https://drafts.csswg.org/css-scroll-snap-1/#snap-axis
cc::ScrollSnapType GetPhysicalSnapType(const LayoutBox& snap_container) {
  cc::ScrollSnapType scroll_snap_type =
      snap_container.Style()->GetScrollSnapType();
  if (scroll_snap_type.axis == cc::SnapAxis::kInline) {
    if (snap_container.Style()->IsHorizontalWritingMode())
      scroll_snap_type.axis = cc::SnapAxis::kX;
    else
      scroll_snap_type.axis = cc::SnapAxis::kY;
  }
  if (scroll_snap_type.axis == cc::SnapAxis::kBlock) {
    if (snap_container.Style()->IsHorizontalWritingMode())
      scroll_snap_type.axis = cc::SnapAxis::kY;
    else
      scroll_snap_type.axis = cc::SnapAxis::kX;
  }
  // Writing mode does not affect the cases where axis is kX, kY or kBoth.
  return scroll_snap_type;
}

// Adding a snap container means that the element's descendant snap areas need
// to be reassigned to it. First we find the snap container ancestor of the new
// snap container, then check its snap areas to see if their closest ancestor is
// changed to the new snap container.
// E.g., if A1 and A2's ancestor, C2, is C1's descendant and becomes scrollable
// then C2 becomes a snap container then the snap area assignments change:
//  Before            After adding C2
//       C1              C1
//       |               |
//   +-------+         +-----+
//   |  |    |         C2    |
//   A1 A2   A3      +---+   A3
//                   |   |
//                   A1  A2
void SnapCoordinator::AddSnapContainer(LayoutBox& snap_container) {
  snap_containers_.insert(&snap_container);

  LayoutBox* ancestor_snap_container = FindSnapContainer(snap_container);
  // If an ancestor doesn't exist then this means that the element is being
  // attached now; this means that it won't have any descendants that are
  // assigned to an ancestor snap container.
  if (!ancestor_snap_container) {
    DCHECK(!snap_container.Parent());
    return;
  }
  SnapAreaSet* snap_areas = ancestor_snap_container->SnapAreas();
  if (!snap_areas)
    return;
  Vector<LayoutBox*> snap_areas_to_reassign;
  for (auto* snap_area : *snap_areas) {
    if (FindSnapContainer(*snap_area) == &snap_container)
      snap_areas_to_reassign.push_back(snap_area);
  }
  for (auto* snap_area : snap_areas_to_reassign)
    snap_area->SetSnapContainer(&snap_container);

  // The new snap container will not have attached its ScrollableArea yet, so we
  // cannot invalidate the snap container data at this point. However, the snap
  // container data is set to needing an update by default, so we only need to
  // update the flag for the ancestor.
  if (snap_areas_to_reassign.size()) {
    ancestor_snap_container->GetScrollableArea()
        ->SetSnapContainerDataNeedsUpdate(true);
  }
}

void SnapCoordinator::RemoveSnapContainer(LayoutBox& snap_container) {
  LayoutBox* ancestor_snap_container = FindSnapContainer(snap_container);

  // We remove the snap container if it is no longer scrollable, or if the
  // element is detached.
  // - If it is no longer scrollable, then we reassign its snap areas to the
  // next ancestor snap container.
  // - If it is detached, then we simply clear its snap areas since they will be
  // detached as well.
  if (ancestor_snap_container) {
    ancestor_snap_container->GetScrollableArea()
        ->SetSnapContainerDataNeedsUpdate(true);
    snap_container.ReassignSnapAreas(*ancestor_snap_container);
  } else {
    DCHECK(!snap_container.Parent());
    snap_container.ClearSnapAreas();
  }
  // We don't need to update the old snap container's data since the
  // corresponding ScrollableArea is being removed, and thus the snap container
  // data is removed too.
  snap_container.SetNeedsPaintPropertyUpdate();
  snap_containers_.erase(&snap_container);
}

void SnapCoordinator::SnapContainerDidChange(LayoutBox& snap_container) {
  // Scroll snap properties have no effect on the document element instead they
  // are propagated to (See Document::PropagateStyleToViewport) and handled by
  // the LayoutView.
  if (snap_container.GetNode() ==
      snap_container.GetDocument().documentElement())
    return;

  PaintLayerScrollableArea* scrollable_area =
      snap_container.GetScrollableArea();
  // Per specification snap positions only affect *scroll containers* [1]. So if
  // the layout box is not a scroll container we ignore it here even if it has
  // non-none scroll-snap-type. Note that in blink, existence of scrollable area
  // directly maps to being a scroll container in the specification. [1]
  // https://drafts.csswg.org/css-scroll-snap/#overview
  if (!scrollable_area) {
    DCHECK(!snap_containers_.Contains(&snap_container));
    return;
  }

  // Note that even if scroll snap type is 'none' we continue to maintain its
  // snap container entry as long as the element is a scroller. This is because
  // while the scroller does not snap, it still captures the snap areas in its
  // subtree for whom it is the nearest ancestor scroll container per spec [1].
  //
  // [1] https://drafts.csswg.org/css-scroll-snap/#overview
  scrollable_area->SetSnapContainerDataNeedsUpdate(true);
}

void SnapCoordinator::SnapAreaDidChange(LayoutBox& snap_area,
                                        cc::ScrollSnapAlign scroll_snap_align) {
  LayoutBox* old_container = snap_area.SnapContainer();
  if (scroll_snap_align.alignment_inline == cc::SnapAlignment::kNone &&
      scroll_snap_align.alignment_block == cc::SnapAlignment::kNone) {
    snap_area.SetSnapContainer(nullptr);
    if (old_container)
      old_container->GetScrollableArea()->SetSnapContainerDataNeedsUpdate(true);
    return;
  }

  // If there is no ancestor snap container then this means that this snap
  // area is being detached. In the worst case, the layout view is the
  // ancestor snap container, which should exist as long as the document is
  // not destroyed.
  if (LayoutBox* new_container = FindSnapContainer(snap_area)) {
    snap_area.SetSnapContainer(new_container);
    // TODO(sunyunjia): consider keep the SnapAreas in a map so it is
    // easier to update.
    new_container->GetScrollableArea()->SetSnapContainerDataNeedsUpdate(true);
    if (old_container && old_container != new_container)
      old_container->GetScrollableArea()->SetSnapContainerDataNeedsUpdate(true);
  }
}

void SnapCoordinator::ResnapAllContainersIfNeeded() {
  for (const auto* container : snap_containers_) {
    if (!container->GetScrollableArea()->NeedsResnap())
      continue;

    auto* scrollable_area = ScrollableArea::GetForScrolling(container);
    ScrollOffset initial_offset = scrollable_area->GetScrollOffset();
    scrollable_area->SnapAfterLayout();
    container->GetScrollableArea()->SetNeedsResnap(false);

    // If this is the first time resnapping all containers then this means this
    // is the initial layout. We record whenever the initial scroll offset
    // changes as a result of snapping.
    // TODO(majidvp): This is here to measure potential web-compat impact of
    // launching this feature. We should remove it once it is launched.
    // crbug.com/866127
    if (!did_first_resnap_all_containers_ &&
        scrollable_area->GetScrollOffset() != initial_offset) {
      UseCounter::Count(container->GetDocument(),
                        WebFeature::kScrollSnapCausesScrollOnInitialLayout);
    }
  }
  did_first_resnap_all_containers_ = true;
}

void SnapCoordinator::UpdateAllSnapContainerDataIfNeeded() {
  for (auto* container : snap_containers_) {
    if (container->GetScrollableArea()->SnapContainerDataNeedsUpdate())
      UpdateSnapContainerData(*container);
  }
  SetAnySnapContainerDataNeedsUpdate(false);
}

void SnapCoordinator::UpdateSnapContainerData(LayoutBox& snap_container) {
  ScrollableArea* scrollable_area =
      ScrollableArea::GetForScrolling(&snap_container);
  const auto* old_snap_container_data = scrollable_area->GetSnapContainerData();
  auto snap_type = GetPhysicalSnapType(snap_container);
  scrollable_area->SetSnapContainerDataNeedsUpdate(false);

  // Scrollers that don't have any snap areas assigned to them and don't snap
  // require no further processing. These are the most common types and thus
  // returning as early as possible ensures efficiency.
  if (!old_snap_container_data && snap_type.is_none)
    return;

  cc::SnapContainerData snap_container_data(snap_type);

  DCHECK(snap_containers_.Contains(&snap_container));

  // When snap type is 'none' we don't perform any snapping so there is no need
  // to keep the area data up to date. So just update the type and skip updating
  // areas as an optimization.
  if (!snap_container_data.scroll_snap_type().is_none) {
    FloatPoint max_position = scrollable_area->ScrollOffsetToPosition(
        scrollable_area->MaximumScrollOffset());
    snap_container_data.set_max_position(
        gfx::ScrollOffset(max_position.X(), max_position.Y()));

    // Scroll-padding represents inward offsets from the corresponding edge of
    // the scrollport.
    // https://drafts.csswg.org/css-scroll-snap-1/#scroll-padding Scrollport is
    // the visual viewport of the scroll container (through which the scrollable
    // overflow region can be viewed) coincides with its padding box.
    // https://drafts.csswg.org/css-overflow-3/#scrollport. So we use the
    // LayoutRect of the padding box here. The coordinate is relative to the
    // container's border box.
    PhysicalRect container_rect(snap_container.PhysicalPaddingBoxRect());

    const ComputedStyle* container_style = snap_container.Style();
    LayoutRectOutsets container_padding(
        // The percentage of scroll-padding is different from that of normal
        // padding, as scroll-padding resolves the percentage against
        // corresponding dimension of the scrollport[1], while the normal
        // padding resolves that against "width".[2,3] We use
        // MinimumValueForLength here to ensure kAuto is resolved to
        // LayoutUnit() which is the correct behavior for padding.
        // [1] https://drafts.csswg.org/css-scroll-snap-1/#scroll-padding
        //     "relative to the corresponding dimension of the scroll
        //     container’s
        //      scrollport"
        // [2] https://drafts.csswg.org/css-box/#padding-props
        // [3] See for example LayoutBoxModelObject::ComputedCSSPadding where it
        //     uses |MinimumValueForLength| but against the "width".
        MinimumValueForLength(container_style->ScrollPaddingTop(),
                              container_rect.Height()),
        MinimumValueForLength(container_style->ScrollPaddingRight(),
                              container_rect.Width()),
        MinimumValueForLength(container_style->ScrollPaddingBottom(),
                              container_rect.Height()),
        MinimumValueForLength(container_style->ScrollPaddingLeft(),
                              container_rect.Width()));
    container_rect.Contract(container_padding);
    snap_container_data.set_rect(FloatRect(container_rect));

    if (snap_container_data.scroll_snap_type().strictness ==
        cc::SnapStrictness::kProximity) {
      PhysicalSize size = container_rect.size;
      size.Scale(kProximityRatio);
      gfx::ScrollOffset range(size.width.ToFloat(), size.height.ToFloat());
      snap_container_data.set_proximity_range(range);
    }

    cc::TargetSnapAreaElementIds new_target_ids;
    const cc::TargetSnapAreaElementIds old_target_ids =
        old_snap_container_data
            ? old_snap_container_data->GetTargetSnapAreaElementIds()
            : cc::TargetSnapAreaElementIds();

    if (SnapAreaSet* snap_areas = snap_container.SnapAreas()) {
      for (const LayoutBox* snap_area : *snap_areas) {
        cc::SnapAreaData snap_area_data =
            CalculateSnapAreaData(*snap_area, snap_container);
        // The target snap elements should be preserved in the new container
        // only if the respective snap areas are still present.
        if (old_target_ids.x == snap_area_data.element_id)
          new_target_ids.x = old_target_ids.x;
        if (old_target_ids.y == snap_area_data.element_id)
          new_target_ids.y = old_target_ids.y;

        snap_container_data.AddSnapAreaData(snap_area_data);
      }
    }
    snap_container_data.SetTargetSnapAreaElementIds(new_target_ids);
  }
  if (!old_snap_container_data ||
      *old_snap_container_data != snap_container_data) {
    snap_container.SetNeedsPaintPropertyUpdate();
    scrollable_area->SetSnapContainerData(snap_container_data);
    // If the snap container data changed then we need to resnap.
    scrollable_area->SetNeedsResnap(true);
  }
}

static cc::ScrollSnapAlign GetPhysicalAlignment(
    const ComputedStyle& area_style,
    const ComputedStyle& container_style) {
  cc::ScrollSnapAlign align = area_style.GetScrollSnapAlign();
  if (container_style.IsHorizontalWritingMode())
    return align;

  cc::SnapAlignment tmp = align.alignment_inline;
  align.alignment_inline = align.alignment_block;
  align.alignment_block = tmp;

  if (container_style.IsFlippedBlocksWritingMode()) {
    if (align.alignment_inline == cc::SnapAlignment::kStart) {
      align.alignment_inline = cc::SnapAlignment::kEnd;
    } else if (align.alignment_inline == cc::SnapAlignment::kEnd) {
      align.alignment_inline = cc::SnapAlignment::kStart;
    }
  }
  return align;
}

cc::SnapAreaData SnapCoordinator::CalculateSnapAreaData(
    const LayoutBox& snap_area,
    const LayoutBox& snap_container) {
  const ComputedStyle* container_style = snap_container.Style();
  const ComputedStyle* area_style = snap_area.Style();
  cc::SnapAreaData snap_area_data;

  // We assume that the snap_container is the snap_area's ancestor in layout
  // tree, as the snap_container is found by walking up the layout tree in
  // FindSnapContainer(). Under this assumption,
  // snap_area.LocalToAncestorRect() returns the snap_area's position relative
  // to the snap container's border box, while ignoring scroll offset.
  PhysicalRect area_rect = snap_area.PhysicalBorderBoxRect();
  area_rect = snap_area.LocalToAncestorRect(
      area_rect, &snap_container,
      kTraverseDocumentBoundaries | kIgnoreScrollOffset);

  LayoutRectOutsets area_margin(
      area_style->ScrollMarginTop(), area_style->ScrollMarginRight(),
      area_style->ScrollMarginBottom(), area_style->ScrollMarginLeft());
  area_rect.Expand(area_margin);
  snap_area_data.rect = FloatRect(area_rect);

  cc::ScrollSnapAlign align =
      GetPhysicalAlignment(*area_style, *container_style);
  snap_area_data.scroll_snap_align = align;

  snap_area_data.must_snap =
      (area_style->ScrollSnapStop() == EScrollSnapStop::kAlways);

  snap_area_data.element_id = CompositorElementIdFromDOMNodeId(
      DOMNodeIds::IdForNode(snap_area.GetNode()));

  return snap_area_data;
}

#ifndef NDEBUG

void SnapCoordinator::ShowSnapAreaMap() {
  for (auto* const container : snap_containers_)
    ShowSnapAreasFor(container);
}

void SnapCoordinator::ShowSnapAreasFor(const LayoutBox* container) {
  LOG(INFO) << *container->GetNode();
  if (SnapAreaSet* snap_areas = container->SnapAreas()) {
    for (auto* const snap_area : *snap_areas) {
      LOG(INFO) << "    " << *snap_area->GetNode();
    }
  }
}

void SnapCoordinator::ShowSnapDataFor(const LayoutBox* snap_container) {
  if (!snap_container)
    return;
  ScrollableArea* scrollable_area =
      ScrollableArea::GetForScrolling(snap_container);
  const auto* optional_data =
      scrollable_area ? scrollable_area->GetSnapContainerData() : nullptr;
  if (optional_data)
    LOG(INFO) << *optional_data;
}

#endif

}  // namespace blink