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// Copyright 2015 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.
#ifndef ScrollAnchor_h
#define ScrollAnchor_h
#include "core/CoreExport.h"
#include "platform/geometry/LayoutPoint.h"
#include "platform/heap/Handle.h"
#include "platform/wtf/text/WTFString.h"
namespace blink {
class LayoutObject;
class ScrollableArea;
static const int kMaxSerializedSelectorLength = 500;
// Scrolls to compensate for layout movements (
class CORE_EXPORT ScrollAnchor final {
explicit ScrollAnchor(ScrollableArea*);
// The scroller that is scrolled to componsate for layout movements. Note
// that the scroller can only be initialized once.
void SetScroller(ScrollableArea*);
// Returns true if the underlying scroller is set.
bool HasScroller() const { return scroller_; }
// The LayoutObject we are currently anchored to. Lazily computed during
// notifyBeforeLayout() and cached until the next call to clear().
LayoutObject* AnchorObject() const { return anchor_object_; }
// Indicates that this ScrollAnchor, and all ancestor ScrollAnchors, should
// compute new anchor nodes on their next notifyBeforeLayout().
void Clear();
// Indicates that this ScrollAnchor should compute a new anchor node on the
// next call to notifyBeforeLayout().
void ClearSelf();
// Records the anchor's location in relation to the scroller. Should be
// called when the scroller is about to be laid out.
void NotifyBeforeLayout();
// Scrolls to compensate for any change in the anchor's relative location.
// Should be called at the end of the animation frame.
void Adjust();
enum class Corner {
kTopLeft = 0,
// Which corner of the anchor object we are currently anchored to.
// Only meaningful if anchorObject() is non-null.
Corner GetCorner() const { return corner_; }
// A set of properties used to save/restore previously used scroll anchors.
struct SerializedAnchor {
SerializedAnchor() : simhash(0) {}
SerializedAnchor(const String& s, const LayoutPoint& p)
: selector(s), relative_offset(p), simhash(0) {}
SerializedAnchor(const String& s, const LayoutPoint& p, uint64_t hash)
: selector(s), relative_offset(p), simhash(hash) {}
bool IsValid() const { return !selector.IsEmpty(); }
// Used to locate an element previously used as a scroll anchor.
const String selector;
// Used to restore the previous offset of the element within its scroller.
const LayoutPoint relative_offset;
// Used to compare the similarity of a prospective anchor's contents to the
// contents at the time the previous anchor was saved.
const uint64_t simhash;
// This enum must remain in sync with the corresponding enum in enums.xml.
enum RestorationStatus {
kSuccess = 0,
kStatusCount // Special value used to count the number of enum values; not
// to be used as an actual status. Add new values above.
// Attempt to restore |serialized_anchor| by scrolling to the element
// identified by its selector, adjusting by its relative_offset.
bool RestoreAnchor(const SerializedAnchor&);
// Get the serialized representation of the current anchor_object_.
// If there is not currently an anchor_object_, this will attempt to find one.
// Repeated calls will re-use the previously calculated selector until the
// anchor_object it corresponds to is cleared.
const SerializedAnchor GetSerializedAnchor();
// Checks if we hold any references to the specified object.
bool RefersTo(const LayoutObject*) const;
// Notifies us that an object will be removed from the layout tree.
void NotifyRemoved(LayoutObject*);
void Trace(blink::Visitor* visitor) { visitor->Trace(scroller_); }
void FindAnchor();
// Returns true if searching should stop. Stores result in m_anchorObject.
bool FindAnchorRecursive(LayoutObject*);
bool ComputeScrollAnchorDisablingStyleChanged();
enum WalkStatus { kSkip = 0, kConstrain, kContinue, kReturn };
struct ExamineResult {
ExamineResult(WalkStatus s)
: status(s), viable(false), corner(Corner::kTopLeft) {}
ExamineResult(WalkStatus s, Corner c)
: status(s), viable(true), corner(c) {}
WalkStatus status;
bool viable;
Corner corner;
ExamineResult Examine(const LayoutObject*) const;
IntSize ComputeAdjustment() const;
// The scroller to be adjusted by this ScrollAnchor. This is also the scroller
// that owns us, unless it is the RootFrameViewport in which case we are owned
// by the layout viewport.
Member<ScrollableArea> scroller_;
// The LayoutObject we should anchor to.
LayoutObject* anchor_object_;
// Which corner of m_anchorObject's bounding box to anchor to.
Corner corner_;
// Location of m_layoutObject relative to scroller at time of
// notifyBeforeLayout().
LayoutPoint saved_relative_offset_;
// Previously calculated css selector that uniquely locates the current
// anchor_object_. Cleared when the anchor_object_ is cleared.
String saved_selector_;
// We suppress scroll anchoring after a style change on the anchor node or
// one of its ancestors, if that change might have caused the node to move.
// This bit tracks whether we have had a scroll-anchor-disabling style
// change since the last layout. It is recomputed in notifyBeforeLayout(),
// and used to suppress adjustment in adjust(). See
bool scroll_anchor_disabling_style_changed_;
// True iff an adjustment check has been queued with the FrameView but not yet
// performed.
bool queued_;
} // namespace blink
#endif // ScrollAnchor_h