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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 <stddef.h>
#include <cstdint>
#include <functional>
#include <iosfwd>
#include <memory>
#include "base/hash.h"
#include "cc/cc_export.h"
namespace base {
class Value;
namespace trace_event {
class TracedValue;
} // namespace trace_event
} // namespace base
namespace cc {
using ElementIdType = uint64_t;
static const ElementIdType kInvalidElementId = 0;
// Element ids are chosen by cc's clients and can be used as a stable identifier
// across updates.
// Historically, the layer tree stored all compositing data but this has been
// refactored over time into auxilliary structures such as property trees.
// In composited scrolling, Layers directly reference scroll tree nodes
// (Layer::scroll_tree_index) but scroll tree nodes are being refactored to
// reference stable element ids instead of layers. Scroll property nodes have
// unique element ids that blink creates from scrollable areas (though this is
// opaque to the compositor). This refactoring of scroll nodes keeping a
// scrolling element id instead of a scrolling layer id allows for more general
// compositing where, for example, multiple layers scroll with one scroll node.
// The animation system (see: ElementAnimations and blink::ElementAnimations) is
// another auxilliary structure to the layer tree and uses element ids as a
// stable identifier for animation targets. A Layer's element id can change over
// the Layer's lifetime because non-default ElementIds are only set during an
// animation's lifetime.
struct CC_EXPORT ElementId {
explicit ElementId(int id) : id_(id) {}
ElementId() : ElementId(kInvalidElementId) {}
bool operator==(const ElementId& o) const;
bool operator!=(const ElementId& o) const;
bool operator<(const ElementId& o) const;
// An ElementId's conversion to a boolean value depends only on its primaryId.
explicit operator bool() const;
void AddToTracedValue(base::trace_event::TracedValue* res) const;
std::unique_ptr<base::Value> AsValue() const;
// Returns the element id as its underlying representation.
// TODO(wkorman): Remove or rename this method.
ElementIdType ToInternalValue() const;
std::string ToString() const;
friend struct ElementIdHash;
// The compositor treats this as an opaque handle and should not know how to
// interpret these bits. Non-blink cc clients typically operate in terms of
// layers and may set this value to match the client's layer id.
ElementIdType id_;
ElementId CC_EXPORT LayerIdToElementIdForTesting(int layer_id);
struct CC_EXPORT ElementIdHash {
size_t operator()(ElementId key) const;
// Stream operator so ElementId can be used in assertion statements.
CC_EXPORT std::ostream& operator<<(std::ostream& out, const ElementId& id);
} // namespace cc