cc/trees/element_id.h

// 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.

#ifndef CC_TREES_ELEMENT_ID_H_
#define CC_TREES_ELEMENT_ID_H_

#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 {

// 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 {
  ElementId(int primaryId, int secondaryId)
      : primaryId(primaryId), secondaryId(secondaryId) {}
  ElementId() : ElementId(0, 0) {}

  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;

  // 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.
  int primaryId;
  int secondaryId;
};

CC_EXPORT ElementId 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

#endif  // CC_TREES_ELEMENT_ID_H_