chromium/cc/base/synced_property.h

// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef CC_BASE_SYNCED_PROPERTY_H_
#define CC_BASE_SYNCED_PROPERTY_H_

#include "base/memory/ref_counted.h"

namespace cc {

// This class is the basic primitive used for impl-thread scrolling.  Its job is
// to sanely resolve the case where both the main and impl thread are
// concurrently updating the same value (for example, when Javascript sets the
// scroll offset during an ongoing impl-side scroll).
//
// There are three trees (main, pending, and active) and therefore also three
// places with their own idea of the scroll offsets (and analogous properties
// like page scale).  Objects of this class are meant to be held on the Impl
// side, and contain the canonical reference for the pending and active trees,
// as well as keeping track of the latest delta sent to the main thread (which
// is necessary for conflict resolution).

template <typename T>
class SyncedProperty : public base::RefCounted<SyncedProperty<T>> { … };

// SyncedProperty's delta-based conflict resolution logic makes sense for any
// mathematical group.  In practice, there are two that are useful:
// 1. Numbers/classes with addition and subtraction operations, and
//    identity = constructor() (like gfx::Vector2dF for scroll offset and
//    scroll delta)
// 2. Real numbers with multiplication and division operations, and
//    identity = 1 (like page scale)

template <typename BaseT, typename DeltaT = BaseT>
class AdditionGroup { … };

class ScaleGroup { … };

}  // namespace cc

#endif  // CC_BASE_SYNCED_PROPERTY_H_