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// Copyright 2014 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 "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() : clobber_active_value_(false) {}
// Returns the canonical value for the specified tree, including the sum of
// all deltas. The pending tree should use this for activation purposes and
// the active tree should use this for drawing.
typename T::ValueType Current(bool is_active_tree) const {
if (is_active_tree)
return active_base_.Combine(active_delta_).get();
return pending_base_.Combine(PendingDelta()).get();
// Sets the value on the impl thread, due to an impl-thread-originating
// action. Returns true if this had any effect. This will remain
// impl-thread-only information at first, and will get pulled back to the main
// thread on the next call of PullDeltaToMainThread (which happens right
// before the commit).
bool SetCurrent(typename T::ValueType current) {
T delta = T(current).InverseCombine(active_base_);
if (active_delta_.get() == delta.get())
return false;
active_delta_ = delta;
return true;
// Returns the difference between the last value that was committed and
// activated from the main thread, and the current total value.
typename T::ValueType Delta() const { return active_delta_.get(); }
// Returns the latest active tree delta and also makes a note that this value
// was sent to the main thread.
typename T::ValueType PullDeltaForMainThread() {
reflected_delta_in_main_tree_ = PendingDelta();
return reflected_delta_in_main_tree_.get();
// Push the latest value from the main thread onto pending tree-associated
// state. Returns true if pushing the value results in different values
// between the main layer tree and the pending tree.
bool PushMainToPending(typename T::ValueType main_thread_value) {
reflected_delta_in_pending_tree_ = reflected_delta_in_main_tree_;
reflected_delta_in_main_tree_ = T::Identity();
pending_base_ = T(main_thread_value);
return Current(false) != main_thread_value;
// Push the value associated with the pending tree to be the active base
// value. As part of this, subtract the delta reflected in the pending tree
// from the active tree delta (which will make the delta zero at steady state,
// or make it contain only the difference since the last send).
// Returns true if pushing the update results in:
// 1) Different values on the pending tree and the active tree.
// 2) An update to the current value on the active tree.
// The reason for considering the second case only when pushing to the active
// tree, as opposed to when pushing to the pending tree, is that only the
// active tree computes state using this value which is not computed on the
// pending tree and not pushed during activation (aka scrollbar geometries).
bool PushPendingToActive() {
typename T::ValueType pending_value_before_push = Current(false);
typename T::ValueType active_value_before_push = Current(true);
active_base_ = pending_base_;
active_delta_ = PendingDelta();
reflected_delta_in_pending_tree_ = T::Identity();
clobber_active_value_ = false;
typename T::ValueType current_active_value = Current(true);
return pending_value_before_push != current_active_value ||
active_value_before_push != current_active_value;
// This simulates the consequences of the sent value getting committed and
// activated.
void AbortCommit() {
pending_base_ = pending_base_.Combine(reflected_delta_in_main_tree_);
active_base_ = active_base_.Combine(reflected_delta_in_main_tree_);
active_delta_ = active_delta_.InverseCombine(reflected_delta_in_main_tree_);
reflected_delta_in_main_tree_ = T::Identity();
// Values as last pushed to the pending or active tree respectively, with no
// impl-thread delta applied.
typename T::ValueType PendingBase() const { return pending_base_.get(); }
typename T::ValueType ActiveBase() const { return active_base_.get(); }
// The new delta we would use if we decide to activate now. This delta
// excludes the amount that we know is reflected in the pending tree.
T PendingDelta() const {
if (clobber_active_value_)
return T::Identity();
return active_delta_.InverseCombine(reflected_delta_in_pending_tree_);
void set_clobber_active_value() { clobber_active_value_ = true; }
// Value last committed to the pending tree.
T pending_base_;
// Value last committed to the active tree on the last activation.
T active_base_;
// The difference between |active_base_| and the user-perceived value.
T active_delta_;
// The value sent to the main thread on the last BeginMainFrame. This is
// always identity outside of the BeginMainFrame to (aborted)commit interval.
T reflected_delta_in_main_tree_;
// The value that was sent to the main thread for BeginMainFrame for the
// current pending tree. This is always identity outside of the
// BeginMainFrame to activation interval.
T reflected_delta_in_pending_tree_;
// When true the pending delta is always identity so that it does not change
// and will clobber the active value on push.
bool clobber_active_value_;
friend class base::RefCounted<SyncedProperty<T>>;
~SyncedProperty() {}
// SyncedProperty's delta-based conflict resolution logic makes sense for any
// mathematical group. In practice, there are two that are useful:
// 1. Numbers/vectors with addition and identity = 0 (like scroll offsets)
// 2. Real numbers with multiplication and identity = 1 (like page scale)
template <class V>
class AdditionGroup {
typedef V ValueType;
AdditionGroup() : value_(Identity().get()) {}
explicit AdditionGroup(V value) : value_(value) {}
V& get() { return value_; }
const V& get() const { return value_; }
static AdditionGroup<V> Identity() { return AdditionGroup(V()); } // zero
AdditionGroup<V> Combine(AdditionGroup<V> p) const {
return AdditionGroup<V>(value_ + p.value_);
AdditionGroup<V> InverseCombine(AdditionGroup<V> p) const {
return AdditionGroup<V>(value_ - p.value_);
V value_;
class ScaleGroup {
typedef float ValueType;
ScaleGroup() : value_(Identity().get()) {}
explicit ScaleGroup(float value) : value_(value) {}
float& get() { return value_; }
const float& get() const { return value_; }
static ScaleGroup Identity() { return ScaleGroup(1.f); }
ScaleGroup Combine(ScaleGroup p) const {
return ScaleGroup(value_ * p.value_);
ScaleGroup InverseCombine(ScaleGroup p) const {
return ScaleGroup(value_ / p.value_);
float value_;
} // namespace cc