Update from https://crrev.com/306901

cc/base/synced_property.h

Index: cc/base/synced_property.h
diff --git a/cc/base/synced_property.h b/cc/base/synced_property.h
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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.
+
+#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>> {
+ public:
+  SyncedProperty() {}
+
+  // 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();
+    else
+      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() {
+    sent_delta_ = active_delta_;
+    return active_delta_.get();
+  }
+
+  // Push the latest value from the main thread onto pending tree-associated
+  // state.  Returns true if this had any effect.
+  bool PushFromMainThread(typename T::ValueType main_thread_value) {
+    if (pending_base_.get() == main_thread_value)
+      return false;
+
+    pending_base_ = T(main_thread_value);
+
+    return true;
+  }
+
+  // Push the value associated with the pending tree to be the active base
+  // value.  As part of this, subtract the last sent value 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).
+  bool PushPendingToActive() {
+    if (active_base_.get() == pending_base_.get() &&
+        sent_delta_.get() == T::Identity().get())
+      return false;
+
+    active_base_ = pending_base_;
+    active_delta_ = PendingDelta();
+    sent_delta_ = T::Identity();
+
+    return true;
+  }
+
+  // This simulates the consequences of the sent value getting committed and
+  // activated.  The value sent to the main thread ends up combined with the
+  // active value, and the sent_delta is subtracted from the delta.
+  void AbortCommit() {
+    active_base_ = active_base_.Combine(sent_delta_);
+    active_delta_ = PendingDelta();
+    sent_delta_ = T::Identity();
+  }
+
+ private:
+  // The new delta we would use if we decide to activate now.  This delta
+  // excludes the amount that we expect the main thread to reflect back at the
+  // impl thread during the commit.
+  T PendingDelta() const { return active_delta_.InverseCombine(sent_delta_); }
+
+  // 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 the active_base_ and the user-perceived value.
+  T active_delta_;
+  // The value sent to the main thread (on the last BeginFrame); this is always
+  // identity outside of the BeginFrame-to-activation interval.
+  T sent_delta_;
+
+  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 {
+ public:
+  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_);
+  }
+
+ private:
+  V value_;
+};
+
+class ScaleGroup {
+ public:
+  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_);
+  }
+
+ private:
+  float value_;
+};
+
+}  // namespace cc
+
+#endif  // CC_BASE_SYNCED_PROPERTY_H_