cc/blimp: Add synchronization for scroll/scale state.

cc/blimp/synced_property_remote.h

Index: cc/blimp/synced_property_remote.h
diff --git a/cc/blimp/synced_property_remote.h b/cc/blimp/synced_property_remote.h
new file mode 100644
index 0000000000000000000000000000000000000000..457d1578add0e65bf0973b5bf5d7c885f7c9c982
--- /dev/null
+++ b/cc/blimp/synced_property_remote.h
@@ -0,0 +1,122 @@
+// 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_BLIMP_SYNCED_PROPERTY_REMOTE_H_
+#define CC_BLIMP_SYNCED_PROPERTY_REMOTE_H_
+
+#include "cc/base/synced_property.h"
+
+namespace cc {
+
+// This class is heavily inspired from SyncedProperty and is the equivalent of
+// SyncedProperty for synchronizing state between the engine and client in
+// remote mode. There are effectively 2 trees on the main thread, the LayerTree
+// on the main thread on the engine, and the tree on the main thread on the
+// client.
+//
+// The client main thread tree receives deltas from the impl thread, but must
+// synchronize them with the engine main thread before applying them onto the
+// the main thread associated state. At the same time there are different sets
+// of deltas, the value that was received from the impl thread but has not yet
+// been reported to the engine main thread, and the value that has been sent
+// to the engine main thread, but a state update with the application of these
+// deltas has not been received from the engine. The purpose of this class is to
+// track these deltas for deciding what needs to be reported to the main thread
+// on the engine, and providing the impl thread with the delta that it should
+// apply to its associated state since it could not be reflected in the local
+// BeginMainFrame on the client main thread.
+//
+// Instances of this class are held on the client main thread and have a 1:1
+// mapping with their corresponding SyncedProperty instances.
+
+// Note: This class currently supports only one set of deltas to be in flight to
+// the engine.
+template <typename T>
+class SyncedPropertyRemote {
+ public:
+  SyncedPropertyRemote() = default;
+
+  SyncedPropertyRemote(SyncedPropertyRemote&& other) = default;
+
+  SyncedPropertyRemote& operator=(SyncedPropertyRemote&& other) = default;
+
+  // Push the main thread state from the engine onto the client main thread
+  // associated state.
+  void PushFromEngineMainThread(
+      typename T::ValueType engine_main_thread_value) {
+    engine_main_base_ = T(engine_main_thread_value);
+  }
+
+  // Called when an update for changes made on the impl thread was received on
+  // the client main thread.
+  // |main_thread_value| holds the updated value on the client main thread.
+  void UpdateDeltaFromImplThread(typename T::ValueType main_thread_value) {
+    T delta_from_impl_thread =
+        T(main_thread_value).InverseCombine(engine_main_base_);
+    unsent_delta_from_impl_thread_ =
+        unsent_delta_from_impl_thread_.Combine(delta_from_impl_thread);
+  }
+
+  // Pull deltas for changes tracked on the client main thread to be sent to the
+  // engine main thread.
+  // Each call to this must be followed with a call to DidApplySentDeltaOnEngine
+  // before another set of deltas can be pulled.
+  typename T::ValueType PullDeltaForEngineUpdate() {
+    DCHECK(sent_but_unapplied_delta_from_impl_thread_.get() ==
+           T::Identity().get());
+
+    T delta_to_send = unsent_delta_from_impl_thread_;
+    sent_but_unapplied_delta_from_impl_thread_ = delta_to_send;
+    unsent_delta_from_impl_thread_ = T::Identity();
+    return delta_to_send.get();
+  }
+
+  // Called when deltas sent to the engine were applied to the main thread state
+  // on the engine.
+  void DidApplySentDeltaOnEngine() {
+    // Pre-emptively apply the deltas that were sent. This is necessary since
+    // the engine may not send a frame update if the deltas were simply
+    // reflected back by the engine main thread (equivalent to
+    // BeginMainFrameAborted between the main and impl thread).
+    // If the engine did send a frame, we expect the frame to come with the ack
+    // for the deltas, so the value will be over-written with the engine update
+    // before being pushed to the impl thread.
+    engine_main_base_ =
+        engine_main_base_.Combine(sent_but_unapplied_delta_from_impl_thread_);
+    sent_but_unapplied_delta_from_impl_thread_ = T::Identity();
+  }
+
+  // Returns the delta that was reported to the main thread on the client but
+  // has not yet been applied to the main thread on the engine.
+  typename T::ValueType DeltaNotAppliedOnEngine() {
+    T total_delta_on_client = unsent_delta_from_impl_thread_.Combine(
+        sent_but_unapplied_delta_from_impl_thread_);
+    return total_delta_on_client.get();
+  }
+
+  typename T::ValueType EngineMain() { return engine_main_base_.get(); }
+
+ private:
+  // The delta that was applied to the state on the impl thread and received on
+  // the main thread on the client during BeginMainFrame, but has not yet been
+  // reported to the main thread on the engine.
+  T unsent_delta_from_impl_thread_;
+
+  // The delta that was applied to the state on the impl thread and has been
+  // sent to the main thread on the engine, but has not yet been applied to the
+  // main thread on the engine.
+  T sent_but_unapplied_delta_from_impl_thread_;
+
+  // The value as last received from the main thread on the engine. The value on
+  // the main thread state on the client should always be set to this value,
+  // outside of the interval when deltas from the impl thread are reported to
+  // the main thread on the client.
+  T engine_main_base_;
+
+  DISALLOW_COPY_AND_ASSIGN(SyncedPropertyRemote);
+};
+
+}  // namespace cc
+
+#endif  // CC_BLIMP_SYNCED_PROPERTY_REMOTE_H_