[Sync] USS: Add EntityChange.

sync/api/entity_change.h

+// Copyright 2015 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 SYNC_API_ENTITY_CHANGE_H_
+#define SYNC_API_ENTITY_CHANGE_H_
+
+#include <string>
+
+#include "sync/api/entity_data.h"
+#include "sync/base/sync_export.h"
+
+namespace syncer_v2 {
+
+class SYNC_EXPORT EntityChange {
+ public:
+  enum ChangeType {
+    ADD,
+    UPDATE,
+    DELETE
+  };
+
+  static EntityChange CreateAdd(std::string client_tag, EntityDataPtr data);
+  static EntityChange CreateUpdate(std::string client_tag, EntityDataPtr data);
+  static EntityChange CreateDelete(std::string client_tag);
+
+  virtual ~EntityChange();
+
+  std::string client_tag() const { return client_tag_; }
+  ChangeType type() const { return type_; }
+  const EntityData& data() const { return data_.value(); }
+
+ private:
+  EntityChange(std::string client_tag, ChangeType type, EntityDataPtr data);
+
+  const std::string client_tag_;
+  const ChangeType type_;
+  const EntityDataPtr data_;
+};
+
+typedef std::vector<EntityChange> EntityChangeList;
+
+}  // namespace syncer_v2
+
+#endif  // SYNC_API_ENTITY_CHANGE_H_