[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);

[stanisc, 2015/11/20 19:01:49]

I am not sure we need these 3 functions as a part of the API. They would make
sense if EntityChange was used both ways but with the current design services
aren't going to ever construct EntityChange.

I am still open to changing the design. The advantage of using this type of
construction on the service side is that it would allow to pass the data without
copying it. 
Let's keep these functions here but file a tracking bug to finalize the API for
submitting local changes once we start making progress with DeviceInfo service
implementation.

[maxbogue, 2015/11/20 19:53:25]

Filed http://crbug.com/559276.

+  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_; }
+  EntityDataPtr data() const { return data_; }

[stanisc, 2015/11/20 19:01:49]

This should be const EntityData& 

[maxbogue, 2015/11/20 19:53:25]

Done.

+
+ 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_