[Sync] Improve MetadataChangeList usage for types using ModelTypeStore.

components/sync/model/simple_metadata_change_list.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 COMPONENTS_SYNC_MODEL_SIMPLE_METADATA_CHANGE_LIST_H_
-#define COMPONENTS_SYNC_MODEL_SIMPLE_METADATA_CHANGE_LIST_H_
-
-#include <map>
-#include <memory>
-#include <string>
-
-#include "components/sync/engine/non_blocking_sync_common.h"
-#include "components/sync/model/metadata_change_list.h"
-#include "components/sync/model/model_type_store.h"
-#include "components/sync/protocol/entity_metadata.pb.h"
-#include "components/sync/protocol/model_type_state.pb.h"
-
-namespace syncer {
-
-// A MetadataChangeList implementation that is meant to be used in combination
-// with a ModelTypeStore. It accumulates changes in member fields, and then when
-// requested transfers them to the store/write batch.
-class SimpleMetadataChangeList : public MetadataChangeList {
- public:
-  enum ChangeType { UPDATE, CLEAR };
-
-  struct MetadataChange {
-    ChangeType type;
-    sync_pb::EntityMetadata metadata;
-  };
-
-  struct ModelTypeStateChange {
-    ChangeType type;
-    sync_pb::ModelTypeState state;
-  };
-
-  typedef std::map<std::string, MetadataChange> MetadataChanges;
-
-  SimpleMetadataChangeList();
-  ~SimpleMetadataChangeList() override;
-
-  void UpdateModelTypeState(
-      const sync_pb::ModelTypeState& model_type_state) override;
-  void ClearModelTypeState() override;
-  void UpdateMetadata(const std::string& storage_key,
-                      const sync_pb::EntityMetadata& metadata) override;
-  void ClearMetadata(const std::string& storage_key) override;
-
-  const MetadataChanges& GetMetadataChanges() const;
-  bool HasModelTypeStateChange() const;
-  const ModelTypeStateChange& GetModelTypeStateChange() const;
-
-  // Moves all currently accumulated changes into the write batch, clear out
-  // local copies. Calling this multiple times will work, but should not be
-  // necessary.
-  void TransferChanges(ModelTypeStore* store,
-                       ModelTypeStore::WriteBatch* write_batch);
-
- private:
-  MetadataChanges metadata_changes_;
-  std::unique_ptr<ModelTypeStateChange> state_change_;
-};
-
-}  // namespace syncer
-
-#endif  // COMPONENTS_SYNC_MODEL_SIMPLE_METADATA_CHANGE_LIST_H_