Add a PersistentStore to FeatureEngagementTracker

components/feature_engagement_tracker/internal/persistent_store.h

Index: components/feature_engagement_tracker/internal/persistent_store.h
diff --git a/components/feature_engagement_tracker/internal/persistent_store.h b/components/feature_engagement_tracker/internal/persistent_store.h
new file mode 100644
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+++ b/components/feature_engagement_tracker/internal/persistent_store.h
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+// Copyright 2017 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_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_PERSISTENT_STORE_H_
+#define COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_PERSISTENT_STORE_H_
+
+#include <memory>
+#include <vector>
+
+#include "base/files/file_path.h"
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "components/feature_engagement_tracker/internal/proto/event.pb.h"
+#include "components/feature_engagement_tracker/internal/store.h"
+#include "components/leveldb_proto/proto_database.h"
+
+namespace feature_engagement_tracker {
+
+// A PersistentStore provides a DB layer that persists the data to disk.  The
+// data is retrieved once during the load process and after that this store is
+// write only.  Data will be persisted asynchronously so it is not guaranteed to
+// always save every write during shutdown.
+class PersistentStore : public Store {
+ public:
+  // Builds a PersistentStore backed by the ProtoDatabase |db|.  The database
+  // will be loaded and/or created at |storage_dir|.
+  PersistentStore(const base::FilePath& storage_dir,
+                  std::unique_ptr<leveldb_proto::ProtoDatabase<Event>> db);
+  ~PersistentStore() override;
+
+  // Store implementation.
+  void Load(const OnLoadedCallback& callback) override;
+  bool IsReady() const override;
+  void WriteEvent(const Event& event) override;
+
+ private:
+  void OnInitComplete(const OnLoadedCallback& callback, bool success);
+  void OnLoadComplete(const OnLoadedCallback& callback,
+                      bool success,
+                      std::unique_ptr<std::vector<Event>> entries);
+
+  const base::FilePath storage_dir_;
+  std::unique_ptr<leveldb_proto::ProtoDatabase<Event>> db_;
+
+  // Whether or not the underlying ProtoDatabase is ready.  This will be false
+  // until the OnLoadedCallback is broadcast.  It will also be false if loading
+  // fails.
+  bool ready_;
+
+  base::WeakPtrFactory<PersistentStore> weak_ptr_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(PersistentStore);
+};
+
+}  // namespace feature_engagement_tracker
+
+#endif  // COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_PERSISTENT_STORE_H_