[NTP Snippets] Re-fetch snippets on ML changes only when the hosts actually change

components/ntp_snippets/ntp_snippets_service.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_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 #define COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 
 #include <stddef.h>
 
+#include <set>
 #include <string>
 #include <vector>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/sequenced_task_runner.h"
 #include "base/timer/timer.h"
 #include "components/keyed_service/core/keyed_service.h"
@@ skipping 91 matching lines @@
  private:
   friend class NTPSnippetsServiceTest;
 
   void OnSuggestionsChanged(const suggestions::SuggestionsProfile& suggestions);
   void OnSnippetsDownloaded(const std::string& snippets_json);
 
   void OnJsonParsed(const std::string& snippets_json,
                     scoped_ptr<base::Value> parsed);
   void OnJsonError(const std::string& snippets_json, const std::string& error);
 
-  void FetchSnippetsImpl(const std::vector<std::string>& hosts);
+  void FetchSnippetsImpl(const std::set<std::string>& hosts);
 
   // Expects a top-level dictionary containing a "recos" list, which will be
   // passed to LoadFromListValue().
   bool LoadFromValue(const base::Value& value);
 
   // Expects a list of dictionaries each containing a "contentInfo" dictionary
   // with keys matching the properties of a snippet (url, title, site_title,
   // etc...). The URL is the only mandatory value.
   bool LoadFromListValue(const base::ListValue& list);
 
   // TODO(treib): Investigate a better storage, maybe LevelDB or SQLite?
   void LoadSnippetsFromPrefs();
   void StoreSnippetsToPrefs();
 
   void LoadDiscardedSnippetsFromPrefs();
   void StoreDiscardedSnippetsToPrefs();
 
+  std::set<std::string> GetSnippetHostsFromPrefs() const;
+  void StoreSnippetHostsToPrefs(const std::set<std::string>& hosts);
+
   bool HasDiscardedSnippet(const GURL& url) const;
 
   void RemoveExpiredSnippets();
 
   PrefService* pref_service_;
 
   suggestions::SuggestionsService* suggestions_service_;
 
   // True if the suggestions are loaded.
   bool loaded_;
@@ skipping 48 matching lines @@
   // Send when the service is shutting down.
   virtual void NTPSnippetsServiceShutdown(NTPSnippetsService* service) = 0;
 
  protected:
   virtual ~NTPSnippetsServiceObserver() {}
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_