Prevent site engagement scores from decaying when Chrome isn't in use.

chrome/browser/engagement/site_engagement_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 CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_H_
 #define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_H_
 
 #include <map>
 #include <memory>
 #include <set>
@@ skipping 108 matching lines @@
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, ClearHistoryForURLs);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, GetMedianEngagement);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, GetTotalNavigationPoints);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, GetTotalUserInputPoints);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, LastShortcutLaunch);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest,
                            CleanupOriginsOnHistoryDeletion);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, IsBootstrapped);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, EngagementLevel);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, ScoreDecayHistograms);
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest,
+                           PersistLastEngagementTime);
   FRIEND_TEST_ALL_PREFIXES(AppBannerSettingsHelperTest, SiteEngagementTrigger);
 
   // Only used in tests.
   SiteEngagementService(Profile* profile, std::unique_ptr<base::Clock> clock);
 
   // Adds the specified number of points to the given origin, respecting the
   // maximum limits for the day and overall.
   void AddPoints(const GURL& url, double points);
 
   // Retrieves the SiteEngagementScore object for |origin|.
   SiteEngagementScore CreateEngagementScore(const GURL& origin);
   const SiteEngagementScore CreateEngagementScore(const GURL& origin) const;
 
-  // Post startup tasks: cleaning up origins which have decayed to 0, and
-  // logging UMA statistics.
+  // Runs site engagement maintenance tasks.
   void AfterStartupTask();
-  void CleanupEngagementScores();
+
+  // Removes any origins which have decayed to 0 engagement. If
+  // |update_last_engagement_time| is true, the last engagement time of all
+  // origins is reset by calculating the delta between the last engagement event
+  // recorded by the site engagement service and the origin. The origin's last
+  // engagement time is then set to clock_->Now() - delta.
+  //
+  // If a user does not use the browser at all for some period of time,
+  // engagement is not decayed, and the state is restored equivalent to how they
+  // left it once they return.
+  void CleanupEngagementScores(bool update_last_engagement_time);
+
+  // Records UMA metrics.
   void RecordMetrics();
 
   // Returns the median engagement score of all recorded origins.
   double GetMedianEngagement(const std::map<GURL, double>& score_map) const;
 
   // Update the engagement score of the origin loaded in |web_contents| for
   // media playing. The points awarded are discounted if the media is being
   // played in a non-visible tab.
   void HandleMediaPlaying(content::WebContents* web_contents, bool is_hidden);
 
   // Update the engagement score of the origin loaded in |web_contents| for
   // navigation.
   void HandleNavigation(content::WebContents* web_contents,
                         ui::PageTransition transition);
 
   // Update the engagement score of the origin loaded in |web_contents| for
   // time-on-site, based on user input.
   void HandleUserInput(content::WebContents* web_contents,
                        SiteEngagementMetrics::EngagementType type);
 
+  // Returns true if the last engagement increasing event seen by the site
+  // engagement service was sufficiently long ago that we need to reset all
+  // scores to be relative to now. This ensures that users who do not use Chrome
+  // at all for an extended period of time do not have their engagement decay.
+  bool IsLastEngagementObsolete();
+
   // Overridden from history::HistoryServiceObserver:
   void OnURLsDeleted(history::HistoryService* history_service,
                      bool all_history,
                      bool expired,
                      const history::URLRows& deleted_rows,
                      const std::set<GURL>& favicon_urls) override;
 
   // Returns the number of origins with maximum daily and total engagement
   // respectively.
   int OriginsWithMaxDailyEngagement() const;
@@ skipping 12 matching lines @@
   // The clock used to vend times.
   std::unique_ptr<base::Clock> clock_;
 
   // Metrics are recorded at non-incognito browser startup, and then
   // approximately once per hour thereafter. Store the local time at which
   // metrics were previously uploaded: the first event which affects any
   // origin's engagement score after an hour has elapsed triggers the next
   // upload.
   base::Time last_metrics_time_;
 
+  // Keep track of the last time any engagement was recorded. If this date is
+  // too far in the past, assume that the user has not been using Chrome, and
+  // ensure that scores are not decayed as usual.
+  base::Time last_engagement_time_;
+
   base::WeakPtrFactory<SiteEngagementService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementService);
 };
 
 #endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_H_