Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_score.h

 // Copyright 2016 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_SCORE_H_
 #define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SCORE_H_
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "base/values.h"
 
 namespace base {
 class Clock;
 }
 
 class SiteEngagementScore {
  public:
-  // The parameters which can be varied via field trial. All "points" values
-  // should be appended to the end of the enum prior to MAX_VARIATION.
+  // The parameters which can be varied via field trial.
+  // Any new point value that increases engagement based on direct user activity
+  // should be added after POINTS_INCREMENT_FIRST and before
+  // POINTS_INCREMENT_LAST.
   enum Variation {
     // The maximum number of points that can be accrued in one day.
     MAX_POINTS_PER_DAY = 0,
 
     // The period over which site engagement decays.
     DECAY_PERIOD_IN_DAYS,
 
     // The number of points to decay per period.
     DECAY_POINTS,
 
     // The number of points given for navigations.
     NAVIGATION_POINTS,
+    POINTS_INCREMENT_FIRST = NAVIGATION_POINTS,

[benwells, 2016/06/01 01:44:45]

Is it just me, or will it be awkward now to add more variations which increment
points?

Update: I just saw calamity's comments and can see this has already been
addressed... but I'm not convinced that inserting values into this enum down the
track will be a good idea. Seems like there is a potential for error there.

You should at least mention in a comment that if anything is inserted field
trials should be updated to target specific versions, but even that feels
fraught to me.

[dominickn, 2016/06/01 02:54:13]

Acknowledged. I'll remove this.

 
     // The number of points given for user input.
     USER_INPUT_POINTS,
 
     // The number of points given for media playing. Initially calibrated such
     // that at least 30 minutes of foreground media would be required to allow a
     // site to reach the daily engagement maximum.
     VISIBLE_MEDIA_POINTS,
     HIDDEN_MEDIA_POINTS,
 
+    // Any new points value that increases engagement directly based on user
+    // input should be inserted above this, and update this enum value
+    // appropriately.
+    POINTS_INCREMENT_LAST = HIDDEN_MEDIA_POINTS,
+
     // The number of points added to engagement when a site is launched from
     // homescreen or added as a bookmark app. This bonus will apply for ten days
     // following a launch; each new launch resets the ten days.
     WEB_APP_INSTALLED_POINTS,
 
     // The number of points given for the first engagement event of the day for
     // each site.
     FIRST_DAILY_ENGAGEMENT,
 
     // The number of points that the engagement service must accumulate to be
@@ skipping 16 matching lines @@
   static double GetDecayPoints();
   static double GetNavigationPoints();
   static double GetUserInputPoints();
   static double GetVisibleMediaPoints();
   static double GetHiddenMediaPoints();
   static double GetWebAppInstalledPoints();
   static double GetFirstDailyEngagementPoints();
   static double GetBootstrapPoints();
   static double GetMediumEngagementBoundary();
   static double GetHighEngagementBoundary();
+  static double GetMinimumEngagementIncrement();
 
   // Update the default engagement settings via variations.
   static void UpdateFromVariations(const char* param_name);
 
   // The SiteEngagementScore does not take ownership of |clock|. It is the
   // responsibility of the caller to make sure |clock| outlives this
   // SiteEngagementScore.
   SiteEngagementScore(base::Clock* clock,
                       const base::DictionaryValue& score_dict);
   ~SiteEngagementScore();
@@ skipping 79 matching lines @@
   base::Time last_engagement_time_;
 
   // The last time the site with this score was launched from an installed
   // shortcut.
   base::Time last_shortcut_launch_time_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementScore);
 };
 
 #endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SCORE_H_