Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_service.h

Index: chrome/browser/engagement/site_engagement_service.h
diff --git a/chrome/browser/engagement/site_engagement_service.h b/chrome/browser/engagement/site_engagement_service.h
index 1d733322e887cced1264f0a6ac062984c06b8a99..66bd8e5da86aa81f1ba387d7d36f7f2a9a8d6829 100644
--- a/chrome/browser/engagement/site_engagement_service.h
+++ b/chrome/browser/engagement/site_engagement_service.h
@@ -9,6 +9,7 @@
 #include <memory>
 #include <set>
 
+#include "base/callback_forward.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
@@ -58,6 +59,14 @@ class SiteEngagementService : public KeyedService,
   // the service of them.
   class Helper;
 
+  // Callback used by the site engagement service to signal that a given origin
+  // has reached a given engagement score. When invoked, the callback will be
+  // passed the score for |url|'s origin at the time of invocation, and the URL
+  // which led to the engagement reaching that score (or the origin if the score
+  // was already at the requested level).
+  using EngagementCallback = base::Callback<void(const GURL&, double)>;
+  using EngagementCallbacks = std::vector<EngagementCallback>;
+
   enum EngagementLevel {
     ENGAGEMENT_LEVEL_NONE,
     ENGAGEMENT_LEVEL_LOW,
@@ -97,6 +106,18 @@ class SiteEngagementService : public KeyedService,
   // Returns whether |url| has at least the given |level| of engagement.
   bool IsEngagementAtLeast(const GURL& url, EngagementLevel level) const;
 
+  // Register |callback| to be run on the UI thread when the engagement for
+  // |url|'s origin reaches |level| or |score|. Callbacks for origins which have

[calamity, 2016/05/19 03:28:22]

It's not obvious that this only happens when engagement goes above the specified
score/level.

[dominickn, 2016/05/19 04:36:15]

I've made this comment more explicit - it's equal to or greater than.

+  // already reached the requested score will be dispatched as soon as they are
+  // registered. Callers are responsible for ensuring that |callback| is safe to
+  // run at any time.
+  void RegisterCallback(const EngagementCallback& callback,
+                        const GURL& url,
+                        EngagementLevel level);
+  void RegisterCallback(const EngagementCallback& callback,
+                        const GURL& url,
+                        double score);
+
   // Resets the engagement score |url| to |score|, clearing daily limits.
   void ResetScoreForURL(const GURL& url, double score);
 
@@ -109,6 +130,13 @@ class SiteEngagementService : public KeyedService,
   double GetTotalEngagementPoints() const override;
 
  private:
+  // Wrapping struct for callbacks.
+  struct EngagementCallbackWrapper {
+    EngagementCallback* callback;
+    GURL origin;
+    double score;
+  };
+
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, CheckHistograms);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, CleanupEngagementScores);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, ClearHistoryForURLs);
@@ -120,6 +148,7 @@ class SiteEngagementService : public KeyedService,
                            CleanupOriginsOnHistoryDeletion);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, IsBootstrapped);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, EngagementLevel);
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, RegisterCallbacks);
   FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, ScoreDecayHistograms);
   FRIEND_TEST_ALL_PREFIXES(AppBannerSettingsHelperTest, SiteEngagementTrigger);
 
@@ -139,6 +168,9 @@ class SiteEngagementService : public KeyedService,
   // Returns the median engagement score of all recorded origins.
   double GetMedianEngagement(const std::map<GURL, double>& score_map) const;
 
+  // Returns the minimum engagement score corresponding to |level|.
+  double GetScoreForEngagementLevel(EngagementLevel level) const;
+
   // Update the engagement score of the origin matching |url| for media playing.
   // The points awarded are discounted if the media is being played in a non-
   // visible tab.
@@ -172,6 +204,16 @@ class SiteEngagementService : public KeyedService,
     bool expired,
     const history::OriginCountAndLastVisitMap& remaining_origin_counts);
 
+  // Runs any callbacks in |engagement_callbacks_| which have been registered to
+  // listen to |url|'s origin reaching at least |score| engagement.
+  void ProcessCallbacks(const GURL& url, double score);
+
+  // Dispatches |callback| to be run on the UI thread, and deletes it once it
+  // has finished running.
+  void PostCallback(const EngagementCallback* callback,
+                    const GURL& url,
+                    double score);
+
   // Resets the engagement score for |url| to |score|, and sets the last
   // engagement time and last shortcut launch time (if it is non-null) to
   // |updated_time|. Clears daily limits.
@@ -179,6 +221,11 @@ class SiteEngagementService : public KeyedService,
                                       double score,
                                       const base::Time* updated_time);
 
+  // Runs |callback| and deletes it once it has finished execution.
+  void RunAndDeleteCallback(const EngagementCallback* callback,
+                            const GURL& origin,
+                            double score) const;
+
   Profile* profile_;
 
   // The clock used to vend times.
@@ -191,6 +238,10 @@ class SiteEngagementService : public KeyedService,
   // upload.
   base::Time last_metrics_time_;
 
+  // A list of callbacks, registered with an origin and an engagement score.
+  // Once a callback is dispatched, its wrapper will be removed from the list.
+  std::vector<EngagementCallbackWrapper> engagement_callbacks_;
+
   base::WeakPtrFactory<SiteEngagementService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementService);