Implement an observer interface for the site engagement service.

chrome/browser/engagement/site_engagement_observer.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_OBSERVER_H_
+#define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_
+
+#include "base/gtest_prod_util.h"
+#include "base/macros.h"
+
+namespace content {
+class WebContents;
+}
+
+class GURL;
+class SiteEngagementService;
+
+class SiteEngagementObserver {
+ public:
+  // Called when the engagement for |url| loaded in |web_contents| increases to
+  // |score|. |is_hidden| is true if the engagement occurred when |web_contents|
+  // was hidden (e.g. in the background).

[benwells, 2016/06/11 09:31:23]

Why is the hidden field necessary?

[dominickn, 2016/06/11 15:05:50]

Passing it through means that clients don't need to keep their own hidden state
if they care - they can just use the one already computed by the site engagement
helper.

[benwells, 2016/06/14 07:25:34]

Let's chat about this. I think it would be better for clients to keep track of
this but don't feel super strongly. It will be easier to explain face to face.

+  // This method may be run on user input, so observers *must not* perform any
+  // expensive tasks here.
+  virtual void OnEngagementIncreased(content::WebContents* web_contents,
+                                     const GURL& url,
+                                     double score,
+                                     bool is_hidden) {}
+
+ protected:
+  explicit SiteEngagementObserver(SiteEngagementService* service);
+
+  SiteEngagementObserver();
+
+  ~SiteEngagementObserver();
+
+  // Returns the site engagement service which this object is observing.
+  SiteEngagementService* GetSiteEngagementService() const;
+
+  // Begin observing |service| for engagement increases.
+  // To stop observing, call Observe(nullptr).
+  void Observe(SiteEngagementService* service);
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(SiteEngagementServiceTest, Observers);
+  SiteEngagementService* service_;
+
+  DISALLOW_COPY_AND_ASSIGN(SiteEngagementObserver);
+};
+
+#endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_OBSERVER_H_