Add desktop engagement metrics.

chrome/browser/metrics/desktop_engagement/audible_contents_tracker.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_METRICS_DESKTOP_ENGAGEMENT_AUDIBLE_CONTENTS_TRACKER_H_
+#define CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_AUDIBLE_CONTENTS_TRACKER_H_
+
+#include <set>
+
+#include "base/callback.h"
+#include "chrome/browser/ui/browser_list_observer.h"
+#include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
+
+namespace metrics {
+
+// BrowserList / TabStripModelObserver used for tracking audio status.
+class AudibleContentsTracker : public chrome::BrowserListObserver,
+                               public TabStripModelObserver {
+ public:
+  // Interface for an observer of the AudibleContentsTracker. The only client
+  // of this class is the DesktopEngagementService, but an observer interface
+  // has been created for ease of testing.
+  class Observer {
+   public:
+    Observer() {}
+    ~Observer() {}

[chrisha, 2016/07/29 15:21:26]

virtual ~Observer() as this is an abstract base class.

[gayane -on leave until 09-2017, 2016/07/29 19:37:39]

Done.

+
+    // Invoked when a first audio source starts playing after a period of no
+    // audio sources. Overridden by tests.
+    virtual void OnAudioStart();
+
+    // Invoked when all audio sources stop playing. Overridden by tests.
+    virtual void OnAudioEnd();

[Alexei Svitkine (slow), 2016/07/28 21:31:45]

Make this a pure virtual class and just have DesktopEngagementService implement
this observer class.

[chrisha, 2016/07/29 15:21:25]

+1

[gayane -on leave until 09-2017, 2016/07/29 19:37:38]

Acknowledged.

[gayane -on leave until 09-2017, 2016/07/29 19:37:39]

Done.

+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(Observer);
+  };
+
+  // Creates an audible contents tracker that dispatches its messages to the
+  // provided |observer|.
+  explicit AudibleContentsTracker(Observer* observer);
+  ~AudibleContentsTracker() override;
+
+  // Creates the |AudibleContentsTracker| instance and its observer which
+  // observe and notify audio status changes.
+  static void Initialize();
+
+ private:
+  // chrome::BrowserListObserver:
+  void OnBrowserAdded(Browser* browser) override;
+  void OnBrowserRemoved(Browser* browser) override;
+
+  // TabStripModelObserver:
+  void TabClosingAt(TabStripModel* model,
+                    content::WebContents* web_contents,
+                    int index) override;
+  void TabChangedAt(content::WebContents* web_contents,
+                    int index,
+                    TabChangeType change_type) override;
+  void TabReplacedAt(TabStripModel* model,
+                     content::WebContents* old_web_contents,
+                     content::WebContents* new_web_contents,
+                     int index) override;
+
+  // Used for managing audible_contents_, and invoking OnAudioStart and
+  // OnAudioEnd callbacks.
+  void AddAudibleWebContents(content::WebContents* web_contents);
+  void RemoveAudibleWebContents(content::WebContents* web_contents);
+
+  Observer* observer_;
+
+  // The set of WebContents that are currently playing audio.
+  std::set<content::WebContents*> audible_contents_;
+
+  DISALLOW_COPY_AND_ASSIGN(AudibleContentsTracker);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_AUDIBLE_CONTENTS_TRACKER_H_