Add a mechanism for collecting Rappor samples on a daily interval.

components/metrics/daily_event.h

+// Copyright 2014 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 COMPONENTS_METRICS_DAILY_EVENT_H_
+#define COMPONENTS_METRICS_DAILY_EVENT_H_
+
+#include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
+#include "base/time/time.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace metrics {
+
+// DailyEvent is used for throttling an event to about once per day, even if
+// the program is restarted more frequently.  It is based on local machine
+// time, so it could be fired more often if the clock is changed.
+//
+// The service using the DailyEvent should first provide all of the Observers
+// for the interval, and then arrange for CheckInterval() to be called
+// periodically to test if the event should be fired.
+class DailyEvent {
+ public:
+  // Observer receives notifications from a DailyEvent.
+  // Observers must be added before the DailyEvent begins checking time,
+  // and will be owned by the DailyEvent.
+  class Observer {
+   public:
+    Observer();
+    virtual ~Observer();
+
+    // Called when the daily event is fired.
+    virtual void OnDailyEvent() = 0;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(Observer);
+  };
+
+  // Constructs DailyEvent monitor which stores the time it last fired in the
+  // preference |pref_name|.
+  // |pref_name| should be registered by calling RegisterPref before using this
+  // object.
+  // Caller is responsible for ensuring |pref_service| outlives the DailyEvent.

[Alexei Svitkine (slow), 2014/09/15 21:32:32]

Nit: Also, pref_name. Update comment and mention the same on the field comment.

[Steven Holte, 2014/09/15 21:43:09]

Done.

+  // |histogram_name| is the name of the UMA metric which record when this
+  // interval fires, and should be registered in histograms.xml
+  DailyEvent(PrefService* pref_service,
+             const char* pref_name,
+             const std::string& histogram_name);
+  ~DailyEvent();
+
+  // Adds a observer to be notified when a day elapses. All observers should
+  // be registered before the the DailyEvent starts checking time.
+  void AddObserver(scoped_ptr<Observer> observer);
+
+  // Check if a day has elapsed. If it has, OnDailyEvent will be called on

[Alexei Svitkine (slow), 2014/09/15 21:32:32]

Nit: Check -> Checks for consistency with verb tenses in other comments.

[Steven Holte, 2014/09/15 21:43:09]

Done.

+  // all observers.
+  void CheckInterval();
+
+  // Registers the preference used by this interval.
+  static void RegisterPref(PrefRegistrySimple* registry, const char* pref_name);
+
+ private:
+  // Handle an interval elapsing.

[Alexei Svitkine (slow), 2014/09/15 21:32:32]

Nit: Handle -> Handles

[Steven Holte, 2014/09/15 21:43:09]

Done.

+  void OnInterval(base::Time now);
+
+  // A weak pointer to the PrefService object to read and write preferences
+  // from. Calling code should ensure this object continues to exist for the
+  // lifetime of the DailyEvent object.
+  PrefService* pref_service_;
+
+  // The name of the preference to store the last fired time in.
+  const char* pref_name_;
+
+  // The name of the histogram to record intervals.
+  std::string histogram_name_;
+
+  // List of observers

[Alexei Svitkine (slow), 2014/09/15 21:32:32]

Nit: Add a .

[Steven Holte, 2014/09/15 21:43:09]

Done.

+  ScopedVector<Observer> observers_;
+
+  // The time that the daily event was last fired.
+  base::Time last_fired_;
+
+  DISALLOW_COPY_AND_ASSIGN(DailyEvent);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_DAILY_EVENT_H_