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

components/metrics/daily_interval.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_INTERVAL_H_
+#define COMPONENTS_METRICS_DAILY_INTERVAL_H_
+
+#include "base/macros.h"
+#include "base/observer_list.h"
+#include "base/time/time.h"
+#include "components/metrics/daily_observer.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace metrics {
+
+// DailyInterval is used for firing an event about once per day, even if chrome

[jwd, 2014/08/29 15:24:16]

Hmm, this implies to me that DailyInterval is causing the event to fire, whereas
it's preventing the event when the interval hasn't elapsed.

[Steven Holte, 2014/09/03 19:33:45]

Done.

+// is restarted more or less frequently.

[jwd, 2014/08/29 15:24:16]

Maybe mention it's based on local machine time, and is subject to skew and
changing.

[Steven Holte, 2014/09/03 19:33:45]

Done.

+class DailyInterval {
+public:
+  // Constructs DailyInterval monitor which stores the time it last fired
+  // in the preference |pref_name|.
+  // Caller is responsible for ensuring |pref_service| outlives the
+  // DailyInterval.

[jwd, 2014/08/29 15:24:15]

Mention that RegisterPref had to be called with the same pref_name.

[Steven Holte, 2014/09/03 19:33:45]

Done.

+  DailyInterval(PrefService* pref_service, const char* pref_name);
+  ~DailyInterval();
+
+  // Registers the preference used by this interval.
+  static void RegisterPref(PrefRegistrySimple* registry, const char* pref_name);
+
+  // Adds a observer to be notified when a day elapses.  All observers should
+  // be registered before the the DailyInterval starts checking time.
+  void AddObserver(DailyObserver* observer);
+
+  // Check if a day has elapsed.

[jwd, 2014/08/29 15:24:15]

Mention the event is called when the day has elapsed.

[Steven Holte, 2014/09/03 19:33:45]

Done.

+  void CheckInterval();
+
+private:
+  // Handle an interval elapsing.
+  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 DailyInterval object.
+  PrefService* pref_service_;
+
+  // The name of the preference to store the last fired time in.
+  const char* pref_name_;
+
+  // List of observers
+  ObserverList<DailyObserver> observers_;
+
+  // True if we've started checking for events.
+  base::Time last_fired_;
+
+  DISALLOW_COPY_AND_ASSIGN(DailyInterval);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_DAILY_INTERVAL_H_