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/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
+#include "base/time/time.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace metrics {
+
+// DailyInterval is used for throttling an event to about once per day, even if
+// chrome is restarted more frequently.  It is based on local machine time, so

[Alexei Svitkine (slow), 2014/09/15 20:29:41]

Nit: This is in a component, so don't say "Chrome" since the component can be
used by different programs - you can say "the program".

[Steven Holte, 2014/09/15 21:27:03]

Done.

+// it could be fired more often if the clock is changed.

[Alexei Svitkine (slow), 2014/09/15 20:29:41]

In the description, mention how this class is intended to be used (e.g. that
CheckInterval() should is expected to be called by the caller in place of
recording the metric, etc).

[Steven Holte, 2014/09/15 21:27:03]

Done.

+class DailyInterval {
+ public:
+  // DailyObserver receives notifications from a DailyInterval.

[Alexei Svitkine (slow), 2014/09/15 20:29:41]

Nit: Update comment (no longer named DailyObserver).

[Steven Holte, 2014/09/15 21:27:03]

Done.

+  // Observers must be added before the DailyInterval begins checking time,
+  // and will be owned by the DailyInterval.
+  class Observer {
+   public:
+    Observer();
+    virtual ~Observer();
+
+    // Called when daily metrics should be collected.
+    virtual void OnDailyInterval() = 0;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(Observer);
+  };
+
+  // Constructs DailyInterval 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
+  // DailyInterval.
+  // |histogram_name| is the name of the UMA metric which record when this
+  // interval fires, and should be registered in histograms.xml
+  DailyInterval(PrefService* pref_service,
+                const char* pref_name,
+                const std::string& histogram_name);
+  ~DailyInterval();
+
+  // Adds a observer to be notified when a day elapses. All observers should
+  // be registered before the the DailyInterval starts checking time.
+  void AddObserver(scoped_ptr<Observer> observer);
+
+  // Check if a day has elapsed. If it has, OnDailyInterval will be called on
+  // 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.
+  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_;
+
+  // The name of the histogram to record intervals.
+  std::string histogram_name_;
+
+  // List of observers
+  ScopedVector<Observer> observers_;
+
+  // True if we've started checking for events.

[Alexei Svitkine (slow), 2014/09/15 20:29:41]

Comment seems to not match the field (it's a time field, not a bool).

[Steven Holte, 2014/09/15 21:27:03]

Done.

+  base::Time last_fired_;
+
+  DISALLOW_COPY_AND_ASSIGN(DailyInterval);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_DAILY_INTERVAL_H_