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

components/metrics/daily_interval.h

Index: components/metrics/daily_interval.h
diff --git a/components/metrics/daily_interval.h b/components/metrics/daily_interval.h
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+// 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_vector.h"
+#include "base/time/time.h"
+#include "components/metrics/daily_observer.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
+// it could be fired more often if the clock is changed.
+class DailyInterval {
+ public:
+  // 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.
+  // The interval takes ownership of all of it's observers.
+  void AddObserver(DailyObserver* 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<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_