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

components/metrics/daily_interval.cc

+// 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.
+
+#include "components/metrics/daily_interval.h"
+
+#include "base/i18n/time_formatting.h"
+#include "base/prefs/pref_registry_simple.h"
+#include "base/prefs/pref_service.h"
+
+namespace metrics {
+
+DailyInterval::DailyInterval(PrefService* pref_service, const char* pref_name)
+    : pref_service_(pref_service), pref_name_(pref_name) {
+}
+
+DailyInterval::~DailyInterval() {
+}
+
+// static
+void DailyInterval::RegisterPref(PrefRegistrySimple* registry,
+                                 const char* pref_name) {
+  registry->RegisterInt64Pref(pref_name, base::Time().ToInternalValue());
+}
+
+void DailyInterval::AddObserver(DailyObserver* observer) {
+  DVLOG(2) << "DailyInterval observer added.";
+  DCHECK(last_fired_.is_null());
+  observers_.AddObserver(observer);
+}
+
+void DailyInterval::CheckInterval() {
+  base::Time now = base::Time::Now();
+  if (last_fired_.is_null()) {
+    // The first time we call CheckInterval, we read the time stored in prefs.
+    last_fired_ = base::Time::FromInternalValue(
+        pref_service_->GetInt64(pref_name_));
+    DVLOG(1) << "DailyInterval time loaded: "
+             << base::TimeFormatShortDateAndTime(last_fired_);
+    if (last_fired_.is_null()) {
+      DVLOG(1) << "DailyInterval first run.";
+      OnInterval(now);
+      return;
+    }
+  }
+  int days_elapsed = (now - last_fired_).InDays();
+  if (days_elapsed > 1) {
+    DVLOG(1) << "DailyInterval day elapsed.";
+    OnInterval(now);
+  } else if (days_elapsed <= -1) {
+    // The "last fired" time is more than a day in the future, so the clock
+    // must have been changed.
+    DVLOG(1) << "DailyInterval clock change detected.";

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

It might be nice to log an uma metric here, to see how often this happens. I'm
not sure if I like the idea of one metric for all DailyInterval instances, or
going through the trouble of suffixing it by pref_name. 

It's an optional change.

[Steven Holte, 2014/09/03 19:33:44]

Done.

+    OnInterval(now);
+  }
+}
+
+void DailyInterval::OnInterval(base::Time now) {
+  DCHECK(!now.is_null());
+  last_fired_ = now;
+  pref_service_->SetInt64(pref_name_, last_fired_.ToInternalValue());
+  FOR_EACH_OBSERVER(
+      DailyObserver, observers_, OnDailyInterval());
+}
+
+}  // namespace metrics