Create MemoryPressureStatsCollector.

components/memory_pressure/memory_pressure_stats_collector.h

Index: components/memory_pressure/memory_pressure_stats_collector.h
diff --git a/components/memory_pressure/memory_pressure_stats_collector.h b/components/memory_pressure/memory_pressure_stats_collector.h
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+// Copyright 2015 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_MEMORY_PRESSURE_MEMORY_PRESSURE_STATS_COLLECTOR_H_
+#define COMPONENTS_MEMORY_PRESSURE_MEMORY_PRESSURE_STATS_COLLECTOR_H_
+
+#include "base/time/tick_clock.h"
+#include "components/memory_pressure/memory_pressure_listener.h"
+
+namespace memory_pressure {
+
+// Enumeration of UMA memory pressure levels. This needs to be kept in sync with
+// histograms.xml and the memory pressure levels defined in
+// MemoryPressureListener. Included in the header so that
+// UMA_MEMORY_PRESSURE_LEVEL_COUNT is available.
+enum MemoryPressureLevelUMA {
+  UMA_MEMORY_PRESSURE_LEVEL_NONE = 0,
+  UMA_MEMORY_PRESSURE_LEVEL_MODERATE = 1,
+  UMA_MEMORY_PRESSURE_LEVEL_CRITICAL = 2,
+  // This must be the last value in the enum.
+  UMA_MEMORY_PRESSURE_LEVEL_COUNT,
+};
+
+// Class that is responsible for collecting and eventually reporting memory
+// pressure statistics.
+//
+// On platforms with a polling memory pressure implementation the
+// UpdateStatistics function will be invoked every time the pressure is polled.
+// On non-polling platforms (Mac, Android) it will be invoked on a periodic
+// timer, and at the moment of pressure level changes.
+class MemoryPressureStatsCollector {
+ public:
+  using MemoryPressureLevel = MemoryPressureListener::MemoryPressureLevel;
+
+  // An abstract reporting delegate used as a testing seam.
+  class ReportingDelegate;
+  // The default reporting delegate, which delivers stats via UMA.
+  class UmaReportingDelegate;
+
+  // The provided |reporting_delegate| and |tick_clock| must outlive this class.
+  MemoryPressureStatsCollector(ReportingDelegate* reporting_delegate,
+                               base::TickClock* tick_clock);
+
+  // This is to be called periodically to ensure that up to date statistics
+  // have been reported.
+  void UpdateStatistics(MemoryPressureLevel current_pressure_level);
+
+ private:
+  friend class TestMemoryPressureStatsCollector;
+
+  // The reporting delegate in use. This indirection creates a test seam.
+  ReportingDelegate* reporting_delegate_;
+
+  // The tick clock in use. This class is intended to be owned by a class with
+  // a tick clock, and ownership remains there. Also intended as a test seam.
+  base::TickClock* tick_clock_;
+
+  // Buckets of time that have been spent in different pressure levels, but
+  // not yet reported. At every call to UpdateStatistics these buckets will be
+  // drained as much as possible and reported.
+  base::TimeDelta unreported_cumulative_time_[UMA_MEMORY_PRESSURE_LEVEL_COUNT];
+
+  // The last observed pressure level and the time at which it was observed, and
+  // the time when this pressure level started.
+  MemoryPressureLevel last_pressure_level_;
+  base::TimeTicks last_update_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureStatsCollector);
+};
+
+// An abstract reporting delegate used as a testing seam.
+class MemoryPressureStatsCollector::ReportingDelegate {

[grt (UTC plus 2), 2015/09/04 19:46:19]

instead of introducing a layer between MPSC and UMA for the sake of testing, can
the test reach into base::StatisticsRecorder to look after the fact to see if
histograms were recorded as desired?

[chrisha, 2015/09/04 20:31:24]

I'm not averse to this, I just didn't even know it was possible. It doesn't
appear that you can directly query a histogram's buckets/values via any
interface. However, I found a nifty thing called a HistogramTester which is
intended exactly for this purpose.

I'll happily refactor and get back to you next week.

+ public:
+  ReportingDelegate() {}

[grt (UTC plus 2), 2015/09/04 19:46:19]

nit: make the ctor and dtors protected via:
 protected:
  ReportingDelegate() = default;
  virtual ~ReportingDelegate() = default;

[chrisha, 2015/09/04 20:31:24]

Grrr... I'm still not used to "= default" (despite having it pointed out to me
repeatedly). 

+  virtual ~ReportingDelegate() {}
+
+  // Sends a report that the system spent the given number of |seconds| at the
+  // given |pressure_level|, cumulatively (potentially split across multiple
+  // pressure changes since the last report was sent).
+  virtual void ReportCumulativeTime(MemoryPressureLevel pressure_level,
+                                    int seconds) = 0;
+
+  // Reports a memory pressure level change.
+  virtual void ReportLevelChange(MemoryPressureLevel old_pressure_level,
+                                 MemoryPressureLevel new_pressure_level) = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ReportingDelegate);
+};
+
+// The default reporting delegate, which delivers stats via UMA.
+class MemoryPressureStatsCollector::UmaReportingDelegate
+    : public ReportingDelegate {
+ public:
+  UmaReportingDelegate() {}

[grt (UTC plus 2), 2015/09/04 19:46:19]

= default;

[chrisha, 2015/09/08 15:47:15]

No longer applicable.

+  ~UmaReportingDelegate() override {}

[grt (UTC plus 2), 2015/09/04 19:46:19]

hmm, can you = default; here as well rather than overriding?

[chrisha, 2015/09/08 15:47:14]

No longer applicable.

+
+  // ReportingDelegate:
+  void ReportCumulativeTime(MemoryPressureLevel pressure_level,
+                            int seconds) override;
+  void ReportLevelChange(MemoryPressureLevel old_pressure_level,
+                         MemoryPressureLevel new_pressure_level) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(UmaReportingDelegate);
+};
+
+}  // namespace memory_pressure
+
+#endif  // COMPONENTS_MEMORY_PRESSURE_MEMORY_PRESSURE_STATS_COLLECTOR_H_