Add a StatisticsDeltaReader class and use cases

base/test/histogram_recorder.h

Index: base/test/histogram_recorder.h
diff --git a/base/test/histogram_recorder.h b/base/test/histogram_recorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..f867b1e0944df5a823d5641ab607576600d2239d
--- /dev/null
+++ b/base/test/histogram_recorder.h
@@ -0,0 +1,44 @@
+// Copyright 2013 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 BASE_TEST_HISTOGRAM_RECORDER_H_
+#define BASE_TEST_HISTOGRAM_RECORDER_H_
+
+#include <map>
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_samples.h"
+
+namespace base {
+
+// This class acts as a differential reader for histogram samples, enabling
+// tests to check that metrics were recorded as they should be.

[ppi, 2013/12/23 16:37:48]

I suggest:

"This class acts as a differential reader for histogram samples recorded in the
global StatisticsRecorder, allowing to inspect samples collected during the
lifetime of the class."

+class HistogramRecorder {

[ppi, 2013/12/23 16:37:48]

Let's rename it to HistogramDeltaReader ?

+ public:
+  // Initializes the HistogramRecorder system.
+  static void Initialize();

[ppi, 2013/12/23 16:37:48]

Clients can call StatisticsRecorder::Initialize() directly, so this seems an
unnecessary wiring - should we drop it?

+  HistogramRecorder();
+  virtual ~HistogramRecorder();
+
+  // Returns whether the HistogramRecorder has been initialized.
+  static bool IsActive();

[ppi, 2013/12/23 16:37:48]

What do we need that for? See also the comment above, if this is available in
StatisticsRecorder(), we shouldn't need to expose it here.

+
+  // Returns the histogram data accumulated since this instance was created.
+  // Returns NULL if no samples are available.
+  scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
+      const std::string& histogram_name);
+
+ private:
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle. This isntance takes ownership of the samples, which are deleted
+  // when the instance is destroyed.
+  std::map<std::string, HistogramSamples*> original_samples_;
+
+  DISALLOW_COPY_AND_ASSIGN(HistogramRecorder);
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_RECORDER_H_