5 files changed, 117 insertions, 3 deletions
diff --git a/base/base.gyp b/base/base.gyp
index e4db68d..c177606 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -596,6 +596,7 @@
         'template_util_unittest.cc',
         'test/expectations/expectation_unittest.cc',
         'test/expectations/parser_unittest.cc',
+        'test/histogram_recorder_unittest.cc',
         'test/trace_event_analyzer_unittest.cc',
         'threading/non_thread_safe_unittest.cc',
         'threading/platform_thread_unittest.cc',
@@ -863,6 +864,8 @@
         'test/expectations/expectation.h',
         'test/expectations/parser.cc',
         'test/expectations/parser.h',
+        'test/histogram_recorder.cc',
+        'test/histogram_recorder.h',
         'test/mock_chrome_application_mac.h',
         'test/mock_chrome_application_mac.mm',
         'test/mock_devices_changed_observer.cc',
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index 9a55225..744be01 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -66,9 +66,9 @@ class BASE_EXPORT StatisticsRecorder {
   static HistogramBase* FindHistogram(const std::string& name);
 
   // GetSnapshot copies some of the pointers to registered histograms into the
-  // caller supplied vector (Histograms).  Only histograms with names matching
-  // query are returned. The query must be a substring of histogram name for its
-  // pointer to be copied.
+  // caller supplied vector (Histograms).  Only histograms which have query as
+  // a substring are copied (an empty string will process all registered
+  // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private:
diff --git a/base/test/histogram_recorder.cc b/base/test/histogram_recorder.cc
new file mode 100644
index 0000000..500f16c
--- /dev/null
+++ b/base/test/histogram_recorder.cc
@@ -0,0 +1,42 @@
+// 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.
+
+#include "base/test/histogram_recorder.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+HistogramRecorder::HistogramRecorder() {
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Initialize();
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot("", &histograms);
+  for (size_t i = 0; i < histograms.size(); i++) {
+    original_samples_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+HistogramRecorder::~HistogramRecorder() {
+  STLDeleteValues(&original_samples_);
+}
+
+scoped_ptr<HistogramSamples>
+    HistogramRecorder::GetHistogramSamplesSinceCreation(
+        const std::string& histogram_name) {
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
+  if (!histogram)
+    return scoped_ptr<HistogramSamples>();
+  scoped_ptr<HistogramSamples> named_samples(histogram->SnapshotSamples());
+  HistogramSamples* named_original_samples = original_samples_[histogram_name];
+  if (named_original_samples)
+    named_samples->Subtract(*named_original_samples);
+  return named_samples.Pass();
+}
+
+}  // namespace base
diff --git a/base/test/histogram_recorder.h b/base/test/histogram_recorder.h
new file mode 100644
index 0000000..85139bd
--- /dev/null
+++ b/base/test/histogram_recorder.h
@@ -0,0 +1,35 @@
+// 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/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 sent as they should be.
+class HistogramRecorder {
+ public:
+  HistogramRecorder();
+  virtual ~HistogramRecorder();
+
+  // 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.
+  std::map<std::string, HistogramSamples*> original_samples_;
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_RECORDER_H_
diff --git a/base/test/histogram_recorder_unittest.cc b/base/test/histogram_recorder_unittest.cc
new file mode 100644
index 0000000..a69c6b0
--- /dev/null
+++ b/base/test/histogram_recorder_unittest.cc
@@ -0,0 +1,34 @@
+// 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.
+
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/test/histogram_recorder.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+class HistogramRecorderTest : public testing::Test {};
+
+TEST_F(HistogramRecorderTest, Scope) {
+  // Send a histogram before the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  HistogramRecorder recorder;
+
+  // Verify that no histogram is recorded.
+  EXPECT_FALSE(recorder.GetHistogramSamplesSinceCreation("Test"));
+
+  // Send a histogram after the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  // Verify that one histogram is recorded.
+  scoped_ptr<HistogramSamples> samples(
+      recorder.GetHistogramSamplesSinceCreation("Test"));
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(1, samples->TotalCount());
+}
+
+}  // namespace base