Add a StatisticsDeltaReader class and use cases

base/test/histogram_recorder_unittest.cc

+// 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 {
+ public:
+  static void SetUpTestCase() {
+    HistogramRecorder::Initialize();
+  }

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

I'm afraid we have to follow suit of existing unit tests - we can't leave the
global state initialized after the tests run or subsequent base tests will
crash.

It should go along the lines:

  virtual void SetUp() {
    statistics_recorder_ = new StatisticsRecorder();
  }

  virtual void TearDown() {
    delete statistics_recorder_;
    statistics_recorder_ = NULL;
  }

  StatisticsRecorder* statistics_recorder_;

[lpromero, 2013/12/23 17:22:54]

Done.

+};
+
+TEST_F(HistogramRecorderTest, Scope) {
+  // Record a histogram before the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  HistogramRecorder recorder;
+
+  // Verify that no histogram is recorded.
+  scoped_ptr<HistogramSamples> samples(
+      recorder.GetHistogramSamplesSinceCreation("Test"));
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(0, samples->TotalCount());
+
+  // Record a histogram after the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  // Verify that one histogram is recorded.
+  samples = recorder.GetHistogramSamplesSinceCreation("Test");
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(1, samples->TotalCount());
+}
+
+}  // namespace base