Add a StatisticsDeltaReader class and use cases

base/test/statistics_delta_reader_unittest.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 "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/test/statistics_delta_reader.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+class StatisticsDeltaReaderTest : public testing::Test {
+ protected:
+  virtual void SetUp() OVERRIDE {
+    // Each test will have a clean state (no Histogram / BucketRanges
+    // registered).
+    statistics_recorder_ = new StatisticsRecorder();
+  }
+
+  virtual void TearDown() OVERRIDE {
+    delete statistics_recorder_;
+    statistics_recorder_ = NULL;
+  }
+
+  StatisticsRecorder* statistics_recorder_;
+};
+
+TEST_F(StatisticsDeltaReaderTest, Scope) {
+  // Record a histogram before the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  StatisticsDeltaReader reader;
+
+  // Verify that no histogram is recorded.
+  scoped_ptr<HistogramSamples> samples(
+      reader.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 = reader.GetHistogramSamplesSinceCreation("Test");
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(1, samples->TotalCount());
+}
+
+}  // namespace base