Add support for single sample metrics.

components/metrics/single_sample_metrics_factory_impl.cc

+// Copyright 2017 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 "components/metrics/single_sample_metrics_factory_impl.h"
+
+#include "base/threading/thread_checker.h"
+
+namespace metrics {
+
+class SingleSampleMetricImpl : public base::SingleSampleMetric {

[Alexei Svitkine (slow), 2017/05/03 21:18:48]

Can this be in the anon namespace?

[DaleCurtis, 2017/05/03 21:37:51]

Done.

+ public:
+  SingleSampleMetricImpl(mojom::SingleSampleMetricPtr metric)
+      : metric_(std::move(metric)) {}
+
+  ~SingleSampleMetricImpl() override {
+    DCHECK(thread_checker_.CalledOnValidThread());
+  }
+
+  void SetSample(base::HistogramBase::Sample sample) override {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    metric_->SetSample(sample);
+  }
+
+ private:
+  base::ThreadChecker thread_checker_;
+  mojom::SingleSampleMetricPtr metric_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleSampleMetricImpl);
+};
+
+SingleSampleMetricsFactoryImpl::SingleSampleMetricsFactoryImpl(
+    CreateProviderCB create_provider_cb)
+    : create_provider_cb_(std::move(create_provider_cb)) {}
+
+SingleSampleMetricsFactoryImpl::~SingleSampleMetricsFactoryImpl() {}
+
+std::unique_ptr<base::SingleSampleMetric>
+SingleSampleMetricsFactoryImpl::CreateCustomCountsMetric(
+    const std::string& histogram_name,
+    base::HistogramBase::Sample min,
+    base::HistogramBase::Sample max,
+    uint32_t bucket_count) {
+  return CreateMetric(histogram_name, min, max, bucket_count,
+                      base::HistogramBase::kUmaTargetedHistogramFlag);
+}
+
+void SingleSampleMetricsFactoryImpl::DestroyProviderForTesting() {
+  if (auto* provider = provider_.Get())
+    delete provider;
+  provider_.Set(nullptr);
+}
+
+std::unique_ptr<base::SingleSampleMetric>
+SingleSampleMetricsFactoryImpl::CreateMetric(const std::string& histogram_name,
+                                             base::HistogramBase::Sample min,
+                                             base::HistogramBase::Sample max,
+                                             uint32_t bucket_count,
+                                             int32_t flags) {
+  mojom::SingleSampleMetricPtr metric;
+  GetProvider()->AcquireSingleSampleMetric(
+      histogram_name, min, max, bucket_count, flags,
+      mojo::MakeRequest<mojom::SingleSampleMetric>(&metric));
+  return base::MakeUnique<SingleSampleMetricImpl>(std::move(metric));
+}
+
+mojom::SingleSampleMetricsProvider*
+SingleSampleMetricsFactoryImpl::GetProvider() {
+  if (auto* provider = provider_.Get())
+    return provider->get();
+
+  // Providers persist per-thread until process shutdown.
+  mojom::SingleSampleMetricsProviderPtr* provider =
+      new mojom::SingleSampleMetricsProviderPtr();
+  provider_.Set(provider);

[Alexei Svitkine (slow), 2017/05/03 21:18:48]

That's a lot of indirection. Might be good to call out this set up in a comment
somewhere. i.e. explaining the three objects (tls pointer to mojo pointer to
actual object)

[DaleCurtis, 2017/05/03 21:37:51]

Comments added.

+
+  // Create a provider for the current thread.
+  create_provider_cb_.Run(mojo::MakeRequest(provider));
+  return provider->get();
+}
+
+}  // namespace metrics