Add support for single sample metrics.

components/metrics/single_value_histograms_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_value_histograms_factory_impl.h"
+
+#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_checker.h"
+#include "content/public/common/service_names.mojom.h"
+#include "services/service_manager/public/cpp/connector.h"
+
+namespace metrics {
+
+// Helper class which ensures we have a provider which is safe to use on the
+// current thread. It trampolines to the thread the service_manager::Connector()
+// in order to create a SingleValueHistogramsProvider.
+class SingleValueHistogramsProviderTrampoline {
+ public:
+  SingleValueHistogramsProviderTrampoline(
+      service_manager::Connector* connector,
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
+    if (task_runner->BelongsToCurrentThread()) {
+      BindOnCorrectThread(connector, mojo::MakeRequest(&provider_));
+      return;
+    }
+
+    task_runner->PostTask(
+        FROM_HERE,
+        base::BindOnce(
+            &SingleValueHistogramsProviderTrampoline::BindOnCorrectThread,
+            base::Unretained(this), connector, mojo::MakeRequest(&provider_)));
+  }
+
+  ~SingleValueHistogramsProviderTrampoline() {
+    // This trampoline persists until process shutdown.
+    NOTREACHED();
+  }
+
+  mojom::SingleValueHistogramsProvider* GetProvider() {
+    return provider_.get();
+  }
+
+ private:
+  void BindOnCorrectThread(
+      service_manager::Connector* connector,
+      mojom::SingleValueHistogramsProviderRequest request) {
+    connector->BindInterface(content::mojom::kBrowserServiceName,
+                             std::move(request));
+  }
+
+  mojom::SingleValueHistogramsProviderPtr provider_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleValueHistogramsProviderTrampoline);
+};
+
+// Implementation of the single value counts histogram variant.
+class SingleValueCountsHistogramImpl : public base::SingleValueCountsHistogram {
+ public:
+  SingleValueCountsHistogramImpl(mojom::SingleValueCountsHistogramPtr metric)
+      : metric_(std::move(metric)) {}
+
+  ~SingleValueCountsHistogramImpl() 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::SingleValueCountsHistogramPtr metric_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleValueCountsHistogramImpl);
+};
+
+SingleValueHistogramsFactoryImpl::SingleValueHistogramsFactoryImpl(
+    service_manager::Connector* connector,
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner)
+    : connector_(connector), task_runner_(std::move(task_runner)) {}
+
+SingleValueHistogramsFactoryImpl::~SingleValueHistogramsFactoryImpl() {
+  // This factory persists until process shutdown.
+  NOTREACHED();
+}
+
+std::unique_ptr<base::SingleValueCountsHistogram>
+SingleValueHistogramsFactoryImpl::CreateSingleValueCountsHistogram(
+    const std::string& name,
+    base::HistogramBase::Sample min,
+    base::HistogramBase::Sample max,
+    uint32_t bucket_count) {
+  mojom::SingleValueCountsHistogramPtr metric;
+  GetProvider()->AcquireSingleValueCountsHistogram(
+      name, min, max, bucket_count,
+      mojo::MakeRequest<mojom::SingleValueCountsHistogram>(&metric));
+  return base::MakeUnique<SingleValueCountsHistogramImpl>(std::move(metric));
+}
+
+mojom::SingleValueHistogramsProvider*
+SingleValueHistogramsFactoryImpl::GetProvider() {
+  if (auto* provider = provider_.Get())
+    return provider->GetProvider();
+
+  // Provider trampolines persist per-thread until process shutdown.
+  SingleValueHistogramsProviderTrampoline* provider =
+      new SingleValueHistogramsProviderTrampoline(connector_, task_runner_);
+  provider_.Set(provider);
+  return provider->GetProvider();
+}
+
+}  // namespace metrics