Initial implementation of FeatureList in base/.

base/feature_list.cc

+// Copyright 2015 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/feature_list.h"
+
+#include <vector>
+
+#include "base/logging.h"
+#include "base/strings/string_split.h"
+
+namespace base {
+
+namespace {
+
+// Pointer to the FeatureList instance singleton that was set via
+// FeatureList::SetInstance(). Does not use base/memory/singleton.h in order to
+// have more control over initialization timing. Leaky.
+FeatureList* g_instance = nullptr;
+
+// Splits a comma-separated string containing feature names into a vector.
+std::vector<std::string> SplitFeatureListString(const std::string& input) {
+  return SplitString(input, ",", TRIM_WHITESPACE, SPLIT_WANT_NONEMPTY);
+}
+
+}  // namespace
+
+FeatureList::FeatureList() : initialized_(false) {}
+
+FeatureList::~FeatureList() {}
+
+void FeatureList::InitializeFromCommandLine(
+    const std::string& enable_features,
+    const std::string& disable_features) {
+  DCHECK(!initialized_);
+
+  // Process disabled features first, so that disabled ones take precedence over
+  // enabled ones (since RegisterOverride() uses insert()).
+  for (const auto& feature_name : SplitFeatureListString(disable_features)) {
+    RegisterOverride(feature_name, OVERRIDE_DISABLE_FEATURE);
+  }
+  for (const auto& feature_name : SplitFeatureListString(enable_features)) {
+    RegisterOverride(feature_name, OVERRIDE_ENABLE_FEATURE);
+  }
+}
+
+// static
+bool FeatureList::IsEnabled(const Feature& feature) {
+  return GetInstance()->IsFeatureEnabled(feature);
+}
+
+// static
+FeatureList* FeatureList::GetInstance() {
+  return g_instance;
+}
+
+// static
+void FeatureList::SetInstance(scoped_ptr<FeatureList> instance) {
+  DCHECK(!g_instance);
+  instance->FinalizeInitialization();
+
+  // Note: Intentional leak of global singleton.
+  g_instance = instance.release();
+}
+
+// static
+void FeatureList::ClearInstanceForTesting() {
+  delete g_instance;
+  g_instance = nullptr;
+}
+
+void FeatureList::FinalizeInitialization() {
+  DCHECK(!initialized_);
+  initialized_ = true;
+}
+
+bool FeatureList::IsFeatureEnabled(const Feature& feature) {
+  DCHECK(initialized_);
+  DCHECK(CheckFeatureIdentity(feature)) << feature.name;
+
+  auto it = overrides_.find(feature.name);
+  if (it != overrides_.end()) {
+    const OverrideEntry& entry = it->second;
+    // TODO(asvitkine) Expand this section as more support is added.
+    return entry.overridden_state == OVERRIDE_ENABLE_FEATURE;
+  }
+  // Otherwise, return the default state.
+  return feature.default_state == FEATURE_ENABLED_BY_DEFAULT;
+}
+
+void FeatureList::RegisterOverride(const std::string& feature_name,
+                                   OverrideState overridden_state) {
+  DCHECK(!initialized_);
+  overrides_.insert(make_pair(feature_name, OverrideEntry(overridden_state)));
+}
+
+bool FeatureList::CheckFeatureIdentity(const Feature& feature) {
+  AutoLock auto_lock(feature_identity_tracker_lock_);
+
+  auto it = feature_identity_tracker_.find(feature.name);
+  if (it == feature_identity_tracker_.end()) {
+    // If it's not tracked yet, register it.
+    feature_identity_tracker_[feature.name] = &feature;
+    return true;
+  }
+  // Compare address of |feature| to the existing tracked entry.
+  return it->second == &feature;
+}
+
+FeatureList::OverrideEntry::OverrideEntry(OverrideState overridden_state)
+    : overridden_state(overridden_state) {}
+
+}  // namespace base