| // 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. |
| |
| #ifndef BASE_FEATURE_LIST_H_ |
| #define BASE_FEATURE_LIST_H_ |
| |
| #include <map> |
| #include <string> |
| #include <vector> |
| |
| #include "base/base_export.h" |
| #include "base/basictypes.h" |
| #include "base/gtest_prod_util.h" |
| #include "base/memory/scoped_ptr.h" |
| #include "base/strings/string_piece.h" |
| #include "base/synchronization/lock.h" |
| |
| namespace base { |
| |
| class FieldTrial; |
| |
| // Specifies whether a given feature is enabled or disabled by default. |
| enum FeatureState { |
| FEATURE_DISABLED_BY_DEFAULT, |
| FEATURE_ENABLED_BY_DEFAULT, |
| }; |
| |
| // The Feature struct is used to define the default state for a feature. See |
| // comment below for more details. There must only ever be one struct instance |
| // for a given feature name - generally defined as a constant global variable or |
| // file static. |
| struct BASE_EXPORT Feature { |
| // The name of the feature. This should be unique to each feature and is used |
| // for enabling/disabling features via command line flags and experiments. |
| const char* const name; |
| |
| // The default state (i.e. enabled or disabled) for this feature. |
| const FeatureState default_state; |
| }; |
| |
| // The FeatureList class is used to determine whether a given feature is on or |
| // off. It provides an authoritative answer, taking into account command-line |
| // overrides and experimental control. |
| // |
| // The basic use case is for any feature that can be toggled (e.g. through |
| // command-line or an experiment) to have a defined Feature struct, e.g.: |
| // |
| // const base::Feature kMyGreatFeature { |
| // "MyGreatFeature", base::FEATURE_ENABLED_BY_DEFAULT |
| // }; |
| // |
| // Then, client code that wishes to query the state of the feature would check: |
| // |
| // if (base::FeatureList::IsEnabled(kMyGreatFeature)) { |
| // // Feature code goes here. |
| // } |
| // |
| // Behind the scenes, the above call would take into account any command-line |
| // flags to enable or disable the feature, any experiments that may control it |
| // and finally its default state (in that order of priority), to determine |
| // whether the feature is on. |
| // |
| // Features can be explicitly forced on or off by specifying a list of comma- |
| // separated feature names via the following command-line flags: |
| // |
| // --enable-features=Feature5,Feature7 |
| // --disable-features=Feature1,Feature2,Feature3 |
| // |
| // After initialization (which should be done single-threaded), the FeatureList |
| // API is thread safe. |
| // |
| // Note: This class is a singleton, but does not use base/memory/singleton.h in |
| // order to have control over its initialization sequence. Specifically, the |
| // intended use is to create an instance of this class and fully initialize it, |
| // before setting it as the singleton for a process, via SetInstance(). |
| class BASE_EXPORT FeatureList { |
| public: |
| FeatureList(); |
| ~FeatureList(); |
| |
| // Initializes feature overrides via command-line flags |enable_features| and |
| // |disable_features|, each of which is a comma-separated list of features to |
| // enable or disable, respectively. If a feature appears on both lists, then |
| // it will be disabled. If a list entry has the format "FeatureName<TrialName" |
| // then this initialization will also associate the feature state override |
| // with the named field trial, if it exists. Must only be invoked during the |
| // initialization phase (before FinalizeInitialization() has been called). |
| void InitializeFromCommandLine(const std::string& enable_features, |
| const std::string& disable_features); |
| |
| // Specifies whether a feature override enables or disables the feature. |
| enum OverrideState { |
| OVERRIDE_DISABLE_FEATURE, |
| OVERRIDE_ENABLE_FEATURE, |
| }; |
| |
| // Returns true if the state of |feature_name| has been overridden via |
| // |InitializeFromCommandLine()|. |
| bool IsFeatureOverriddenFromCommandLine(const std::string& feature_name, |
| OverrideState state) const; |
| |
| // Associates a field trial for reporting purposes corresponding to the |
| // command-line setting the feature state to |for_overridden_state|. The trial |
| // will be activated when the state of the feature is first queried. This |
| // should be called during registration, after InitializeFromCommandLine() has |
| // been called but before the instance is registered via SetInstance(). |
| void AssociateReportingFieldTrial(const std::string& feature_name, |
| OverrideState for_overridden_state, |
| FieldTrial* field_trial); |
| |
| // Registers a field trial to override the enabled state of the specified |
| // feature to |override_state|. Command-line overrides still take precedence |
| // over field trials, so this will have no effect if the feature is being |
| // overridden from the command-line. The associated field trial will be |
| // activated when the feature state for this feature is queried. This should |
| // be called during registration, after InitializeFromCommandLine() has been |
| // called but before the instance is registered via SetInstance(). |
| void RegisterFieldTrialOverride(const std::string& feature_name, |
| OverrideState override_state, |
| FieldTrial* field_trial); |
| |
| // Returns comma-separated lists of feature names (in the same format that is |
| // accepted by InitializeFromCommandLine()) corresponding to features that |
| // have been overridden - either through command-line or via FieldTrials. For |
| // those features that have an associated FieldTrial, the output entry will be |
| // of the format "FeatureName<TrialName", where "TrialName" is the name of the |
| // FieldTrial. Must be called only after the instance has been initialized and |
| // registered. |
| void GetFeatureOverrides(std::string* enable_overrides, |
| std::string* disable_overrides); |
| |
| // Returns whether the given |feature| is enabled. Must only be called after |
| // the singleton instance has been registered via SetInstance(). Additionally, |
| // a feature with a given name must only have a single corresponding Feature |
| // struct, which is checked in builds with DCHECKs enabled. |
| static bool IsEnabled(const Feature& feature); |
| |
| // Splits a comma-separated string containing feature names into a vector. |
| static std::vector<std::string> SplitFeatureListString( |
| const std::string& input); |
| |
| // Initializes and sets a default instance of FeatureList if one has not yet |
| // already been set. No-op otherwise. |
| static void InitializeInstance(); |
| |
| // Returns the singleton instance of FeatureList. Will return null until an |
| // instance is registered via SetInstance(). |
| static FeatureList* GetInstance(); |
| |
| // Registers the given |instance| to be the singleton feature list for this |
| // process. This should only be called once and |instance| must not be null. |
| static void SetInstance(scoped_ptr<FeatureList> instance); |
| |
| // Clears the previously-registered singleton instance for tests. |
| static void ClearInstanceForTesting(); |
| |
| private: |
| FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity); |
| |
| struct OverrideEntry { |
| // The overridden enable (on/off) state of the feature. |
| const OverrideState overridden_state; |
| |
| // An optional associated field trial, which will be activated when the |
| // state of the feature is queried for the first time. Weak pointer to the |
| // FieldTrial object that is owned by the FieldTrialList singleton. |
| base::FieldTrial* field_trial; |
| |
| // Specifies whether the feature's state is overridden by |field_trial|. |
| // If it's not, and |field_trial| is not null, it means it is simply an |
| // associated field trial for reporting purposes (and |overridden_state| |
| // came from the command-line). |
| const bool overridden_by_field_trial; |
| |
| // TODO(asvitkine): Expand this as more support is added. |
| |
| // Constructs an OverrideEntry for the given |overridden_state|. If |
| // |field_trial| is not null, it implies that |overridden_state| comes from |
| // the trial, so |overridden_by_field_trial| will be set to true. |
| OverrideEntry(OverrideState overridden_state, FieldTrial* field_trial); |
| }; |
| |
| // Finalizes the initialization state of the FeatureList, so that no further |
| // overrides can be registered. This is called by SetInstance() on the |
| // singleton feature list that is being registered. |
| void FinalizeInitialization(); |
| |
| // Returns whether the given |feature| is enabled. This is invoked by the |
| // public FeatureList::IsEnabled() static function on the global singleton. |
| // Requires the FeatureList to have already been fully initialized. |
| bool IsFeatureEnabled(const Feature& feature); |
| |
| // For each feature name in comma-separated list of strings |feature_list|, |
| // registers an override with the specified |overridden_state|. Also, will |
| // associate an optional named field trial if the entry is of the format |
| // "FeatureName<TrialName". |
| void RegisterOverridesFromCommandLine(const std::string& feature_list, |
| OverrideState overridden_state); |
| |
| // Registers an override for feature |feature_name|. The override specifies |
| // whether the feature should be on or off (via |overridden_state|), which |
| // will take precedence over the feature's default state. If |field_trial| is |
| // not null, registers the specified field trial object to be associated with |
| // the feature, which will activate the field trial when the feature state is |
| // queried. If an override is already registered for the given feature, it |
| // will not be changed. |
| void RegisterOverride(StringPiece feature_name, |
| OverrideState overridden_state, |
| FieldTrial* field_trial); |
| |
| // Verifies that there's only a single definition of a Feature struct for a |
| // given feature name. Keeps track of the first seen Feature struct for each |
| // feature. Returns false when called on a Feature struct with a different |
| // address than the first one it saw for that feature name. Used only from |
| // DCHECKs and tests. |
| bool CheckFeatureIdentity(const Feature& feature); |
| |
| // Map from feature name to an OverrideEntry struct for the feature, if it |
| // exists. |
| std::map<std::string, OverrideEntry> overrides_; |
| |
| // Locked map that keeps track of seen features, to ensure a single feature is |
| // only defined once. This verification is only done in builds with DCHECKs |
| // enabled. |
| Lock feature_identity_tracker_lock_; |
| std::map<std::string, const Feature*> feature_identity_tracker_; |
| |
| // Whether this object has been fully initialized. This gets set to true as a |
| // result of FinalizeInitialization(). |
| bool initialized_; |
| |
| DISALLOW_COPY_AND_ASSIGN(FeatureList); |
| }; |
| |
| } // namespace base |
| |
| #endif // BASE_FEATURE_LIST_H_ |