concurrent::AsyncEventChannel< Args > Class Template Reference

#include <userver/concurrent/async_event_channel.hpp>

Detailed Description

template<typename... Args>
class concurrent::AsyncEventChannel< Args >

AsyncEventChannel is an in-process pub-sub with strict FIFO serialization, i.e. only after the event was processed a new event may appear for processing, same listener is never called concurrently.

Example usage:

enum class WeatherKind { kSunny, kRainy };
 
class WeatherStorage final {
public:
    explicit WeatherStorage(WeatherKind value) : value_(value), channel_("weather") {}
 
    WeatherKind Get() const { return value_.load(); }
 
    concurrent::AsyncEventSource<WeatherKind>& GetSource() { return channel_; }
 
    template <typename Class>
    concurrent::AsyncEventSubscriberScope
    UpdateAndListen(Class* obj, std::string_view name, void (Class::*func)(WeatherKind)) {
        return channel_.DoUpdateAndListen(obj, name, func, [&] { (obj->*func)(Get()); });
    }
 
    void Set(WeatherKind value) {
        value_.store(value);
        channel_.SendEvent(value);
    }
 
private:
    std::atomic<WeatherKind> value_;
    concurrent::AsyncEventChannel<WeatherKind> channel_;
};
 
enum class CoatKind { kJacket, kRaincoat };
 
class CoatStorage final {
public:
    explicit CoatStorage(WeatherStorage& weather_storage) {
        weather_subscriber_ = weather_storage.UpdateAndListen(this, "coats", &CoatStorage::OnWeatherUpdate);
    }
 
    ~CoatStorage() { weather_subscriber_.Unsubscribe(); }
 
    CoatKind Get() const { return value_.load(); }
 
private:
    void OnWeatherUpdate(WeatherKind weather) { value_.store(ComputeCoat(weather)); }
 
    static CoatKind ComputeCoat(WeatherKind weather);
 
    std::atomic<CoatKind> value_{};
    concurrent::AsyncEventSubscriberScope weather_subscriber_;
};
 
UTEST(AsyncEventChannel, UpdateAndListenSample) {
    WeatherStorage weather_storage(WeatherKind::kSunny);
    CoatStorage coat_storage(weather_storage);
    EXPECT_EQ(coat_storage.Get(), CoatKind::kJacket);
    weather_storage.Set(WeatherKind::kRainy);
    EXPECT_EQ(coat_storage.Get(), CoatKind::kRaincoat);
}

Definition at line 77 of file async_event_channel.hpp.

Inheritance diagram for concurrent::AsyncEventChannel< Args >:

Public Types
using	Function = typename AsyncEventSource<Args...>::Function
using	OnRemoveCallback = std::function<void(Function&)>

Public Member Functions
	AsyncEventChannel (std::string_view name)
	The primary constructor.
	AsyncEventChannel (std::string_view name, OnRemoveCallback on_listener_removal)
	The constructor with AsyncEventSubscriberScope usage checking.
template<typename UpdaterFunc>
AsyncEventSubscriberScope	DoUpdateAndListen (FunctionId id, std::string_view name, Function &&func, UpdaterFunc &&updater)
	For use in UpdateAndListen of specific event channels.
template<typename Class, typename UpdaterFunc>
AsyncEventSubscriberScope	DoUpdateAndListen (Class *obj, std::string_view name, void(Class::*func)(Args...), UpdaterFunc &&updater)
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
void	SendEvent (Args... args) const
const std::string &	Name () const noexcept
AsyncEventSubscriberScope	AddListener (Class *obj, std::string_view name, void(Class::*func)(Args...))
	Subscribes to updates from this event source.
AsyncEventSubscriberScope	AddListener (FunctionId id, std::string_view name, Function &&func)
	A type-erased version of AddListener.

Member Typedef Documentation

Function

template<typename... Args>

using concurrent::AsyncEventChannel< Args >::Function = typename AsyncEventSource<Args...>::Function

Definition at line 79 of file async_event_channel.hpp.

OnRemoveCallback

template<typename... Args>

using concurrent::AsyncEventChannel< Args >::OnRemoveCallback = std::function<void(Function&)>

Definition at line 80 of file async_event_channel.hpp.

Constructor & Destructor Documentation

AsyncEventChannel() [1/2]

template<typename... Args>

concurrent::AsyncEventChannel< Args >::AsyncEventChannel ( std::string_view name )

inlineexplicit

The primary constructor.

Parameters

name	used for diagnostic purposes and is also accessible with Name

Definition at line 84 of file async_event_channel.hpp.

AsyncEventChannel() [2/2]

template<typename... Args>

concurrent::AsyncEventChannel< Args >::AsyncEventChannel ( std::string_view name, OnRemoveCallback on_listener_removal )

inline

The constructor with AsyncEventSubscriberScope usage checking.

The constructor with a callback that is called on listener removal. The callback takes a reference to Function as input. This is useful for checking the lifetime of data captured by the listener update function.

Note

Works only in debug mode.

Warning

Data captured by on_listener_removal function must be valid until the AsyncEventChannel object is completely destroyed.

Example usage:

    auto on_remove = [](std::function<void(int)> func) { func(1); };
 
    concurrent::AsyncEventChannel<int> channel("channel", on_remove);
 
    int value = 0;
    {
        concurrent::AsyncEventSubscriberScope sub =
            channel.AddListener(concurrent::FunctionId(&sub), "sub", [&value](int new_value) { value = new_value; });
    }
 
    if constexpr (concurrent::impl::kCheckSubscriptionUB) {
        EXPECT_EQ(value, 1);
    } else {
        EXPECT_EQ(value, 0);
    }

Parameters

name	used for diagnostic purposes and is also accessible with Name
on_listener_removal	the callback used for check

See also

impl::CheckDataUsedByCallbackHasNotBeenDestroyedBeforeUnsubscribing

Definition at line 104 of file async_event_channel.hpp.

Member Function Documentation

AddListener() [1/2]

AsyncEventSubscriberScope concurrent::AsyncEventSource< Args >::AddListener ( Class * obj, std::string_view name, void(Class::* func )(Args...) )

inlineinherited

Subscribes to updates from this event source.

The listener won't be called immediately. To process the current value and then listen to updates, use UpdateAndListen of specific event channels.

Warning

Listeners should not be added or removed while processing the event inside another listener.

Example usage:

UTEST(AsyncEventChannel, AddListenerSample) {
    WeatherStorage weather_storage(WeatherKind::kSunny);
    std::vector<WeatherKind> recorded_weather;
 
    concurrent::AsyncEventSubscriberScope recorder = weather_storage.GetSource().AddListener(
        concurrent::FunctionId(&recorder), "recorder", [&](WeatherKind weather) { recorded_weather.push_back(weather); }
    );
 
    weather_storage.Set(WeatherKind::kRainy);
    weather_storage.Set(WeatherKind::kSunny);
    weather_storage.Set(WeatherKind::kSunny);
    EXPECT_EQ(recorded_weather, (std::vector{WeatherKind::kRainy, WeatherKind::kSunny, WeatherKind::kSunny}));
 
    recorder.Unsubscribe();
}

Parameters

obj	the subscriber, which is the owner of the listener method, and is also used as the unique identifier of the subscription for this AsyncEventSource
name	the name of the subscriber, for diagnostic purposes
func	the listener method, usually called On<DataName>Update, e.g. OnConfigUpdate or OnCacheUpdate

Returns

a AsyncEventSubscriberScope controlling the subscription, which should be stored as a member in the subscriber; Unsubscribe should be called explicitly

Definition at line 130 of file async_event_source.hpp.

AddListener() [2/2]

AsyncEventSubscriberScope concurrent::AsyncEventSource< Args >::AddListener ( FunctionId id, std::string_view name, Function && func )

inlineinherited

A type-erased version of AddListener.

Parameters

id	the unique identifier of the subscription
name	the name of the subscriber, for diagnostic purposes
func	the callback used for event notifications

Definition at line 139 of file async_event_source.hpp.

DoUpdateAndListen() [1/2]

template<typename... Args>

template<typename Class, typename UpdaterFunc>

AsyncEventSubscriberScope concurrent::AsyncEventChannel< Args >::DoUpdateAndListen ( Class * obj, std::string_view name, void(Class::* func )(Args...), UpdaterFunc && updater )

inline

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Definition at line 129 of file async_event_channel.hpp.

DoUpdateAndListen() [2/2]

template<typename... Args>

template<typename UpdaterFunc>

AsyncEventSubscriberScope concurrent::AsyncEventChannel< Args >::DoUpdateAndListen ( FunctionId id, std::string_view name, Function && func, UpdaterFunc && updater )

inline

For use in UpdateAndListen of specific event channels.

Atomically calls updater, which should invoke func with the previously sent event, and subscribes to new events as if using AddListener.

Parameters

id	the subscriber class instance, see also a simpler DoUpdateAndListen overload below
name	the name of the subscriber
func	the callback that is called on each update
updater	the initial () -> void callback that should call func with the current value

See also

AsyncEventSource::AddListener

Definition at line 120 of file async_event_channel.hpp.

Name()

template<typename... Args>

const std::string & concurrent::AsyncEventChannel< Args >::Name ( ) const

inlinenoexcept

Returns

the name of this event channel

Definition at line 163 of file async_event_channel.hpp.

SendEvent()

template<typename... Args>

void concurrent::AsyncEventChannel< Args >::SendEvent ( Args... args ) const

inline

Send the next event and wait until all the listeners process it.

Strict FIFO serialization is guaranteed, i.e. only after this event is processed a new event may be delivered for the subscribers, same listener/subscriber is never called concurrently.

Definition at line 143 of file async_event_channel.hpp.

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The documentation for this class was generated from the following file:

• userver/concurrent/async_event_channel.hpp