microbus - A simple event-bus and event-loop header-only library

The microbus namespace provides a simple yet efficient event-bus and event-loop implementation in C++17. This library includes classes to manage subscriptions, event notifications, and asynchronous event processing. The bus component can be used in a simple subscribe/trigger scenario, or, in a complex multi-threading scenario.

Key Features

• Thread-Safe: Both event_bus and event_loop classes use mutexes to ensure thread-safety.
• Type Erasure: Handlers are type-erased, allowing storage of various callable objects in a unified manner.
• Asynchronous Processing: Events can be processed asynchronously using the event_loop class.
• Automatic Cleanup: Event handlers are automatically cleaned up when unsubscribed or when the event bus is cleared.
• Flexible Subscription Management: Supports multiple subscribers per event and manages them using unique subscription IDs.
• Context Helper Class: Helps operate the bus and loop pair in common application use-cases.

Overview

Classes

• type_erased_handler: An abstract base class for type-erased handlers.
• concrete_handler: A concrete handler that binds a function to a given set of arguments.
• event_bus: Manages event subscriptions and notifications.
• event_loop: Processes asynchronous events.

Detailed Description

type_erased_handler

This is a base class for handlers that erase type information. It provides a virtual invoke method which derived classes must implement.

concrete_handler

A templated class that derives from type_erased_handler. It binds a specific function (handler) to a set of arguments. It overrides the invoke method to call the stored function with the provided arguments.

event_bus

This class manages event subscriptions and notifications. It allows users to subscribe to events, unsubscribe, and trigger events.

• subscribe: Allows a client to subscribe to an event with a specified handler. Returns a unique subscription ID.
• unsubscribe: Unsubscribes a client from an event using the event name and subscription ID.
• trigger: Triggers an event, calling all subscribed handlers with provided arguments.
• clear: Clears all event subscriptions.

event_loop

This class manages the processing of asynchronous events. It runs an internal thread to process events queued for execution.

• enqueue_event: Enqueues an event for asynchronous processing. The event will be processed by calling the relevant handlers from the event_bus.
• wait_until_finished: Blocks until all events in the queue are processed.
• stop: Stops the event loop and joins the internal thread.

shared_context

This class is optional, it helps to operate the bus nad loop pair in common development scenarios. It allows subscribing and queuing event as well as operating the loop.

Shared Context Example

#include <iostream>
#include "microbus.hpp"

using my_event_handler = microbus::event_bus::event_handler<int>;

auto context = microbus::shared_context();

// You can also use the event handler inline (and create macros).
int on_number_id = context.subscribe("OnNumber", (my_event_handler)[](int number) {
    auto result = factorial(number);
    std::cout << "Number " << number << " was passed to event." << std::endl;
});

context.enqueue_event("OnNumber", 69);

context.wait_until_finished();

context.unsubscribe("OnNumber", on_number_id);

Simple/Async Usage Example

This example demonstrates subscribing to an event, triggering it synchronously and asynchronously, and then stopping the event loop.

#include <iostream>
#include <memory>
#include "microbus.hpp"

int main() {
    auto bus = std::make_shared<microbus::event_bus>();
    microbus::event_loop loop;

    // Define event handler
    using my_event_handler = microbus::event_bus::event_handler<int>;

    // Event handler callback
    my_event_handler my_event_callback = (my_event_handler)[](int value) {
        std::cout << "Received event with value: " << value << std::endl;
    };

    // Subscribe callback to event
    int subscription_id = bus->subscribe<int>("OnTest", my_event_callback);

    // Queue the event for asynchronous processing
    loop.enqueue_event(bus, "OnTest", 100);

    // Trigger the event synchronously
    bus->trigger("OnTest", 42);

    // Unsubscribe event (auto when out of scope).
    bus->unsubscribe("OnTest", subscription_id);

    // Wait for all events to be processed
    loop.wait_until_finished();

    return 0;
}