calloop

Calloop, a Callback-based Event Loop

This crate provides an EventLoop type, which is a small abstraction over an mio-based event loop. The main difference between this crate and other traditional rust event loops is that it is based on callbacks: you can register several event sources, each being associated with a callback closure that will be invoked whenever the associated event source generates events.

This crate was initially an implementation detail of wayland-server, and has been split-off for reuse. I expect it to be more useful for GUI programs or graphical servers (like wayland-based apps) than performance critial networking code, which are more versed towards tokio and async-await. It mostly shines in the conception of modular infrastructures, allowing different modules to use the same event loop without needing to know about each other.

How to use it

extern crate calloop;

use std::time::Duration;

fn main() {
    // Create the event loop
    let mut event_loop = calloop::EventLoop::new().expect("Failed to initialize the event loop!");
    // Retrieve an handle. It is used to insert new sources into the event loop
    // It can be cloned, allowing you to insert sources from within sources
    let handle = event_loop.handle();

    /*
     * Setup your program, inserting event sources in the loop
     */

    // Actual run of your loop
    loop {
        // Dispatch received events to their callbacks, waiting at most 20 ms for
        // new events
        //
        // The `&mut shared_data` is a mutable reference that will be forwarded to all
        // your callbacks, allowing them to easily share some state
        event_loop.dispatch(Some(Duration::from_millis(20)), &mut shared_data);

        /*
         * Insert here the processing you need to do do between each event loop run
         * like your drawing logic if you're doing a GUI app for example.
         */
    }
}

Event source types

The event loop is backed by mio, as such anything implementing the mio::Evented trait can be used as an event source, with some adapter code (see the EventSource trait).

This crate also provide some adapters for common event sources such as:

• MPSC channels
• Timers
• unix signals

As well as generic mio::Evented objects.

It is also possible to insert "idle" callbacks. These callbacks represent computations that need to be done at some point, but are not as urgent as processing the events. These callbacks are stored and then executed during EventLoop::dispatch(..), once all events from the sources have been processed.

Custom event sources

You can create custom event sources can will be inserted in the event loop by implementing the EventSource trait. This notably involves the creation of an internal type implementing the EventDispatcher trait, that will convert readiness notifications from mio into events generated by your sources. For example, by reading the new messages from a display server socket and producing an event and calling the user callback for each of them.

License: MIT