calloop

Calloop, a Callback-based Event Loop

This crate provides an EventLoop type, which is a small abstraction over a polling system. 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.

The main target use of this event loop is thus for apps that expect to spend most of their time waiting for events and wishes to do so in a cheap and convenient way. It is not meant for large scale high performance IO.

How to use it

Below is a quick usage example of calloop. For a more in-depth tutorial, see the calloop book.

For simple uses, you can just add event sources with callbacks to the event loop. For example, here's a runnable program that exits after five seconds:

use calloop::{timer::{Timer, TimeoutAction}, EventLoop, LoopSignal};

fn main() {
    // Create the event loop. The loop is parameterised by the kind of shared
    // data you want the callbacks to use. In this case, we want to be able to
    // stop the loop when the timer fires, so we provide the loop with a
    // LoopSignal, which has the ability to stop the loop from within events. We
    // just annotate the type here; the actual data is provided later in the
    // run() call.
    let mut event_loop: EventLoop<LoopSignal> =
        EventLoop::try_new().expect("Failed to initialize the event loop!");

    // Retrieve a handle. It is used to insert new sources into the event loop
    // It can be cloned, allowing you to insert sources from within source
    // callbacks.
    let handle = event_loop.handle();

    // Create our event source, a timer, that will expire in 2 seconds
    let source = Timer::from_duration(std::time::Duration::from_secs(2));

    // Inserting an event source takes this general form. It can also be done
    // from within the callback of another event source.
    handle
        .insert_source(
            // a type which implements the EventSource trait
            source,
            // a callback that is invoked whenever this source generates an event
            |event, _metadata, shared_data| {
                // This callback is given 3 values:
                // - the event generated by the source (in our case, timer events are the Instant
                //   representing the deadline for which it has fired)
                // - &mut access to some metadata, specific to the event source (in our case, a
                //   timer handle)
                // - &mut access to the global shared data that was passed to EventLoop::run or
                //   EventLoop::dispatch (in our case, a LoopSignal object to stop the loop)
                //
                // The return type is just () because nothing uses it. Some
                // sources will expect a Result of some kind instead.
                println!("Timeout for {:?} expired!", event);
                // notify the event loop to stop running using the signal in the shared data
                // (see below)
                shared_data.stop();
                // The timer event source requires us to return a TimeoutAction to
                // specify if the timer should be rescheduled. In our case we just drop it.
                TimeoutAction::Drop
            },
        )
        .expect("Failed to insert event source!");

    // Create the shared data for our loop.
    let mut shared_data = event_loop.get_signal();

    // Actually run the event loop. This will dispatch received events to their
    // callbacks, waiting at most 20ms for new events between each invocation of
    // the provided callback (pass None for the timeout argument if you want to
    // wait indefinitely between events).
    //
    // This is where we pass the *value* of the shared data, as a mutable
    // reference that will be forwarded to all your callbacks, allowing them to
    // share some state
    event_loop
        .run(
            std::time::Duration::from_millis(20),
            &mut shared_data,
            |_shared_data| {
                // Finally, this is where you can insert the processing you need
                // to do do between each waiting event eg. drawing logic if
                // you're doing a GUI app.
            },
        )
        .expect("Error during event loop!");
}

Event source types

The event loop is backed by an OS provided polling selector (epoll on Linux).

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

MPSC channels
Timers
unix signals on Linux

As well as generic objects backed by file descriptors.

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.

Async/Await compatibility

calloop can be used with futures, both as an executor and for monitoring Async IO.

Activating the executor cargo feature will add the futures module, which provides a future executor that can be inserted into an EventLoop as yet another EventSource.

IO objects can be made Async-aware via the LoopHandle::adapt_io method. Waking up the futures using these objects is handled by the associated EventLoop directly.

Custom event sources

You can create custom event sources can will be inserted in the event loop by implementing the EventSource trait. This can be done either directly from the file descriptors of your source of interest, or by wrapping an other event source and further processing its events. An EventSource can register more than one file descriptor and aggregate them.

Platforms support

Currently, calloop is tested on Linux, FreeBSD and macOS.

The following platforms are also enabled at compile time but not tested: Android, NetBSD, OpenBSD, DragonFlyBSD.

Those platforms should work based on the fact that they have the same polling mechanism as tested platforms, but some subtle bugs might still occur.