async-winit

Use winit like the async runtime you've always wanted.

winit is actually asynchronous, contrary to popular belief; it's just not async. It uses an event loop to handle events, which is an good fit for some cases but not others. The maintainers of winit have referred to this type of event loop as "poor man's async"; a system that is not async but is still asynchronous.

This crate builds an async interface on top of this event loop.

Example

Consider the following winit program, which creates a window and prints the size of the window when it is resized:

use winit::event::{Event, WindowEvent};
use winit::event_loop::EventLoop;
use winit::window::Window;

fn main2(evl: EventLoop<()>) {
    let mut window = None;

    evl.run(move |event, elwt, flow| {
        match event {
            Event::Resumed => {
                // Application is active; create a window.
                window = Some(Window::new(elwt).unwrap());
            },

            Event::Suspended => {
                // Application is inactive; destroy the window.
                window = None;
            },

            Event::WindowEvent { event, .. } => match event {
                WindowEvent::CloseRequested => {
                    // Window is closed; exit the application.
                    flow.set_exit();
                },

                WindowEvent::Resized(size) => {
                    println!("{:?}", size);
                }

                _ => {},
            },

            _ => {},
        }
    });
}

fn main() {
#   return;
    let evl = EventLoop::new();
    main2(evl);
}

This strategy is a bit long winded. Now, compare against the equivalent async-winit program:

use async_winit::event_loop::EventLoop;
use async_winit::window::Window;
use async_winit::ThreadUnsafe;
use futures_lite::prelude::*;

fn main2(evl: EventLoop<ThreadUnsafe>) {
    let window_target = evl.window_target().clone();

    evl.block_on(async move {
        loop {
            // Wait for the application to be active.
            window_target.resumed().await;

            // Create a window.
            let window = Window::<ThreadUnsafe>::new().await.unwrap();

            // Print the size of the window when it is resized.
            let print_size = async {
                window
                    .resized()
                    .wait()
                    .for_each(|size| {
                        println!("{:?}", size);
                    })
                    .await;

                true
            };

            // Wait until the window is closed.
            let close = async {
                window.close_requested().wait().await;
                println!("Close");
                true
            };

            // Wait until the application is suspended.
            let suspend = async {
                window_target.suspended().wait().await;
                false
            };

            // Run all of these at once.
            let needs_exit = print_size.or(close).or(suspend).await;

            // If we need to exit, exit. Otherwise, loop again, destroying the window.
            if needs_exit {
                window_target.exit().await;
            } else {
                drop(window);
            }
        }
    });
}

fn main() {
#   return;
    let evl = EventLoop::new();
    main2(evl);
}

In my opinion, the flatter async style is much easier to read and understand. Your mileage may vary.

Pros

In many cases it may make more sense to think of a program as an async task, rather than an event loop.
You don't need to tie everything to the EventLoopWindowTarget; Window::new() and other functions take no parameters and can be called from anywhere as long as an EventLoop is running somewhere.
You can use the async ecosystem to its full potential here.

Cons

There is a not insignificant amount of overhead involved in using async-winit. This is because async-winit is built on top of winit, which is built on top of winit's event loop. This means that async-winit has to convert between async and winit's event loop, which is not free.
async-winit is not as low level as winit. This means that you can't do everything that you can do with winit.
- For instance, data cannot be shared mutable between individual tasks. This can be easily worked around with RefCell in simple cases, but still requires additional thought for shared state.

Credits

async-winit was created by John Nunley (@notgull).

This project is heavily based on async-io by Stjepan Glavina et al, as well as winit by Pierre Kreiger et al.

License

async-winit is free software: you can redistribute it and/or modify it under the terms of one of the following licenses:

GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
Mozilla Public License as published by the Mozilla Foundation, version 2.

async-winit is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License or the Mozilla Public License for more details.