Bevy Defer

A simple asynchronous runtime for executing async coroutines.

Motivation

bevy_defer is an async runtime for bevy with world access. Think of a future in bevy_defer as a Command that can take as long as it wants to complete. They can even wait for each other or pause for a few seconds, which is pretty powerful!

• So who needs bevy_defer?

bevy_defer is not for every use case as it is somewhat opposite to ecss design goal, if you have a game that might not want ecs only, bevy_defer might help you make your life easier.

Common use cases

• Powerful Abstractions

Pin<Box<dyn Future>> in bevy_defer is the strongest possible abstraction in bevy. Put that on a button's OnClick and it can do anything you want, much more powerful than a command or a system.

• Turn based gameplay and event orchestration

In turn based games, you generally want to sequence actions after the player's input. in bevy_defer this is incredibly simple:

move_to(position).await;
let damage = attack(enemy).await;
show_damage(damage).await;

• UI reactivity

bevy_defer provides an alternative model to handle reactivity using async rust.

Getting Started

First add AsyncPlugin

app.add_plugins(AsyncPlugin::default_settings())

If you want to react to events, states, or other things, add their corresponding react_to functions.

app.react_to_state::<PlayerAlive>();
app.react_to_event::<PlayerAttack>();

Spawning a Task

You can spawn a task onto bevy_defer from World, App, Commands, AsyncWorld or AsyncExecutor.

Here is an example:

commands.spawn_task(|| async move {
    AsyncWorld.sleep(500.0).await;
    println!("Hello, World!");
    Ok(())
});

Accessing the world

The entry point of all world access is AsyncWorld, for example a Component can be accessed by

let translation = AsyncWorld
    .entity(entity)
    .component::<Transform>()
    .get(|t| {
        t.translation
    }).unwrap()

This is similar to World but all validation are deferred to the access function.

• New in 0.13: the fetch! macro can be used:

let translation = fetch!(entity, Transform).get(|t| t.translation).unwrap()

To set some data:

let richard = AsyncWorld.entity(richard_entity);
richard.component::<HP>().set(|hp| hp.set(500))?;

This works for all the bevy things you expect, Resource, Query, etc. See the access module and the AsyncAccess trait for more detail.

You can add extension methods to these accessors via Deref if you own the underlying types. See the access::deref module for more detail. The async_access derive macro can be useful for adding method to async accessors.

Run Systems

You might have noticed AsyncSystemParam does not exist. In order to run more complicated logic, use one of the one-shot system based API, like:

AsyncWorld.run_cached_system(my_system)
AsyncWorld.run_cached_system_with_input(my_system2, 4)

Event Orchestration

• Coroutines

AsyncWorld.yield_now() yields execution for the current frame, with default settings this makes code like this run once per frame.

loop {
    transform.set(|t| t.translation.x += delta_time)?;
    AsyncWorld.yield_now().await
}

• Pausing

AsyncWorld.sleep(4.0) pauses the future for 4 seconds, while AsyncWorld.sleep_frames(4) pauses the future for 4 frames.

• Concurrency

Use futures::join! or futures_lite::zip to achieve concurrency:

// Do both at the same time.
join! {
    dance(),
    play_music(),
}

• Cancellation

Use futures::select! or futures_lite::or to achieve cancellation:

// Dance for no more than 5 seconds.
select! {
    _ = dance() => (),
    _ = sleep(5.0) => (),
}

Bridging Sync and Async

Communicating between sync and async can be daunting for new users. See this amazing tokio article: https://tokio.rs/tokio/topics/bridging.

Communicating from sync to async is simple, async code can provide channels to sync code and await on them, pausing the task. Once sync code sends data through the channel, it will wake and resume the corresponding task.

Communicating from async to sync require more thought. This usually means mutating the world or sending an event in an async function, then a system can listen for that particular change in sync code.

async {
    fetch!(entity, IsJumping).set(|j| *j == true);
    AsyncWold.send_event(Jump(entity))
}

pub fn jump_system(query: Query<Name, Changed<IsJumping>>) {
    for name in &query {
        println!("{} is jumping!", name);
    }
}

States is particularly powerful for this type of communication:

AsyncWorld.set_state(GameState::Loading);

The core principle is async code should help sync code to do less work, and vice versa!

Comparison with bevy_tasks

bevy_tasks has no direct world access, which makes it difficult to write game logic in it.

The core idea behind bevy_defer is simple:

// Pseudocode
static WORLD_CELL: Mutex<&mut World>;

fn run_async_executor(world: &mut World) {
    let executor = world.get_executor();
    WORLD_CELL.set(world);
    executor.run();
    WORLD_CELL.clear();
}

Futures spawned onto the executor can access the World via access functions, similar to how database transaction works:

WORLD_CELL.with(|world: &mut World| {
    world.entity(entity).get::<Transform>().clone()
})

As long as no references can be borrowed from the world, and the executor is single threaded, this is perfectly sound!

Implementation Details

bevy_defer uses a single threaded runtime that always runs on bevy's main thread inside the main schedule, this is ideal for simple game logic, wait heavy or IO heavy tasks, but CPU heavy tasks should not be run in bevy_defer. Unlike multithreaded runtimes, single threaded runtimes are more vulnerable to blocking tasks, so pay extra attention to this.

AsyncWorld::unblock is an abstraction for using AsyncComputeTaskPool to run cpu intensive tasks. for blocking io, checkout crates like async-fs which are more suited for these tasks.

Versions

License

License under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0) MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT) at your option.

Contribution

Contributions are welcome!

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.