Bevy Defer

A simple asynchronous runtime for executing async coroutines.

Motivation

Async rust is incredible for modelling wait centric tasks like coroutines. Not utilizing async in game development is a huge waste of potential.

Imagine we want to model a rapid sword attack animation, in async rust this is straightforward:

swing_animation().await;
show_damage_number().await;
damage_vfx().await;

swing_animation().await;
show_damage_number().await;
damage_vfx().await;

At each await point we wait for something to complete, without wasting resources spin looping a thread or defining a complex state machine in a system.

What if we want damage number and damage vfx to run concurrently and wait for both before our next attack? It's simple with async semantics!

futures::join! {
    show_damage_number(),
    damage_vfx()
};

swing_animation().await;

Why not 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.remove(world);
}

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!

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 {
    // Wait for state to be `GameState::Animating`.
    AsyncWorld.state_stream::<GameState>().filter(|x| x == &GameState::Animating).next().await;
    // Obtain info from a resource.
    // Since the `World` stored as a thread local, 
    // a closure is the preferable syntax to access it.
    let richard_entity = AsyncWorld.resource::<NamedEntities>()
        .get(|res| *res.get("Richard").unwrap())?;
    // Move to an entity's scope, does not verify the entity exists.
    let richard = AsyncWorld.entity(richard_entity);
    // We can also mutate the world directly.
    richard.component::<HP>().set(|hp| hp.set(500))?;
    // Move to a component's scope, does not verify the entity or component exists.
    let animator = AsyncWorld.component::<Animator>();
    // Implementing `AsyncComponentDeref` allows you to add extension methods to `AsyncComponent`.
    animator.animate("Wave").await?;
    // Spawn another future on the executor.
    let audio = AsyncWorld.spawn(sound_routine(richard_entity));
    // Dance for 5 seconds with `select`.
    futures::select!(
        _ = animator.animate("Dance").fuse() => (),
        _ = AsyncWorld.sleep(Duration::from_secs(5)) => println!("Dance cancelled"),
    );
    // animate back to idle
    richard.component::<Animator>().animate("Idle").await?;
    // Wait for spawned future to complete
    audio.await?;
    // Tell the bevy App to quit.
    AsyncWorld.quit();
    Ok(())
});

World Accessors

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
    }
)

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.

We do not provide a AsyncSystemParam, instead you should use one-shot system based API on AsyncWorld. They can cover all uses cases where you need to running systems in bevy_defer.

Async Basics

Here are some common utilities you might find useful from an async ecosystem.

• AsyncWorld.spawn() spawns a future.

• AsyncWorld.spawn_scoped() spawns a future with a handle to get result from.

• AsyncWorld.yield_now() yields execution for the current frame, similar to how coroutines work.

• AsyncWorld.sleep(4.0) pauses the future for 4 seconds.

• AsyncWorld.sleep_frames(4) pauses the future for 4 frames.

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 in an async function, then a system can listen for that particular change in sync code.

async {
    entity.component::<IsJumping>().set(|j| *j == true);
}

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

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

Signals and AsyncSystems

AsyncSystems and Signals provides per-entity reactivity for user interfaces. Checkout their respective modules for more information.

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. The AsyncComputeTaskPool in bevy_tasks is ideal for this use case. We can use AsyncComputeTaskPool::get().spawn() to spawn a future on task pool and call await in bevy_defer.

Usage Tips

The futures and/or futures_lite crate has excellent tools to for us to use.

For example futures::join! can be used to run tasks concurrently, and futures::select! can be used to cancel tasks, for example despawning a task if a level has finished.

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.