bevy/crates/bevy_render/src/pipelined_rendering.rs

use async_channel::{Receiver, Sender};

use bevy_app::{App, AppLabel, Main, Plugin, SubApp};
use bevy_ecs::{
    schedule::MainThreadExecutor,
    system::Resource,
    world::{Mut, World},
};
use bevy_tasks::ComputeTaskPool;

use crate::RenderApp;

/// A Label for the sub app that runs the parts of pipelined rendering that need to run on the main thread.
///
/// The Main schedule of this app can be used to run logic after the render schedule starts, but
/// before I/O processing. This can be useful for something like frame pacing.
#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, AppLabel)]
pub struct RenderExtractApp;

/// Channels used by the main app to send and receive the render app.
#[derive(Resource)]
pub struct RenderAppChannels {
    app_to_render_sender: Sender<SubApp>,
    render_to_app_receiver: Receiver<SubApp>,
    render_app_in_render_thread: bool,
}

impl RenderAppChannels {
    /// Create a `RenderAppChannels` from a [`async_channel::Receiver`] and [`async_channel::Sender`]
    pub fn new(
        app_to_render_sender: Sender<SubApp>,
        render_to_app_receiver: Receiver<SubApp>,
    ) -> Self {
        Self {
            app_to_render_sender,
            render_to_app_receiver,
            render_app_in_render_thread: false,
        }
    }

    /// Send the `render_app` to the rendering thread.
    pub fn send_blocking(&mut self, render_app: SubApp) {
        self.app_to_render_sender.send_blocking(render_app).unwrap();
        self.render_app_in_render_thread = true;
    }

    /// Receive the `render_app` from the rendering thread.
    pub async fn recv(&mut self) -> SubApp {
        let render_app = self.render_to_app_receiver.recv().await.unwrap();
        self.render_app_in_render_thread = false;
        render_app
    }
}

impl Drop for RenderAppChannels {
    fn drop(&mut self) {
        if self.render_app_in_render_thread {
            // Any non-send data in the render world was initialized on the main thread.
            // So on dropping the main world and ending the app, we block and wait for
            // the render world to return to drop it. Which allows the non-send data
            // drop methods to run on the correct thread.
            self.render_to_app_receiver.recv_blocking().ok();
        }
    }
}

/// The [`PipelinedRenderingPlugin`] can be added to your application to enable pipelined rendering.
/// This moves rendering into a different thread, so that the Nth frame's rendering can
/// be run at the same time as the N + 1 frame's simulation.
///
/// ```text
/// |--------------------|--------------------|--------------------|--------------------|
/// | simulation thread  | frame 1 simulation | frame 2 simulation | frame 3 simulation |
/// |--------------------|--------------------|--------------------|--------------------|
/// | rendering thread   |                    | frame 1 rendering  | frame 2 rendering  |
/// |--------------------|--------------------|--------------------|--------------------|
/// ```
///
/// The plugin is dependent on the [`RenderApp`] added by [`crate::RenderPlugin`] and so must
/// be added after that plugin. If it is not added after, the plugin will do nothing.
///
/// A single frame of execution looks something like below    
///
/// ```text
/// |--------------------------------------------------------------------|
/// |         | RenderExtractApp schedule | winit events | main schedule |
/// | extract |----------------------------------------------------------|
/// |         | extract commands | rendering schedule                    |
/// |--------------------------------------------------------------------|
/// ```
///
/// - `extract` is the step where data is copied from the main world to the render world.
/// This is run on the main app's thread.
/// - On the render thread, we first apply the `extract commands`. This is not run during extract, so the
/// main schedule can start sooner.
/// - Then the `rendering schedule` is run. See [`RenderSet`](crate::RenderSet) for the standard steps in this process.
/// - In parallel to the rendering thread the [`RenderExtractApp`] schedule runs. By
/// default this schedule is empty. But it is useful if you need something to run before I/O processing.
/// - Next all the `winit events` are processed.
/// - And finally the `main app schedule` is run.
/// - Once both the `main app schedule` and the `render schedule` are finished running, `extract` is run again.
#[derive(Default)]
pub struct PipelinedRenderingPlugin;

impl Plugin for PipelinedRenderingPlugin {
    fn build(&self, app: &mut App) {
        // Don't add RenderExtractApp if RenderApp isn't initialized.
        if app.get_sub_app(RenderApp).is_err() {
            return;
        }
        app.insert_resource(MainThreadExecutor::new());

        let mut sub_app = App::empty();
        sub_app.init_schedule(Main);
        app.insert_sub_app(RenderExtractApp, SubApp::new(sub_app, update_rendering));
    }

    // Sets up the render thread and inserts resources into the main app used for controlling the render thread.
    fn cleanup(&self, app: &mut App) {
        // skip setting up when headless
        if app.get_sub_app(RenderExtractApp).is_err() {
            return;
        }

        let (app_to_render_sender, app_to_render_receiver) = async_channel::bounded::<SubApp>(1);
        let (render_to_app_sender, render_to_app_receiver) = async_channel::bounded::<SubApp>(1);

        let mut render_app = app
            .remove_sub_app(RenderApp)
            .expect("Unable to get RenderApp. Another plugin may have removed the RenderApp before PipelinedRenderingPlugin");

        // clone main thread executor to render world
        let executor = app.world.get_resource::<MainThreadExecutor>().unwrap();
        render_app.app.world.insert_resource(executor.clone());

        render_to_app_sender.send_blocking(render_app).unwrap();

        app.insert_resource(RenderAppChannels::new(
            app_to_render_sender,
            render_to_app_receiver,
        ));

        std::thread::spawn(move || {
            #[cfg(feature = "trace")]
            let _span = bevy_utils::tracing::info_span!("render thread").entered();

            let compute_task_pool = ComputeTaskPool::get();
            loop {
                // run a scope here to allow main world to use this thread while it's waiting for the render app
                let sent_app = compute_task_pool
                    .scope(|s| {
                        s.spawn(async { app_to_render_receiver.recv().await });
                    })
                    .pop();
                let Some(Ok(mut render_app)) = sent_app else {
                    break;
                };

                {
                    #[cfg(feature = "trace")]
                    let _sub_app_span =
                        bevy_utils::tracing::info_span!("sub app", name = ?RenderApp).entered();
                    render_app.run();
                }

                if render_to_app_sender.send_blocking(render_app).is_err() {
                    break;
                }
            }

            bevy_utils::tracing::debug!("exiting pipelined rendering thread");
        });
    }
}

// This function waits for the rendering world to be received,
// runs extract, and then sends the rendering world back to the render thread.
fn update_rendering(app_world: &mut World, _sub_app: &mut App) {
    app_world.resource_scope(|world, main_thread_executor: Mut<MainThreadExecutor>| {
        world.resource_scope(|world, mut render_channels: Mut<RenderAppChannels>| {
            // we use a scope here to run any main thread tasks that the render world still needs to run
            // while we wait for the render world to be received.
            let mut render_app = ComputeTaskPool::get()
                .scope_with_executor(true, Some(&*main_thread_executor.0), |s| {
                    s.spawn(async { render_channels.recv().await });
                })
                .pop()
                .unwrap();

            render_app.extract(world);

            render_channels.send_blocking(render_app);
        });
    });
}