bevy/crates/bevy_render/src/render_phase/draw.rs

use crate::render_phase::{PhaseItem, TrackedRenderPass};
use bevy_app::App;
use bevy_ecs::{
    all_tuples,
    entity::Entity,
    query::{QueryState, ROQueryItem, ReadOnlyWorldQuery},
    system::{ReadOnlySystemParam, Resource, SystemParam, SystemParamItem, SystemState},
    world::World,
};
use bevy_utils::HashMap;
use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
use std::{any::TypeId, fmt::Debug, hash::Hash};

/// A draw function used to draw [`PhaseItem`]s.
///
/// The draw function can retrieve and query the required ECS data from the render world.
///
/// This trait can either be implemented directly or implicitly composed out of multiple modular
/// [`RenderCommand`]s. For more details and an example see the [`RenderCommand`] documentation.
pub trait Draw<P: PhaseItem>: Send + Sync + 'static {
    /// Prepares the draw function to be used. This is called once and only once before the phase
    /// begins. There may be zero or more `draw` calls following a call to this function.
    /// Implementing this is optional.
    #[allow(unused_variables)]
    fn prepare(&mut self, world: &'_ World) {}

    /// Draws a [`PhaseItem`] by issuing zero or more `draw` calls via the [`TrackedRenderPass`].
    fn draw<'w>(
        &mut self,
        world: &'w World,
        pass: &mut TrackedRenderPass<'w>,
        view: Entity,
        item: &P,
    );
}

// TODO: make this generic?
/// An identifier for a [`Draw`] function stored in [`DrawFunctions`].
#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
pub struct DrawFunctionId(u32);

/// Stores all [`Draw`] functions for the [`PhaseItem`] type.
///
/// For retrieval, the [`Draw`] functions are mapped to their respective [`TypeId`]s.
pub struct DrawFunctionsInternal<P: PhaseItem> {
    pub draw_functions: Vec<Box<dyn Draw<P>>>,
    pub indices: HashMap<TypeId, DrawFunctionId>,
}

impl<P: PhaseItem> DrawFunctionsInternal<P> {
    /// Prepares all draw function. This is called once and only once before the phase begins.
    pub fn prepare(&mut self, world: &World) {
        for function in &mut self.draw_functions {
            function.prepare(world);
        }
    }

    /// Adds the [`Draw`] function and maps it to its own type.
    pub fn add<T: Draw<P>>(&mut self, draw_function: T) -> DrawFunctionId {
        self.add_with::<T, T>(draw_function)
    }

    /// Adds the [`Draw`] function and maps it to the type `T`
    pub fn add_with<T: 'static, D: Draw<P>>(&mut self, draw_function: D) -> DrawFunctionId {
        let id = DrawFunctionId(self.draw_functions.len().try_into().unwrap());
        self.draw_functions.push(Box::new(draw_function));
        self.indices.insert(TypeId::of::<T>(), id);
        id
    }

    /// Retrieves the [`Draw`] function corresponding to the `id` mutably.
    pub fn get_mut(&mut self, id: DrawFunctionId) -> Option<&mut dyn Draw<P>> {
        self.draw_functions.get_mut(id.0 as usize).map(|f| &mut **f)
    }

    /// Retrieves the id of the [`Draw`] function corresponding to their associated type `T`.
    pub fn get_id<T: 'static>(&self) -> Option<DrawFunctionId> {
        self.indices.get(&TypeId::of::<T>()).copied()
    }

    /// Retrieves the id of the [`Draw`] function corresponding to their associated type `T`.
    ///
    /// Fallible wrapper for [`Self::get_id()`]
    ///
    /// ## Panics
    /// If the id doesn't exist, this function will panic.
    pub fn id<T: 'static>(&self) -> DrawFunctionId {
        self.get_id::<T>().unwrap_or_else(|| {
            panic!(
                "Draw function {} not found for {}",
                std::any::type_name::<T>(),
                std::any::type_name::<P>()
            )
        })
    }
}

/// Stores all draw functions for the [`PhaseItem`] type hidden behind a reader-writer lock.
///
/// To access them the [`DrawFunctions::read`] and [`DrawFunctions::write`] methods are used.
#[derive(Resource)]
pub struct DrawFunctions<P: PhaseItem> {
    internal: RwLock<DrawFunctionsInternal<P>>,
}

impl<P: PhaseItem> Default for DrawFunctions<P> {
    fn default() -> Self {
        Self {
            internal: RwLock::new(DrawFunctionsInternal {
                draw_functions: Vec::new(),
                indices: HashMap::default(),
            }),
        }
    }
}

impl<P: PhaseItem> DrawFunctions<P> {
    /// Accesses the draw functions in read mode.
    pub fn read(&self) -> RwLockReadGuard<'_, DrawFunctionsInternal<P>> {
        self.internal.read()
    }

    /// Accesses the draw functions in write mode.
    pub fn write(&self) -> RwLockWriteGuard<'_, DrawFunctionsInternal<P>> {
        self.internal.write()
    }
}

/// [`RenderCommand`]s are modular standardized pieces of render logic that can be composed into
/// [`Draw`] functions.
///
/// To turn a stateless render command into a usable draw function it has to be wrapped by a
/// [`RenderCommandState`].
/// This is done automatically when registering a render command as a [`Draw`] function via the
/// [`AddRenderCommand::add_render_command`] method.
///
/// Compared to the draw function the required ECS data is fetched automatically
/// (by the [`RenderCommandState`]) from the render world.
/// Therefore the three types [`Param`](RenderCommand::Param),
/// [`ViewWorldQuery`](RenderCommand::ViewWorldQuery) and
/// [`ItemWorldQuery`](RenderCommand::ItemWorldQuery) are used.
/// They specify which information is required to execute the render command.
///
/// Multiple render commands can be combined together by wrapping them in a tuple.
///
/// # Example
/// The `DrawPbr` draw function is created from the following render command
/// tuple. Const generics are used to set specific bind group locations:
///
/// ```ignore
/// pub type DrawPbr = (
///     SetItemPipeline,
///     SetMeshViewBindGroup<0>,
///     SetStandardMaterialBindGroup<1>,
///     SetTransformBindGroup<2>,
///     DrawMesh,
/// );
/// ```
pub trait RenderCommand<P: PhaseItem> {
    /// Specifies the general ECS data (e.g. resources) required by [`RenderCommand::render`].
    ///
    /// All parameters have to be read only.
    type Param: SystemParam + 'static;
    /// Specifies the ECS data of the view entity required by [`RenderCommand::render`].
    ///
    /// The view entity refers to the camera, or shadow-casting light, etc. from which the phase
    /// item will be rendered from.
    /// All components have to be accessed read only.
    type ViewWorldQuery: ReadOnlyWorldQuery;
    /// Specifies the ECS data of the item entity required by [`RenderCommand::render`].
    ///
    /// The item is the entity that will be rendered for the corresponding view.
    /// All components have to be accessed read only.
    type ItemWorldQuery: ReadOnlyWorldQuery;

    /// Renders a [`PhaseItem`] by recording commands (e.g. setting pipelines, binding bind groups,
    /// issuing draw calls, etc.) via the [`TrackedRenderPass`].
    fn render<'w>(
        item: &P,
        view: ROQueryItem<'w, Self::ViewWorldQuery>,
        entity: ROQueryItem<'w, Self::ItemWorldQuery>,
        param: SystemParamItem<'w, '_, Self::Param>,
        pass: &mut TrackedRenderPass<'w>,
    ) -> RenderCommandResult;
}

/// The result of a [`RenderCommand`].
pub enum RenderCommandResult {
    Success,
    Failure,
}

macro_rules! render_command_tuple_impl {
    ($(($name: ident, $view: ident, $entity: ident)),*) => {
        impl<P: PhaseItem, $($name: RenderCommand<P>),*> RenderCommand<P> for ($($name,)*) {
            type Param = ($($name::Param,)*);
            type ViewWorldQuery = ($($name::ViewWorldQuery,)*);
            type ItemWorldQuery = ($($name::ItemWorldQuery,)*);

            #[allow(non_snake_case)]
            fn render<'w>(
                _item: &P,
                ($($view,)*): ROQueryItem<'w, Self::ViewWorldQuery>,
                ($($entity,)*): ROQueryItem<'w, Self::ItemWorldQuery>,
                ($($name,)*): SystemParamItem<'w, '_, Self::Param>,
                _pass: &mut TrackedRenderPass<'w>,
            ) -> RenderCommandResult {
                $(if let RenderCommandResult::Failure = $name::render(_item, $view, $entity, $name, _pass) {
                    return RenderCommandResult::Failure;
                })*
                RenderCommandResult::Success
            }
        }
    };
}

all_tuples!(render_command_tuple_impl, 0, 15, C, V, E);

/// Wraps a [`RenderCommand`] into a state so that it can be used as a [`Draw`] function.
///
/// The [`RenderCommand::Param`], [`RenderCommand::ViewWorldQuery`] and
/// [`RenderCommand::ItemWorldQuery`] are fetched from the ECS and passed to the command.
pub struct RenderCommandState<P: PhaseItem + 'static, C: RenderCommand<P>> {
    state: SystemState<C::Param>,
    view: QueryState<C::ViewWorldQuery>,
    entity: QueryState<C::ItemWorldQuery>,
}

impl<P: PhaseItem, C: RenderCommand<P>> RenderCommandState<P, C> {
    /// Creates a new [`RenderCommandState`] for the [`RenderCommand`].
    pub fn new(world: &mut World) -> Self {
        Self {
            state: SystemState::new(world),
            view: world.query(),
            entity: world.query(),
        }
    }
}

impl<P: PhaseItem, C: RenderCommand<P> + Send + Sync + 'static> Draw<P> for RenderCommandState<P, C>
where
    C::Param: ReadOnlySystemParam,
{
    /// Prepares the render command to be used. This is called once and only once before the phase
    /// begins. There may be zero or more `draw` calls following a call to this function.
    fn prepare(&mut self, world: &'_ World) {
        self.state.update_archetypes(world);
        self.view.update_archetypes(world);
        self.entity.update_archetypes(world);
    }

    /// Fetches the ECS parameters for the wrapped [`RenderCommand`] and then renders it.
    fn draw<'w>(
        &mut self,
        world: &'w World,
        pass: &mut TrackedRenderPass<'w>,
        view: Entity,
        item: &P,
    ) {
        let param = self.state.get_manual(world);
        let view = self.view.get_manual(world, view).unwrap();
        let entity = self.entity.get_manual(world, item.entity()).unwrap();
        // TODO: handle/log `RenderCommand` failure
        C::render(item, view, entity, param, pass);
    }
}

/// Registers a [`RenderCommand`] as a [`Draw`] function.
/// They are stored inside the [`DrawFunctions`] resource of the app.
pub trait AddRenderCommand {
    /// Adds the [`RenderCommand`] for the specified [`RenderPhase`](super::RenderPhase) to the app.
    fn add_render_command<P: PhaseItem, C: RenderCommand<P> + Send + Sync + 'static>(
        &mut self,
    ) -> &mut Self
    where
        C::Param: ReadOnlySystemParam;
}

impl AddRenderCommand for App {
    fn add_render_command<P: PhaseItem, C: RenderCommand<P> + Send + Sync + 'static>(
        &mut self,
    ) -> &mut Self
    where
        C::Param: ReadOnlySystemParam,
    {
        let draw_function = RenderCommandState::<P, C>::new(&mut self.world);
        let draw_functions = self
            .world
            .get_resource::<DrawFunctions<P>>()
            .unwrap_or_else(|| {
                panic!(
                    "DrawFunctions<{}> must be added to the world as a resource \
                     before adding render commands to it",
                    std::any::type_name::<P>(),
                );
            });
        draw_functions.write().add_with::<C, _>(draw_function);
        self
    }
}