Refactor the render instance logic in #9903 so that it's easier for other components to adopt. (#10002)

# Objective

Currently, the only way for custom components that participate in
rendering to opt into the higher-performance extraction method in #9903
is to implement the `RenderInstances` data structure and the extraction
logic manually. This is inconvenient compared to the `ExtractComponent`
API.

## Solution

This commit creates a new `RenderInstance` trait that mirrors the
existing `ExtractComponent` method but uses the higher-performance
approach that #9903 uses. Additionally, `RenderInstance` is more
flexible than `ExtractComponent`, because it can extract multiple
components at once. This makes high-performance rendering components
essentially as easy to write as the existing ones based on component
extraction.

---

## Changelog

### Added

A new `RenderInstance` trait is available mirroring `ExtractComponent`,
but using a higher-performance method to extract one or more components
to the render world. If you have custom components that rendering takes
into account, you may consider migration from `ExtractComponent` to
`RenderInstance` for higher performance.

crates/bevy_pbr/src/material.rs

@@ -20,6 +20,7 @@ use bevy_render::{
     mesh::{Mesh, MeshVertexBufferLayout},
     prelude::Image,
     render_asset::{prepare_assets, RenderAssets},
+    render_instances::{RenderInstancePlugin, RenderInstances},
     render_phase::{
         AddRenderCommand, DrawFunctions, PhaseItem, RenderCommand, RenderCommandResult,
         RenderPhase, SetItemPipeline, TrackedRenderPass,
@@ -31,10 +32,10 @@ use bevy_render::{
     },
     renderer::RenderDevice,
     texture::FallbackImage,
-    view::{ExtractedView, Msaa, ViewVisibility, VisibleEntities},
+    view::{ExtractedView, Msaa, VisibleEntities},
     Extract, ExtractSchedule, Render, RenderApp, RenderSet,
 };
-use bevy_utils::{tracing::error, EntityHashMap, HashMap, HashSet};
+use bevy_utils::{tracing::error, HashMap, HashSet};
 use std::hash::Hash;
 use std::marker::PhantomData;
 
@@ -176,7 +177,8 @@ where
     M::Data: PartialEq + Eq + Hash + Clone,
 {
     fn build(&self, app: &mut App) {
-        app.init_asset::<M>();
+        app.init_asset::<M>()
+            .add_plugins(RenderInstancePlugin::<AssetId<M>>::extract_visible());
 
         if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
             render_app
@@ -187,12 +189,8 @@ where
                 .add_render_command::<AlphaMask3d, DrawMaterial<M>>()
                 .init_resource::<ExtractedMaterials<M>>()
                 .init_resource::<RenderMaterials<M>>()
-                .init_resource::<RenderMaterialInstances<M>>()
                 .init_resource::<SpecializedMeshPipelines<MaterialPipeline<M>>>()
-                .add_systems(
-                    ExtractSchedule,
-                    (extract_materials::<M>, extract_material_meshes::<M>),
-                )
+                .add_systems(ExtractSchedule, extract_materials::<M>)
                 .add_systems(
                     Render,
                     (
@@ -372,26 +370,7 @@ impl<P: PhaseItem, M: Material, const I: usize> RenderCommand<P> for SetMaterial
     }
 }
 
-#[derive(Resource, Deref, DerefMut)]
-pub struct RenderMaterialInstances<M: Material>(EntityHashMap<Entity, AssetId<M>>);
-
-impl<M: Material> Default for RenderMaterialInstances<M> {
-    fn default() -> Self {
-        Self(Default::default())
-    }
-}
-
-fn extract_material_meshes<M: Material>(
-    mut material_instances: ResMut<RenderMaterialInstances<M>>,
-    query: Extract<Query<(Entity, &ViewVisibility, &Handle<M>)>>,
-) {
-    material_instances.clear();
-    for (entity, view_visibility, handle) in &query {
-        if view_visibility.get() {
-            material_instances.insert(entity, handle.id());
-        }
-    }
-}
+pub type RenderMaterialInstances<M> = RenderInstances<AssetId<M>>;
 
 const fn alpha_mode_pipeline_key(alpha_mode: AlphaMode) -> MeshPipelineKey {
     match alpha_mode {

crates/bevy_render/src/lib.rs

@@ -18,6 +18,7 @@ pub mod pipelined_rendering;
 pub mod primitives;
 pub mod render_asset;
 pub mod render_graph;
+pub mod render_instances;
 pub mod render_phase;
 pub mod render_resource;
 pub mod renderer;

crates/bevy_render/src/render_instances.rs

@@ -0,0 +1,153 @@
+//! Convenience logic for turning components from the main world into render
+//! instances in the render world.
+//!
+//! This is essentially the same as the `extract_component` module, but
+//! higher-performance because it avoids the ECS overhead.
+
+use std::marker::PhantomData;
+
+use bevy_app::{App, Plugin};
+use bevy_asset::{Asset, AssetId, Handle};
+use bevy_derive::{Deref, DerefMut};
+use bevy_ecs::{
+    prelude::Entity,
+    query::{QueryItem, ReadOnlyWorldQuery, WorldQuery},
+    system::{lifetimeless::Read, Query, ResMut, Resource},
+};
+use bevy_utils::EntityHashMap;
+
+use crate::{prelude::ViewVisibility, Extract, ExtractSchedule, RenderApp};
+
+/// Describes how to extract data needed for rendering from a component or
+/// components.
+///
+/// Before rendering, any applicable components will be transferred from the
+/// main world to the render world in the [`ExtractSchedule`] step.
+///
+/// This is essentially the same as
+/// [`ExtractComponent`](crate::extract_component::ExtractComponent), but
+/// higher-performance because it avoids the ECS overhead.
+pub trait RenderInstance: Send + Sync + Sized + 'static {
+    /// ECS [`WorldQuery`] to fetch the components to extract.
+    type Query: WorldQuery + ReadOnlyWorldQuery;
+    /// Filters the entities with additional constraints.
+    type Filter: WorldQuery + ReadOnlyWorldQuery;
+
+    /// Defines how the component is transferred into the "render world".
+    fn extract_to_render_instance(item: QueryItem<'_, Self::Query>) -> Option<Self>;
+}
+
+/// This plugin extracts one or more components into the "render world" as
+/// render instances.
+///
+/// Therefore it sets up the [`ExtractSchedule`] step for the specified
+/// [`RenderInstances`].
+#[derive(Default)]
+pub struct RenderInstancePlugin<RI>
+where
+    RI: RenderInstance,
+{
+    only_extract_visible: bool,
+    marker: PhantomData<fn() -> RI>,
+}
+
+/// Stores all render instances of a type in the render world.
+#[derive(Resource, Deref, DerefMut)]
+pub struct RenderInstances<RI>(EntityHashMap<Entity, RI>)
+where
+    RI: RenderInstance;
+
+impl<RI> Default for RenderInstances<RI>
+where
+    RI: RenderInstance,
+{
+    fn default() -> Self {
+        Self(Default::default())
+    }
+}
+
+impl<RI> RenderInstancePlugin<RI>
+where
+    RI: RenderInstance,
+{
+    /// Creates a new [`RenderInstancePlugin`] that unconditionally extracts to
+    /// the render world, whether the entity is visible or not.
+    pub fn new() -> Self {
+        Self {
+            only_extract_visible: false,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<RI> RenderInstancePlugin<RI>
+where
+    RI: RenderInstance,
+{
+    /// Creates a new [`RenderInstancePlugin`] that extracts to the render world
+    /// if and only if the entity it's attached to is visible.
+    pub fn extract_visible() -> Self {
+        Self {
+            only_extract_visible: true,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<RI> Plugin for RenderInstancePlugin<RI>
+where
+    RI: RenderInstance,
+{
+    fn build(&self, app: &mut App) {
+        if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
+            render_app.init_resource::<RenderInstances<RI>>();
+            if self.only_extract_visible {
+                render_app.add_systems(ExtractSchedule, extract_visible_to_render_instances::<RI>);
+            } else {
+                render_app.add_systems(ExtractSchedule, extract_to_render_instances::<RI>);
+            }
+        }
+    }
+}
+
+fn extract_to_render_instances<RI>(
+    mut instances: ResMut<RenderInstances<RI>>,
+    query: Extract<Query<(Entity, RI::Query), RI::Filter>>,
+) where
+    RI: RenderInstance,
+{
+    instances.clear();
+    for (entity, other) in &query {
+        if let Some(render_instance) = RI::extract_to_render_instance(other) {
+            instances.insert(entity, render_instance);
+        }
+    }
+}
+
+fn extract_visible_to_render_instances<RI>(
+    mut instances: ResMut<RenderInstances<RI>>,
+    query: Extract<Query<(Entity, &ViewVisibility, RI::Query), RI::Filter>>,
+) where
+    RI: RenderInstance,
+{
+    instances.clear();
+    for (entity, view_visibility, other) in &query {
+        if view_visibility.get() {
+            if let Some(render_instance) = RI::extract_to_render_instance(other) {
+                instances.insert(entity, render_instance);
+            }
+        }
+    }
+}
+
+impl<A> RenderInstance for AssetId<A>
+where
+    A: Asset,
+{
+    type Query = Read<Handle<A>>;
+    type Filter = ();
+
+    fn extract_to_render_instance(item: QueryItem<'_, Self::Query>) -> Option<Self> {
+        Some(item.id())
+    }
+}