From 8de03b0839661c24cca19cf2998ad342d13549ae Mon Sep 17 00:00:00 2001
From: Daniel McNab <36049421+DJMcNab@users.noreply.github.com>
Date: Sun, 10 Jul 2022 21:04:35 +0000
Subject: [PATCH] Extract resources into their target location (#5271)

# Objective

- Extracting resources currently always uses commands, which requires *at least* one additional move of the extracted value, as well as dynamic dispatch.
- Addresses https://github.com/bevyengine/bevy/pull/4402#discussion_r911634931

## Solution

- Write the resource into a `ResMut<R>` directly.
- Fall-back to commands if the resource hasn't been added yet.
---
 crates/bevy_render/Cargo.toml              |  1 +
 crates/bevy_render/src/extract_resource.rs | 24 +++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/crates/bevy_render/Cargo.toml b/crates/bevy_render/Cargo.toml
index a1643eafd5895..8d646a6d6f64a 100644
--- a/crates/bevy_render/Cargo.toml
+++ b/crates/bevy_render/Cargo.toml
@@ -34,6 +34,7 @@ bevy_core = { path = "../bevy_core", version = "0.8.0-dev" }
 bevy_derive = { path = "../bevy_derive", version = "0.8.0-dev" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.8.0-dev" }
 bevy_encase_derive = { path = "../bevy_encase_derive", version = "0.8.0-dev" }
+bevy_log = { path = "../bevy_log", version = "0.8.0-dev" }
 bevy_math = { path = "../bevy_math", version = "0.8.0-dev" }
 bevy_mikktspace = { path = "../bevy_mikktspace", version = "0.8.0-dev" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.8.0-dev", features = ["bevy"] }
diff --git a/crates/bevy_render/src/extract_resource.rs b/crates/bevy_render/src/extract_resource.rs
index 57be9ab7e1c82..0e57d1d123307 100644
--- a/crates/bevy_render/src/extract_resource.rs
+++ b/crates/bevy_render/src/extract_resource.rs
@@ -1,7 +1,7 @@
 use std::marker::PhantomData;
 
 use bevy_app::{App, Plugin};
-use bevy_ecs::system::{Commands, Res, Resource};
+use bevy_ecs::system::{Commands, Local, Res, ResMut, Resource};
 pub use bevy_render_macros::ExtractResource;
 
 use crate::{Extract, RenderApp, RenderStage};
@@ -38,12 +38,26 @@ impl<R: ExtractResource> Plugin for ExtractResourcePlugin<R> {
 }
 
 /// This system extracts the resource of the corresponding [`Resource`] type
-/// by cloning it.
 pub fn extract_resource<R: ExtractResource>(
     mut commands: Commands,
-    resource: Extract<Res<R::Source>>,
+    main_resource: Extract<Res<R::Source>>,
+    target_resource: Option<ResMut<R>>,
+    #[cfg(debug_assertions)] mut has_warned_on_remove: Local<bool>,
 ) {
-    if resource.is_changed() {
-        commands.insert_resource(R::extract_resource(&*resource));
+    if let Some(mut target_resource) = target_resource {
+        if main_resource.is_changed() {
+            *target_resource = R::extract_resource(&*main_resource);
+        }
+    } else {
+        #[cfg(debug_assertions)]
+        if !main_resource.is_added() && !*has_warned_on_remove {
+            *has_warned_on_remove = true;
+            bevy_log::warn!(
+                "Removing resource {} from render world not expected, adding using `Commands`.
+                This may decrease performance",
+                std::any::type_name::<R>()
+            );
+        }
+        commands.insert_resource(R::extract_resource(&*main_resource));
     }
 }