From 3f614535b99b6cda366c93c16107303eaebecec7 Mon Sep 17 00:00:00 2001
From: TheRawMeatball <therawmeatball@gmail.com>
Date: Tue, 5 Apr 2022 20:04:34 +0000
Subject: [PATCH] Add more FromWorld implementations (#3945)

# Objective

Make `FromWorld` more useful for abstractions with a form similar to
```rs
trait FancyAbstraction {
  type PreInitializedData: FromWorld;
}
```

## Solution

Add a `FromWorld` implementation for `SystemState` as well as a way to group together multiple `FromWorld` implementing types as one.

Note: I plan to follow up this PR with another to add `Local` support to exclusive systems, which should get a fair amount of use from the `FromWorld` implementation on `SystemState`.
---
 crates/bevy_ecs/src/query/state.rs            | 10 ++++++++++
 crates/bevy_ecs/src/system/function_system.rs |  7 +++++++
 2 files changed, 17 insertions(+)

diff --git a/crates/bevy_ecs/src/query/state.rs b/crates/bevy_ecs/src/query/state.rs
index 3decc6def908a..5956e69564661 100644
--- a/crates/bevy_ecs/src/query/state.rs
+++ b/crates/bevy_ecs/src/query/state.rs
@@ -2,6 +2,7 @@ use crate::{
     archetype::{Archetype, ArchetypeComponentId, ArchetypeGeneration, ArchetypeId},
     component::ComponentId,
     entity::Entity,
+    prelude::FromWorld,
     query::{
         Access, Fetch, FetchState, FilterFetch, FilteredAccess, NopFetch, QueryCombinationIter,
         QueryIter, WorldQuery,
@@ -32,6 +33,15 @@ where
     pub(crate) filter_state: F::State,
 }
 
+impl<Q: WorldQuery, F: WorldQuery> FromWorld for QueryState<Q, F>
+where
+    F::Fetch: FilterFetch,
+{
+    fn from_world(world: &mut World) -> Self {
+        world.query_filtered()
+    }
+}
+
 impl<Q: WorldQuery, F: WorldQuery> QueryState<Q, F>
 where
     F::Fetch: FilterFetch,
diff --git a/crates/bevy_ecs/src/system/function_system.rs b/crates/bevy_ecs/src/system/function_system.rs
index c03d09684f844..f3cade700fce5 100644
--- a/crates/bevy_ecs/src/system/function_system.rs
+++ b/crates/bevy_ecs/src/system/function_system.rs
@@ -1,6 +1,7 @@
 use crate::{
     archetype::{Archetype, ArchetypeComponentId, ArchetypeGeneration, ArchetypeId},
     component::ComponentId,
+    prelude::FromWorld,
     query::{Access, FilteredAccessSet},
     schedule::SystemLabel,
     system::{
@@ -229,6 +230,12 @@ impl<Param: SystemParam> SystemState<Param> {
     }
 }
 
+impl<Param: SystemParam> FromWorld for SystemState<Param> {
+    fn from_world(world: &mut World) -> Self {
+        Self::new(world)
+    }
+}
+
 /// Conversion trait to turn something into a [`System`].
 ///
 /// Use this to get a system from a function. Also note that every system implements this trait as