From c8d30edf1a031e9ce3b79b45597a916f35cf4b43 Mon Sep 17 00:00:00 2001
From: databasedav <31483365+databasedav@users.noreply.github.com>
Date: Mon, 12 Aug 2024 09:11:06 -0700
Subject: [PATCH] add `SystemIdMarker` `Component` to enable filtering for
 `SystemId` `Entity`s (#14584)

# Objective

Enables writing queries like `Query<Entity, With<SystemIdMarker>>` to
filter `Entity`s that are, or are not (with `Without`), `SystemId`s.

## Solution

Simple unit struct `SystemIdMarker` added during
`World::register_boxed_system`; `World::remove_system` already despawns
the entity, removing the marker.

## Testing

No tests, but happy to write some with direction.

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
---
 crates/bevy_ecs/src/system/system_registry.rs | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/crates/bevy_ecs/src/system/system_registry.rs b/crates/bevy_ecs/src/system/system_registry.rs
index 10b5790318..f6cf792f71 100644
--- a/crates/bevy_ecs/src/system/system_registry.rs
+++ b/crates/bevy_ecs/src/system/system_registry.rs
@@ -12,6 +12,10 @@ struct RegisteredSystem<I, O> {
     system: BoxedSystem<I, O>,
 }
 
+/// Marker [`Component`](bevy_ecs::component::Component) for identifying [`SystemId`] [`Entity`]s.
+#[derive(Component)]
+pub struct SystemIdMarker;
+
 /// A system that has been removed from the registry.
 /// It contains the system and whether or not it has been initialized.
 ///
@@ -125,10 +129,13 @@ impl World {
     ) -> SystemId<I, O> {
         SystemId {
             entity: self
-                .spawn(RegisteredSystem {
-                    initialized: false,
-                    system,
-                })
+                .spawn((
+                    RegisteredSystem {
+                        initialized: false,
+                        system,
+                    },
+                    SystemIdMarker,
+                ))
                 .id(),
             marker: std::marker::PhantomData,
         }