One Shot Systems (#8963)

I'm adopting this ~~child~~ PR.

# Objective

- Working with exclusive world access is not always easy: in many cases,
a standard system or three is more ergonomic to write, and more
modularly maintainable.
- For small, one-off tasks (commonly handled with scripting), running an
event-reader system incurs a small but flat overhead cost and muddies
the schedule.
- Certain forms of logic (e.g. turn-based games) want very fine-grained
linear and/or branching control over logic.
- SystemState is not automatically cached, and so performance can suffer
and change detection breaks.
- Fixes #2192.
- Partial workaround for #279.

## Solution

- Adds a SystemRegistry resource to the World, which stores initialized
systems keyed by their SystemSet.
- Allows users to call world.run_system(my_system) and
commands.run_system(my_system), without re-initializing or losing state
(essential for change detection).
- Add a Callback type to enable convenient use of dynamic one shot
systems and reduce the mental overhead of working with Box<dyn
SystemSet>.
- Allow users to run systems based on their SystemSet, enabling more
complex user-made abstractions.

## Future work

- Parameterized one-shot systems would improve reusability and bring
them closer to events and commands. The API could be something like
run_system_with_input(my_system, my_input) and use the In SystemParam.
- We should evaluate the unification of commands and one-shot systems
since they are two different ways to run logic on demand over a World.

### Prior attempts

- #2234
- #2417
- #4090
- #7999

This PR continues the work done in
#7999.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Federico Rinaldi <gisquerin@gmail.com>
Co-authored-by: MinerSebas <66798382+MinerSebas@users.noreply.github.com>
Co-authored-by: Aevyrie <aevyrie@gmail.com>
Co-authored-by: Alejandro Pascual Pozo <alejandro.pascual.pozo@gmail.com>
Co-authored-by: Rob Parrett <robparrett@gmail.com>
Co-authored-by: François <mockersf@gmail.com>
Co-authored-by: Dmytro Banin <banind@cs.washington.edu>
Co-authored-by: James Liu <contact@jamessliu.com>

Cargo.toml

@@ -1284,6 +1284,16 @@ description = "Shows how to iterate over combinations of query results"
 category = "ECS (Entity Component System)"
 wasm = true
 
+[[example]]
+name = "one_shot_systems"
+path = "examples/ecs/one_shot_systems.rs"
+
+[package.metadata.example.one_shot_systems]
+name = "One Shot Systems"
+description = "Shows how to flexibly run systems without scheduling them"
+category = "ECS (Entity Component System)"
+wasm = false
+
 [[example]]
 name = "parallel_query"
 path = "examples/ecs/parallel_query.rs"

crates/bevy_ecs/src/schedule/set.rs

@@ -81,6 +81,7 @@ impl<T> Hash for SystemTypeSet<T> {
         // all systems of a given type are the same
     }
 }
+
 impl<T> Clone for SystemTypeSet<T> {
     fn clone(&self) -> Self {
         *self

crates/bevy_ecs/src/system/commands/mod.rs

@@ -5,6 +5,7 @@ use crate::{
     self as bevy_ecs,
     bundle::Bundle,
     entity::{Entities, Entity},
+    system::{RunSystem, SystemId},
     world::{EntityWorldMut, FromWorld, World},
 };
 use bevy_ecs_macros::SystemParam;
@@ -517,11 +518,19 @@ impl<'w, 's> Commands<'w, 's> {
         self.queue.push(RemoveResource::<R>::new());
     }
 
+    /// Runs the system corresponding to the given [`SystemId`].
+    /// Systems are ran in an exclusive and single threaded way.
+    /// Running slow systems can become a bottleneck.
+    ///
+    /// Calls [`World::run_system`](crate::system::World::run_system).
+    pub fn run_system(&mut self, id: SystemId) {
+        self.queue.push(RunSystem::new(id));
+    }
+
     /// Pushes a generic [`Command`] to the command queue.
     ///
     /// `command` can be a built-in command, custom struct that implements [`Command`] or a closure
     /// that takes [`&mut World`](World) as an argument.
-    ///
     /// # Example
     ///
     /// ```

crates/bevy_ecs/src/system/function_system.rs

@@ -72,8 +72,10 @@ impl SystemMeta {
 // (to avoid the need for unwrapping to retrieve SystemMeta)
 /// Holds on to persistent state required to drive [`SystemParam`] for a [`System`].
 ///
-/// This is a very powerful and convenient tool for working with exclusive world access,
+/// This is a powerful and convenient tool for working with exclusive world access,
 /// allowing you to fetch data from the [`World`] as if you were running a [`System`].
+/// However, simply calling `world::run_system(my_system)` using a [`World::run_system`](crate::system::World::run_system)
+/// can be significantly simpler and ensures that change detection and command flushing work as expected.
 ///
 /// Borrow-checking is handled for you, allowing you to mutably access multiple compatible system parameters at once,
 /// and arbitrary system parameters (like [`EventWriter`](crate::event::EventWriter)) can be conveniently fetched.
@@ -89,6 +91,8 @@ impl SystemMeta {
 /// - [`Local`](crate::system::Local) variables that hold state
 /// - [`EventReader`](crate::event::EventReader) system parameters, which rely on a [`Local`](crate::system::Local) to track which events have been seen
 ///
+/// Note that this is automatically handled for you when using a [`World::run_system`](crate::system::World::run_system).
+///
 /// # Example
 ///
 /// Basic usage:

crates/bevy_ecs/src/system/mod.rs

@@ -112,6 +112,7 @@ mod query;
 #[allow(clippy::module_inception)]
 mod system;
 mod system_param;
+mod system_registry;
 
 use std::borrow::Cow;
 
@@ -124,6 +125,7 @@ pub use function_system::*;
 pub use query::*;
 pub use system::*;
 pub use system_param::*;
+pub use system_registry::*;
 
 use crate::world::World;
 

crates/bevy_ecs/src/system/system.rs

@@ -165,6 +165,30 @@ impl<In: 'static, Out: 'static> Debug for dyn System<In = In, Out = Out> {
 /// This function is not an efficient method of running systems and its meant to be used as a utility
 /// for testing and/or diagnostics.
 ///
+/// Systems called through [`run_system_once`](crate::system::RunSystemOnce::run_system_once) do not hold onto any state,
+/// as they are created and destroyed every time [`run_system_once`](crate::system::RunSystemOnce::run_system_once) is called.
+/// Practically, this means that [`Local`](crate::system::Local) variables are
+/// reset on every run and change detection does not work.
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// # use bevy_ecs::system::RunSystemOnce;
+/// #[derive(Resource, Default)]
+/// struct Counter(u8);
+///
+/// fn increment(mut counter: Local<Counter>) {
+///    counter.0 += 1;
+///    println!("{}", counter.0);
+/// }
+///
+/// let mut world = World::default();
+/// world.run_system_once(increment); // prints 1
+/// world.run_system_once(increment); // still prints 1
+/// ```
+///
+/// If you do need systems to hold onto state between runs, use the [`World::run_system`](crate::system::World::run_system)
+/// and run the system by their [`SystemId`](crate::system::SystemId).
+///
 /// # Usage
 /// Typically, to test a system, or to extract specific diagnostics information from a world,
 /// you'd need a [`Schedule`](crate::schedule::Schedule) to run the system. This can create redundant boilerplate code
@@ -180,9 +204,9 @@ impl<In: 'static, Out: 'static> Debug for dyn System<In = In, Out = Out> {
 /// This usage is helpful when trying to test systems or functions that operate on [`Commands`](crate::system::Commands):
 /// ```
 /// # use bevy_ecs::prelude::*;
-/// # use bevy_ecs::system::RunSystem;
+/// # use bevy_ecs::system::RunSystemOnce;
 /// let mut world = World::default();
-/// let entity = world.run_system(|mut commands: Commands| {
+/// let entity = world.run_system_once(|mut commands: Commands| {
 ///     commands.spawn_empty().id()
 /// });
 /// # assert!(world.get_entity(entity).is_some());
@@ -193,7 +217,7 @@ impl<In: 'static, Out: 'static> Debug for dyn System<In = In, Out = Out> {
 /// This usage is helpful when trying to run an arbitrary query on a world for testing or debugging purposes:
 /// ```
 /// # use bevy_ecs::prelude::*;
-/// # use bevy_ecs::system::RunSystem;
+/// # use bevy_ecs::system::RunSystemOnce;
 ///
 /// #[derive(Component)]
 /// struct T(usize);
@@ -202,7 +226,7 @@ impl<In: 'static, Out: 'static> Debug for dyn System<In = In, Out = Out> {
 /// world.spawn(T(0));
 /// world.spawn(T(1));
 /// world.spawn(T(1));
-/// let count = world.run_system(|query: Query<&T>| {
+/// let count = world.run_system_once(|query: Query<&T>| {
 ///     query.iter().filter(|t| t.0 == 1).count()
 /// });
 ///
@@ -213,7 +237,7 @@ impl<In: 'static, Out: 'static> Debug for dyn System<In = In, Out = Out> {
 ///
 /// ```
 /// # use bevy_ecs::prelude::*;
-/// # use bevy_ecs::system::RunSystem;
+/// # use bevy_ecs::system::RunSystemOnce;
 ///
 /// #[derive(Component)]
 /// struct T(usize);
@@ -226,26 +250,26 @@ impl<In: 'static, Out: 'static> Debug for dyn System<In = In, Out = Out> {
 /// world.spawn(T(0));
 /// world.spawn(T(1));
 /// world.spawn(T(1));
-/// let count = world.run_system(count);
+/// let count = world.run_system_once(count);
 ///
 /// # assert_eq!(count, 2);
 /// ```
-pub trait RunSystem: Sized {
+pub trait RunSystemOnce: Sized {
     /// Runs a system and applies its deferred parameters.
-    fn run_system<T: IntoSystem<(), Out, Marker>, Out, Marker>(self, system: T) -> Out {
-        self.run_system_with((), system)
+    fn run_system_once<T: IntoSystem<(), Out, Marker>, Out, Marker>(self, system: T) -> Out {
+        self.run_system_once_with((), system)
     }
 
     /// Runs a system with given input and applies its deferred parameters.
-    fn run_system_with<T: IntoSystem<In, Out, Marker>, In, Out, Marker>(
+    fn run_system_once_with<T: IntoSystem<In, Out, Marker>, In, Out, Marker>(
         self,
         input: In,
         system: T,
     ) -> Out;
 }
 
-impl RunSystem for &mut World {
-    fn run_system_with<T: IntoSystem<In, Out, Marker>, In, Out, Marker>(
+impl RunSystemOnce for &mut World {
+    fn run_system_once_with<T: IntoSystem<In, Out, Marker>, In, Out, Marker>(
         self,
         input: In,
         system: T,
@@ -261,10 +285,11 @@ impl RunSystem for &mut World {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate as bevy_ecs;
     use crate::prelude::*;
 
     #[test]
-    fn run_system() {
+    fn run_system_once() {
         struct T(usize);
 
         impl Resource for T {}
@@ -275,8 +300,53 @@ mod tests {
         }
 
         let mut world = World::default();
-        let n = world.run_system_with(1, system);
+        let n = world.run_system_once_with(1, system);
         assert_eq!(n, 2);
         assert_eq!(world.resource::<T>().0, 1);
     }
+
+    #[derive(Resource, Default, PartialEq, Debug)]
+    struct Counter(u8);
+
+    #[allow(dead_code)]
+    fn count_up(mut counter: ResMut<Counter>) {
+        counter.0 += 1;
+    }
+
+    #[test]
+    fn run_two_systems() {
+        let mut world = World::new();
+        world.init_resource::<Counter>();
+        assert_eq!(*world.resource::<Counter>(), Counter(0));
+        world.run_system_once(count_up);
+        assert_eq!(*world.resource::<Counter>(), Counter(1));
+        world.run_system_once(count_up);
+        assert_eq!(*world.resource::<Counter>(), Counter(2));
+    }
+
+    #[allow(dead_code)]
+    fn spawn_entity(mut commands: Commands) {
+        commands.spawn_empty();
+    }
+
+    #[test]
+    fn command_processing() {
+        let mut world = World::new();
+        assert_eq!(world.entities.len(), 0);
+        world.run_system_once(spawn_entity);
+        assert_eq!(world.entities.len(), 1);
+    }
+
+    #[test]
+    fn non_send_resources() {
+        fn non_send_count_down(mut ns: NonSendMut<Counter>) {
+            ns.0 -= 1;
+        }
+
+        let mut world = World::new();
+        world.insert_non_send_resource(Counter(10));
+        assert_eq!(*world.non_send_resource::<Counter>(), Counter(10));
+        world.run_system_once(non_send_count_down);
+        assert_eq!(*world.non_send_resource::<Counter>(), Counter(9));
+    }
 }