[Merged by Bors] - Add a SystemParam primitive for deferred mutations; allow #[derive]ing more types of SystemParam

crates/bevy_ecs/src/lib.rs

@@ -40,8 +40,9 @@ pub mod prelude {
         system::{
             adapter as system_adapter,
             adapter::{dbg, error, ignore, info, unwrap, warn},
-            Commands, In, IntoPipeSystem, IntoSystem, Local, NonSend, NonSendMut, ParallelCommands,
-            ParamSet, Query, RemovedComponents, Res, ResMut, Resource, System, SystemParamFunction,
+            Commands, Deferred, In, IntoPipeSystem, IntoSystem, Local, NonSend, NonSendMut,
+            ParallelCommands, ParamSet, Query, RemovedComponents, Res, ResMut, Resource, System,
+            SystemParamFunction,
         },
         world::{FromWorld, Mut, World},
     };

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

@@ -2,16 +2,18 @@ mod command_queue;
 mod parallel_scope;
 
 use crate::{
+    self as bevy_ecs,
     bundle::Bundle,
     entity::{Entities, Entity},
     world::{FromWorld, World},
 };
+use bevy_ecs_macros::SystemParam;
 use bevy_utils::tracing::{error, info};
 pub use command_queue::CommandQueue;
 pub use parallel_scope::*;
 use std::marker::PhantomData;
 
-use super::Resource;
+use super::{Deferred, Resource, SystemBuffer, SystemMeta};
 
 /// A [`World`] mutation.
 ///
@@ -95,22 +97,31 @@ pub trait Command: Send + 'static {
 ///
 /// [stage]: crate::schedule::SystemStage
 /// [`System::apply_buffers`]: crate::system::System::apply_buffers
+#[derive(SystemParam)]
 pub struct Commands<'w, 's> {
-    queue: &'s mut CommandQueue,
+    queue: Deferred<'s, CommandQueue>,
     entities: &'w Entities,
 }
 
+impl SystemBuffer for CommandQueue {
+    #[inline]
+    fn apply(&mut self, _system_meta: &SystemMeta, world: &mut World) {
+        #[cfg(feature = "trace")]
+        let _system_span =
+            bevy_utils::tracing::info_span!("system_commands", name = _system_meta.name())
+                .entered();
+        self.apply(world);
+    }
+}
+
 impl<'w, 's> Commands<'w, 's> {
     /// Returns a new `Commands` instance from a [`CommandQueue`] and a [`World`].
     ///
     /// It is not required to call this constructor when using `Commands` as a [system parameter].
     ///
     /// [system parameter]: crate::system::SystemParam
     pub fn new(queue: &'s mut CommandQueue, world: &'w World) -> Self {
-        Self {
-            queue,
-            entities: world.entities(),
-        }
+        Self::new_from_entities(queue, world.entities())
     }
 
     /// Returns a new `Commands` instance from a [`CommandQueue`] and an [`Entities`] reference.
@@ -119,7 +130,10 @@ impl<'w, 's> Commands<'w, 's> {
     ///
     /// [system parameter]: crate::system::SystemParam
     pub fn new_from_entities(queue: &'s mut CommandQueue, entities: &'w Entities) -> Self {
-        Self { queue, entities }
+        Self {
+            queue: Deferred(queue),
+            entities,
+        }
     }
 
     /// Pushes a [`Command`] to the queue for creating a new empty [`Entity`],

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

@@ -3,17 +3,16 @@ use std::cell::Cell;
 use thread_local::ThreadLocal;
 
 use crate::{
+    self as bevy_ecs,
     entity::Entities,
     prelude::World,
-    system::{SystemMeta, SystemParam},
+    system::{Deferred, SystemBuffer, SystemMeta, SystemParam},
 };
 
 use super::{CommandQueue, Commands};
 
-/// The internal [`SystemParam`] state of the [`ParallelCommands`] type
-#[doc(hidden)]
 #[derive(Default)]
-pub struct ParallelCommandsState {
+struct ParallelCommandQueue {
     thread_local_storage: ThreadLocal<Cell<CommandQueue>>,
 }
 
@@ -43,41 +42,23 @@ pub struct ParallelCommandsState {
 /// }
 /// # bevy_ecs::system::assert_is_system(parallel_command_system);
 ///```
+#[derive(SystemParam)]
 pub struct ParallelCommands<'w, 's> {
-    state: &'s mut ParallelCommandsState,
+    state: Deferred<'s, ParallelCommandQueue>,
     entities: &'w Entities,
 }
 
-// SAFETY: no component or resource access to report
-unsafe impl SystemParam for ParallelCommands<'_, '_> {
-    type State = ParallelCommandsState;
-    type Item<'w, 's> = ParallelCommands<'w, 's>;
-
-    fn init_state(_: &mut World, _: &mut crate::system::SystemMeta) -> Self::State {
-        ParallelCommandsState::default()
-    }
-
-    fn apply(state: &mut Self::State, _system_meta: &SystemMeta, world: &mut World) {
+impl SystemBuffer for ParallelCommandQueue {
+    #[inline]
+    fn apply(&mut self, _system_meta: &SystemMeta, world: &mut World) {
         #[cfg(feature = "trace")]
         let _system_span =
             bevy_utils::tracing::info_span!("system_commands", name = _system_meta.name())
                 .entered();
-        for cq in &mut state.thread_local_storage {
+        for cq in &mut self.thread_local_storage {
             cq.get_mut().apply(world);
         }
     }
-
-    unsafe fn get_param<'w, 's>(
-        state: &'s mut Self::State,
-        _: &crate::system::SystemMeta,
-        world: &'w World,
-        _: u32,
-    ) -> Self::Item<'w, 's> {
-        ParallelCommands {
-            state,
-            entities: world.entities(),
-        }
-    }
 }
 
 impl<'w, 's> ParallelCommands<'w, 's> {

crates/bevy_ecs/src/system/system_param.rs

@@ -8,7 +8,7 @@ use crate::{
     query::{
         Access, FilteredAccess, FilteredAccessSet, QueryState, ReadOnlyWorldQuery, WorldQuery,
     },
-    system::{CommandQueue, Commands, Query, SystemMeta},
+    system::{Query, SystemMeta},
     world::{FromWorld, World},
 };
 pub use bevy_ecs_macros::Resource;
@@ -151,6 +151,8 @@ pub unsafe trait SystemParam: Sized {
 
     /// Applies any deferred mutations stored in this [`SystemParam`]'s state.
     /// This is used to apply [`Commands`] at the end of a stage.
+    ///
+    /// [`Commands`]: crate::prelude::Commands
     #[inline]
     #[allow(unused_variables)]
     fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World) {}
@@ -674,37 +676,6 @@ unsafe impl<'a, T: Resource> SystemParam for Option<ResMut<'a, T>> {
     }
 }
 
-// SAFETY: Commands only accesses internal state
-unsafe impl<'w, 's> ReadOnlySystemParam for Commands<'w, 's> {}
-
-// SAFETY: only local state is accessed
-unsafe impl SystemParam for Commands<'_, '_> {
-    type State = CommandQueue;
-    type Item<'w, 's> = Commands<'w, 's>;
-
-    fn init_state(_world: &mut World, _system_meta: &mut SystemMeta) -> Self::State {
-        Default::default()
-    }
-
-    fn apply(state: &mut Self::State, _system_meta: &SystemMeta, world: &mut World) {
-        #[cfg(feature = "trace")]
-        let _system_span =
-            bevy_utils::tracing::info_span!("system_commands", name = _system_meta.name())
-                .entered();
-        state.apply(world);
-    }
-
-    #[inline]
-    unsafe fn get_param<'w, 's>(
-        state: &'s mut Self::State,
-        _system_meta: &SystemMeta,
-        world: &'w World,
-        _change_tick: u32,
-    ) -> Self::Item<'w, 's> {
-        Commands::new(state, world)
-    }
-}
-
 /// SAFETY: only reads world
 unsafe impl<'w> ReadOnlySystemParam for &'w World {}
 
@@ -868,6 +839,142 @@ unsafe impl<'a, T: FromWorld + Send + 'static> SystemParam for Local<'a, T> {
     }
 }
 
+/// Types that can be used with [`Deferred<T>`] in systems.
+/// This allows storing system-local data which is used to defer [`World`] mutations.
+///
+/// When implementing `SystemBuffer`, you should take care to perform as many
+/// computations up-front as possible. Buffers cannot be applied in parallel,
+/// so you should try to minimize the time spent in [`SystemBuffer::apply`].
+pub trait SystemBuffer: FromWorld + Send + 'static {
+    /// Applies any deferred mutations to the [`World`].
+    fn apply(&mut self, system_meta: &SystemMeta, world: &mut World);
+}
+
+/// A [`SystemParam`] that stores a buffer which gets applied to the [`World`] at the end of a stage.
+/// By using this to defer mutations, you can avoid mutable `World` data access within a system
+/// and allow it to run in parallel with more systems.
+///
+/// The most common example of this pattern is [`Commands`], which uses [`Deferred<T>`] to defer mutations.
+///
+/// [`Commands`]: crate::system::Commands
+///
+/// # Examples
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// // Tracks whether or not there is a threat the player should be aware of.
+/// #[derive(Resource, Default)]
+/// pub struct Alarm(bool);
+///
+/// // A threat from inside the settlement.
+/// #[derive(Component)]
+/// pub struct Criminal;
+///
+/// // A threat from outside the settlement.
+/// #[derive(Component)]
+/// pub struct Monster;
+///
+/// use bevy_ecs::system::{Deferred, SystemBuffer, SystemMeta};
+///
+/// // Uses deferred mutations to allow signalling the alarm from multiple systems in parallel.
+/// #[derive(Resource, Default)]
+/// struct AlarmFlag(bool);
+///
+/// impl SystemBuffer for AlarmFlag {
+///     fn apply(&mut self, system_meta: &SystemMeta, world: &mut World) {
+///         if self.0 {
+///             world.resource_mut::<Alarm>().0 = true;
+///             self.0 = false;
+///         }
+///     }
+/// }
+///
+/// // Sound the alarm if there is a criminal.
+/// fn alert_criminal(
+///     criminals: Query<&Criminal>,
+///     mut alarm: Deferred<AlarmFlag>
+/// ) {
+///     if criminals.iter().next().is_some() {
+///         alarm.0 = true;
+///     }
+/// }
+///
+/// // Sound the alarm if there is a monster.
+/// fn alert_monster(
+///     monsters: Query<&Monster>,
+///     mut alarm: Deferred<AlarmFlag>
+/// ) {
+///     if monsters.iter().next().is_some() {
+///         alarm.0 = true;
+///     }
+/// }
+///
+/// let mut world = World::new();
+/// world.init_resource::<Alarm>();
+///
+/// let mut stage = SystemStage::parallel();
+/// stage
+///     // These two systems have no conflicts and will run in parallel.
+///     .add_system(alert_criminal)
+///     .add_system(alert_monster);
+///
+/// // There are no criminals or monsters, so the alarm is not sounded.
+/// stage.run(&mut world);
+/// assert_eq!(world.resource::<Alarm>().0, false);
+///
+/// // Spawn a monster, which will cause the alarm to be sounded.
+/// world.spawn(Monster);
+/// stage.run(&mut world);
+/// assert_eq!(world.resource::<Alarm>().0, true);
+///
+/// // Reset the alarm.
+/// world.resource_mut::<Alarm>().0 = false;
+///
+/// // Spawn a criminal, which will cause the alarm to be sounded.
+/// world.spawn(Criminal);
+/// stage.run(&mut world);
+/// assert_eq!(world.resource::<Alarm>().0, true);
+/// ```
+pub struct Deferred<'a, T: SystemBuffer>(pub(crate) &'a mut T);
+
+impl<'a, T: SystemBuffer> Deref for Deferred<'a, T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl<'a, T: SystemBuffer> DerefMut for Deferred<'a, T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.0
+    }
+}
+
+// SAFETY: Only local state is accessed.
+unsafe impl<T: SystemBuffer> ReadOnlySystemParam for Deferred<'_, T> {}
+
+// SAFETY: Only local state is accessed.
+unsafe impl<T: SystemBuffer> SystemParam for Deferred<'_, T> {
+    type State = SyncCell<T>;
+    type Item<'w, 's> = Deferred<'s, T>;
+    fn init_state(world: &mut World, _system_meta: &mut SystemMeta) -> Self::State {
+        SyncCell::new(T::from_world(world))
+    }
+    fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World) {
+        state.get().apply(system_meta, world);
+    }
+    unsafe fn get_param<'w, 's>(
+        state: &'s mut Self::State,
+        _system_meta: &SystemMeta,
+        _world: &'w World,
+        _change_tick: u32,
+    ) -> Self::Item<'w, 's> {
+        Deferred(state.get())
+    }
+}
+
 /// A [`SystemParam`] that grants access to the entities that had their `T` [`Component`] removed.
 ///
 /// Note that this does not allow you to see which data existed before removal.