bevy_ecs/system/
combinator.rs

1use std::{borrow::Cow, cell::UnsafeCell, marker::PhantomData};
2
3use bevy_ptr::UnsafeCellDeref;
4
5use crate::{
6    archetype::ArchetypeComponentId,
7    component::{ComponentId, Tick},
8    prelude::World,
9    query::Access,
10    schedule::InternedSystemSet,
11    world::unsafe_world_cell::UnsafeWorldCell,
12};
13
14use super::{ReadOnlySystem, System};
15
16/// Customizes the behavior of a [`CombinatorSystem`].
17///
18/// # Examples
19///
20/// ```
21/// use bevy_ecs::prelude::*;
22/// use bevy_ecs::system::{CombinatorSystem, Combine};
23///
24/// // A system combinator that performs an exclusive-or (XOR)
25/// // operation on the output of two systems.
26/// pub type Xor<A, B> = CombinatorSystem<XorMarker, A, B>;
27///
28/// // This struct is used to customize the behavior of our combinator.
29/// pub struct XorMarker;
30///
31/// impl<A, B> Combine<A, B> for XorMarker
32/// where
33///     A: System<In = (), Out = bool>,
34///     B: System<In = (), Out = bool>,
35/// {
36///     type In = ();
37///     type Out = bool;
38///
39///     fn combine(
40///         _input: Self::In,
41///         a: impl FnOnce(A::In) -> A::Out,
42///         b: impl FnOnce(B::In) -> B::Out,
43///     ) -> Self::Out {
44///         a(()) ^ b(())
45///     }
46/// }
47///
48/// # #[derive(Resource, PartialEq, Eq)] struct A(u32);
49/// # #[derive(Resource, PartialEq, Eq)] struct B(u32);
50/// # #[derive(Resource, Default)] struct RanFlag(bool);
51/// # let mut world = World::new();
52/// # world.init_resource::<RanFlag>();
53/// #
54/// # let mut app = Schedule::default();
55/// app.add_systems(my_system.run_if(Xor::new(
56///     IntoSystem::into_system(resource_equals(A(1))),
57///     IntoSystem::into_system(resource_equals(B(1))),
58///     // The name of the combined system.
59///     std::borrow::Cow::Borrowed("a ^ b"),
60/// )));
61/// # fn my_system(mut flag: ResMut<RanFlag>) { flag.0 = true; }
62/// #
63/// # world.insert_resource(A(0));
64/// # world.insert_resource(B(0));
65/// # app.run(&mut world);
66/// # // Neither condition passes, so the system does not run.
67/// # assert!(!world.resource::<RanFlag>().0);
68/// #
69/// # world.insert_resource(A(1));
70/// # app.run(&mut world);
71/// # // Only the first condition passes, so the system runs.
72/// # assert!(world.resource::<RanFlag>().0);
73/// # world.resource_mut::<RanFlag>().0 = false;
74/// #
75/// # world.insert_resource(B(1));
76/// # app.run(&mut world);
77/// # // Both conditions pass, so the system does not run.
78/// # assert!(!world.resource::<RanFlag>().0);
79/// #
80/// # world.insert_resource(A(0));
81/// # app.run(&mut world);
82/// # // Only the second condition passes, so the system runs.
83/// # assert!(world.resource::<RanFlag>().0);
84/// # world.resource_mut::<RanFlag>().0 = false;
85/// ```
86#[diagnostic::on_unimplemented(
87    message = "`{Self}` can not combine systems `{A}` and `{B}`",
88    label = "invalid system combination",
89    note = "the inputs and outputs of `{A}` and `{B}` are not compatible with this combiner"
90)]
91pub trait Combine<A: System, B: System> {
92    /// The [input](System::In) type for a [`CombinatorSystem`].
93    type In;
94
95    /// The [output](System::Out) type for a [`CombinatorSystem`].
96    type Out;
97
98    /// When used in a [`CombinatorSystem`], this function customizes how
99    /// the two composite systems are invoked and their outputs are combined.
100    ///
101    /// See the trait-level docs for [`Combine`] for an example implementation.
102    fn combine(
103        input: Self::In,
104        a: impl FnOnce(A::In) -> A::Out,
105        b: impl FnOnce(B::In) -> B::Out,
106    ) -> Self::Out;
107}
108
109/// A [`System`] defined by combining two other systems.
110/// The behavior of this combinator is specified by implementing the [`Combine`] trait.
111/// For a full usage example, see the docs for [`Combine`].
112pub struct CombinatorSystem<Func, A, B> {
113    _marker: PhantomData<fn() -> Func>,
114    a: A,
115    b: B,
116    name: Cow<'static, str>,
117    component_access: Access<ComponentId>,
118    archetype_component_access: Access<ArchetypeComponentId>,
119}
120
121impl<Func, A, B> CombinatorSystem<Func, A, B> {
122    /// Creates a new system that combines two inner systems.
123    ///
124    /// The returned system will only be usable if `Func` implements [`Combine<A, B>`].
125    pub const fn new(a: A, b: B, name: Cow<'static, str>) -> Self {
126        Self {
127            _marker: PhantomData,
128            a,
129            b,
130            name,
131            component_access: Access::new(),
132            archetype_component_access: Access::new(),
133        }
134    }
135}
136
137impl<A, B, Func> System for CombinatorSystem<Func, A, B>
138where
139    Func: Combine<A, B> + 'static,
140    A: System,
141    B: System,
142{
143    type In = Func::In;
144    type Out = Func::Out;
145
146    fn name(&self) -> Cow<'static, str> {
147        self.name.clone()
148    }
149
150    fn component_access(&self) -> &Access<ComponentId> {
151        &self.component_access
152    }
153
154    fn archetype_component_access(&self) -> &Access<ArchetypeComponentId> {
155        &self.archetype_component_access
156    }
157
158    fn is_send(&self) -> bool {
159        self.a.is_send() && self.b.is_send()
160    }
161
162    fn is_exclusive(&self) -> bool {
163        self.a.is_exclusive() || self.b.is_exclusive()
164    }
165
166    fn has_deferred(&self) -> bool {
167        self.a.has_deferred() || self.b.has_deferred()
168    }
169
170    unsafe fn run_unsafe(&mut self, input: Self::In, world: UnsafeWorldCell) -> Self::Out {
171        Func::combine(
172            input,
173            // SAFETY: The world accesses for both underlying systems have been registered,
174            // so the caller will guarantee that no other systems will conflict with `a` or `b`.
175            // Since these closures are `!Send + !Sync + !'static`, they can never be called
176            // in parallel, so their world accesses will not conflict with each other.
177            // Additionally, `update_archetype_component_access` has been called,
178            // which forwards to the implementations for `self.a` and `self.b`.
179            |input| unsafe { self.a.run_unsafe(input, world) },
180            // SAFETY: See the comment above.
181            |input| unsafe { self.b.run_unsafe(input, world) },
182        )
183    }
184
185    fn run<'w>(&mut self, input: Self::In, world: &'w mut World) -> Self::Out {
186        // SAFETY: Converting `&mut T` -> `&UnsafeCell<T>`
187        // is explicitly allowed in the docs for `UnsafeCell`.
188        let world: &'w UnsafeCell<World> = unsafe { std::mem::transmute(world) };
189        Func::combine(
190            input,
191            // SAFETY: Since these closures are `!Send + !Sync + !'static`, they can never
192            // be called in parallel. Since mutable access to `world` only exists within
193            // the scope of either closure, we can be sure they will never alias one another.
194            |input| self.a.run(input, unsafe { world.deref_mut() }),
195            #[allow(clippy::undocumented_unsafe_blocks)]
196            |input| self.b.run(input, unsafe { world.deref_mut() }),
197        )
198    }
199
200    fn apply_deferred(&mut self, world: &mut World) {
201        self.a.apply_deferred(world);
202        self.b.apply_deferred(world);
203    }
204
205    #[inline]
206    fn queue_deferred(&mut self, mut world: crate::world::DeferredWorld) {
207        self.a.queue_deferred(world.reborrow());
208        self.b.queue_deferred(world);
209    }
210
211    fn initialize(&mut self, world: &mut World) {
212        self.a.initialize(world);
213        self.b.initialize(world);
214        self.component_access.extend(self.a.component_access());
215        self.component_access.extend(self.b.component_access());
216    }
217
218    fn update_archetype_component_access(&mut self, world: UnsafeWorldCell) {
219        self.a.update_archetype_component_access(world);
220        self.b.update_archetype_component_access(world);
221
222        self.archetype_component_access
223            .extend(self.a.archetype_component_access());
224        self.archetype_component_access
225            .extend(self.b.archetype_component_access());
226    }
227
228    fn check_change_tick(&mut self, change_tick: Tick) {
229        self.a.check_change_tick(change_tick);
230        self.b.check_change_tick(change_tick);
231    }
232
233    fn default_system_sets(&self) -> Vec<InternedSystemSet> {
234        let mut default_sets = self.a.default_system_sets();
235        default_sets.append(&mut self.b.default_system_sets());
236        default_sets
237    }
238
239    fn get_last_run(&self) -> Tick {
240        self.a.get_last_run()
241    }
242
243    fn set_last_run(&mut self, last_run: Tick) {
244        self.a.set_last_run(last_run);
245        self.b.set_last_run(last_run);
246    }
247}
248
249/// SAFETY: Both systems are read-only, so any system created by combining them will only read from the world.
250unsafe impl<A, B, Func> ReadOnlySystem for CombinatorSystem<Func, A, B>
251where
252    Func: Combine<A, B> + 'static,
253    A: ReadOnlySystem,
254    B: ReadOnlySystem,
255{
256}
257
258impl<Func, A, B> Clone for CombinatorSystem<Func, A, B>
259where
260    A: Clone,
261    B: Clone,
262{
263    /// Clone the combined system. The cloned instance must be `.initialize()`d before it can run.
264    fn clone(&self) -> Self {
265        CombinatorSystem::new(self.a.clone(), self.b.clone(), self.name.clone())
266    }
267}
268
269/// A [`System`] created by piping the output of the first system into the input of the second.
270///
271/// This can be repeated indefinitely, but system pipes cannot branch: the output is consumed by the receiving system.
272///
273/// Given two systems `A` and `B`, A may be piped into `B` as `A.pipe(B)` if the output type of `A` is
274/// equal to the input type of `B`.
275///
276/// Note that for [`FunctionSystem`](crate::system::FunctionSystem)s the output is the return value
277/// of the function and the input is the first [`SystemParam`](crate::system::SystemParam) if it is
278/// tagged with [`In`](crate::system::In) or `()` if the function has no designated input parameter.
279///
280/// # Examples
281///
282/// ```
283/// use std::num::ParseIntError;
284///
285/// use bevy_ecs::prelude::*;
286///
287/// fn main() {
288///     let mut world = World::default();
289///     world.insert_resource(Message("42".to_string()));
290///
291///     // pipe the `parse_message_system`'s output into the `filter_system`s input
292///     let mut piped_system = parse_message_system.pipe(filter_system);
293///     piped_system.initialize(&mut world);
294///     assert_eq!(piped_system.run((), &mut world), Some(42));
295/// }
296///
297/// #[derive(Resource)]
298/// struct Message(String);
299///
300/// fn parse_message_system(message: Res<Message>) -> Result<usize, ParseIntError> {
301///     message.0.parse::<usize>()
302/// }
303///
304/// fn filter_system(In(result): In<Result<usize, ParseIntError>>) -> Option<usize> {
305///     result.ok().filter(|&n| n < 100)
306/// }
307/// ```
308pub type PipeSystem<SystemA, SystemB> = CombinatorSystem<Pipe, SystemA, SystemB>;
309
310#[doc(hidden)]
311pub struct Pipe;
312
313impl<A, B> Combine<A, B> for Pipe
314where
315    A: System,
316    B: System<In = A::Out>,
317{
318    type In = A::In;
319    type Out = B::Out;
320
321    fn combine(
322        input: Self::In,
323        a: impl FnOnce(A::In) -> A::Out,
324        b: impl FnOnce(B::In) -> B::Out,
325    ) -> Self::Out {
326        let value = a(input);
327        b(value)
328    }
329}
bevy_ecs/system/combinator.rs

bevy_ecs/system/
combinator.rs
