pebble/ecs/
system_condition.rs

1use crate::ecs::{
2    resources::Resources,
3    system::{IntoSystem, System},
4    system_set::IntoSystemSet,
5};
6
7/// A predicate checked before a system runs. If `should_run` returns
8/// `false`, the wrapped system's body is skipped entirely for that tick —
9/// its `SystemParam`s are never fetched.
10///
11/// Re-checked every tick. This is deliberate: a resource that exists now
12/// is not guaranteed to exist forever (e.g. if something explicitly removes
13/// it later), so conditions should not assume "ready once" means "ready
14/// forever".
15pub trait RunCondition: 'static {
16    fn should_run(world: &hecs::World, resources: &Resources) -> bool;
17}
18
19/// Runs only while resource `T` exists.
20pub struct ResourceExists<T>(std::marker::PhantomData<T>);
21impl<T: 'static + Send + Sync> RunCondition for ResourceExists<T> {
22    fn should_run(world: &hecs::World, resources: &Resources) -> bool {
23        resources.has_resource::<T>(world)
24    }
25}
26
27/// Runs only while both `A` and `B` would run.
28pub struct And<A, B>(std::marker::PhantomData<(A, B)>);
29impl<A: RunCondition, B: RunCondition> RunCondition for And<A, B> {
30    fn should_run(world: &hecs::World, resources: &Resources) -> bool {
31        A::should_run(world, resources) && B::should_run(world, resources)
32    }
33}
34
35/// Runs while either `A` or `B` would run.
36pub struct Or<A, B>(std::marker::PhantomData<(A, B)>);
37impl<A: RunCondition, B: RunCondition> RunCondition for Or<A, B> {
38    fn should_run(world: &hecs::World, resources: &Resources) -> bool {
39        A::should_run(world, resources) || B::should_run(world, resources)
40    }
41}
42
43/// Wraps a [`System`], skipping it for a tick whenever `C::should_run`
44/// returns `false`.
45pub struct Conditional<S, C> {
46    inner: S,
47    _marker: std::marker::PhantomData<C>,
48}
49
50impl<S: System, C: RunCondition> System for Conditional<S, C> {
51    fn run(&mut self, world: &hecs::World, resources: &Resources) {
52        if C::should_run(world, resources) {
53            self.inner.run(world, resources);
54        }
55    }
56}
57
58/// Adds [`.run_if`](RunIfExt::run_if) to anything convertible into a
59/// [`System`], gating it on a [`RunCondition`].
60pub trait RunIfExt<Marker>: IntoSystem<Marker> + Sized {
61    fn run_if<C: RunCondition>(self) -> RunIfSystem<Self, Marker, C> {
62        RunIfSystem {
63            inner: self,
64            _marker: std::marker::PhantomData,
65        }
66    }
67}
68
69impl<Marker, T: IntoSystem<Marker>> RunIfExt<Marker> for T {}
70
71pub struct RunIfSystem<T, Marker, C> {
72    inner: T,
73    _marker: std::marker::PhantomData<(Marker, C)>,
74}
75
76impl<T, Marker, C> IntoSystem<Marker> for RunIfSystem<T, Marker, C>
77where
78    T: IntoSystem<Marker>,
79    C: RunCondition,
80{
81    type System = Conditional<T::System, C>;
82
83    fn into_system(self) -> Self::System {
84        Conditional {
85            inner: self.inner.into_system(),
86            _marker: std::marker::PhantomData,
87        }
88    }
89}
90
91/// Adds [`.run_if`](SystemSetRunIfExt::run_if) to an entire
92/// [`IntoSystemSet`] tuple at once, applying the same condition to every
93/// system in it.
94pub trait SystemSetRunIfExt<M>: IntoSystemSet<M> + Sized {
95    fn run_if<C: RunCondition>(self) -> ConditionalSet<Self, M, C> {
96        ConditionalSet {
97            inner: self,
98            _marker: std::marker::PhantomData,
99        }
100    }
101}
102
103impl<M, T: IntoSystemSet<M>> SystemSetRunIfExt<M> for T {}
104
105pub struct ConditionalSet<T, M, C> {
106    inner: T,
107    _marker: std::marker::PhantomData<(M, C)>,
108}
109
110/// Wraps an already-boxed, type-erased [`System`] with a [`RunCondition`].
111/// Used internally by [`ConditionalSet`], since by the time a system set is
112/// flattened into `Vec<Box<dyn System>>`, individual marker types are gone.
113struct BoxedConditional<C> {
114    inner: Box<dyn System>,
115    _marker: std::marker::PhantomData<C>,
116}
117
118impl<C: RunCondition> System for BoxedConditional<C> {
119    fn run(&mut self, world: &hecs::World, resources: &Resources) {
120        if C::should_run(world, resources) {
121            self.inner.run(world, resources);
122        }
123    }
124}
125
126impl<T, M, C> IntoSystemSet<M> for ConditionalSet<T, M, C>
127where
128    T: IntoSystemSet<M>,
129    C: RunCondition,
130{
131    fn into_system_set(self) -> Vec<Box<dyn System>> {
132        self.inner
133            .into_system_set()
134            .into_iter()
135            .map(|system| {
136                Box::new(BoxedConditional::<C> {
137                    inner: system,
138                    _marker: std::marker::PhantomData,
139                }) as Box<dyn System>
140            })
141            .collect()
142    }
143}
pebble/ecs/system_condition.rs

pebble/ecs/
system_condition.rs
