docs/bevy/examples/ecs/contiguous_query.rs 2.4 K · 67 lines · raw · history

1 //! Demonstrates how contiguous queries work.
2 //!
3 //! Contiguous iteration enables getting slices of contiguously lying components (which lie in the same table), which for example
4 //! may be used for simd-operations, which may accelerate an algorithm.
5 //!
6 //! Contiguous iteration may be used for example via [`Query::contiguous_iter`], [`Query::contiguous_iter_mut`],
7 //! both of which return an option which is only [`None`] when the query doesn't support contiguous
8 //! iteration due to it not being dense (iteration happens on archetypes, not tables) or filters not being archetypal.
9 //!
10 //! For further documentation refer to:
11 //! - [`Query::contiguous_iter`]
12 //! - [`ContiguousQueryData`](`bevy::ecs::query::ContiguousQueryData`)
13 //! - [`ArchetypeFilter`](`bevy::ecs::query::ArchetypeFilter`)
14
15 use bevy::prelude::*;
16
17 #[derive(Component)]
18 /// When the value reaches 0.0 the entity dies
19 pub struct Health(pub f32);
20
21 #[derive(Component)]
22 /// Each tick an entity will have its health multiplied by the factor, which
23 /// for a big amount of entities can be accelerated using contiguous queries
24 pub struct HealthDecay(pub f32);
25
26 fn apply_health_decay(mut query: Query<(&mut Health, &HealthDecay)>) {
27 // contiguous_iter_mut() would return None if query couldn't be iterated contiguously
28 for (mut health, decay) in query.contiguous_iter_mut().unwrap() {
29 // all data slices returned by component queries are the same size
30 assert!(health.len() == decay.len());
31 // we could also bypass change detection via bypass_change_detection() because we do not
32 // use it anyways.
33 for (health, decay) in health.iter_mut().zip(decay) {
34 health.0 *= decay.0;
35 }
36 }
37 }
38
39 fn finish_off_first(mut commands: Commands, mut query: Query<(Entity, &mut Health)>) {
40 if let Some((entity, mut health)) = query.iter_mut().next() {
41 health.0 -= 1.0;
42 if health.0 <= 0.0 {
43 commands.entity(entity).despawn();
44 println!("Finishing off {entity:?}");
45 }
46 }
47 }
48
49 fn main() {
50 App::new()
51 .add_plugins(DefaultPlugins)
52 .add_systems(Update, (apply_health_decay, finish_off_first).chain())
53 .add_systems(Startup, setup)
54 .run();
55 }
56
57 fn setup(mut commands: Commands) {
58 let mut i = 0;
59 commands.spawn_batch(std::iter::from_fn(move || {
60 i += 1;
61 if i == 10_000 {
62 None
63 } else {
64 Some((Health(i as f32 * 5.0), HealthDecay(0.9)))
65 }
66 }));
67 }